testing 1985-89 M.A.F. sensor


Staff member
http://www.mamotorworks.com/corvette-19 ... 5-893.html






"Check terminal C on the MAF sensor - it should be about 0.5 volts at idle and 4.7-volts at wide open throttle. Trace the wire from terminal C and look for a short to ground. All else fails, replace the MAF sensor."

['84-'91 Haynes manual.]

this infos bound to be helpful at times
Sensor Locations


Engine Coolant Temperature Sensor. Front of engine, below Throttle Body.
Engine Oil Temperature Sensor. Left rear of engine, just above the oil filter.
Oil Pressure Sender/Switch. Top, left hand rear of engine.
Fuel Quantity Sender. Top of fuel tank, beneath filler pipe escutcheon panel.
MAT (Manifold Absolute Temperature Sensor). Underside of manifold air plenum at rear.
Outside Temperature Sensor. Right side of engine, top right corner of radiator.
In Car Temp Temperature Sensor. Coupe: above left seat near interior courtesy light, Convertible: center of cargo compartment lid.
MAF (Mass Air Flow) Sensor. Front of engine ahead of throttle body.
Oxygen (O2) Sensor. Left side of engine, in exhaust pipe.
TPS (Throttle Position Sensor). Right side of throttle body at the front.

Sensor Outputs:

Measured Value

Engine Coolant Temperature Sensor. 185 Ohms @ 210F, 3400 Ohms @ 68F, 7,500 Ohms @ 39 F.
Engine Oil Temperature Sensor. 185 Ohms @ 210 F, 3400 Ohms @ 68 F, 7,500 Ohms @39 F.
Oil Pressure Sender/Switch. 1 Ohms @ 0 PSI, 43 Ohms @ 30 PSI, 86 Ohms @ 60 PSI.
Fuel Quantity Sender. 0 Ohms @ Empty, 45 Ohms @ 1/2 Full, 90 Ohms @ Full.
MAT (Manifold Absolute Temperature Sensor). 185 Ohms @ 210 F, 3400 Ohms @ 70 F, 15,000 Ohms @ 40 F.
Outside Temperature Sensor. 4400 Ohms @ 60 F, 2200 Ohms @ 85 F.
In Car Temp Temperature Sensor. 4400 Ohms @ 60 F, 2200 Ohms @ 85 F.
MAF (Mass Air Flow) Sensor. .4 Volts @ idle, 5 Volts @ Full Throttle.
Oxygen (O2) Sensor. .1 Volt Lean Mixture, .9 Volt Rich Mixture.
TPS (Throttle Position Sensor). .54 Volts Idle, ~ 5 Volts Full Throttle.







a factory shop manual and a v.o.m. meter come in handy for testing!
http://www.chevythunder.com/Flow chart index.htm




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Staff member


Mass Air Flow Sensor Facts
youll find many guys claiming that aftermarket CPU chip upgrades help a great deal
Ive found some help marginally but many actually hurt performance

http://www.iroc-z.com/articles/articlepages/1990-3Chips Article by Cliff Gromer, Super Stock, 1991.htm
Most of the C4 Corvettes used a MAF (Mass Air Flow) sensor to determine how much air is being pulled into the intake manifold. The exceptions are the 1984 Corvette that used a speed density system--a sort of predictive method of measurement---and the 1990 through 1993 C4 models which were also speed density based. In 1994, Corvette went back to the MAF based system but used the speed density approach as a back up. (1989 Bosch MAF installation shown at right).

A Mass Air Flow sensor has an extremely fine wire inside its bore. The 1985 through 1989 C4 engines used a Bosch MAF sensor that heated the wire to 100 C. The 1994 and later C4 models used a AC/Delco MAF that heated the wire to 200 C. The amount of current required to reach the temperature is measured in each case. (Note: the LT-5 engine used in the ZR-1 used a speed density system and continued to use that system in 1994 and 1995 since the engines had already been made prior to the last two years of production. The ZR-1 therefore has no MAF even after Corvette went back to the MAF based system).

Theory of Operation

As the air travels past the heated wire enroute to the intake manifold, it will cool the wire and additional current is added to again heat the wire to the design temperature. Since the amount of air moving past the sensor is directly related to the amount of cooling experienced by the heated wire, a feedback condition is established whereby the exact amount of moving air is directly related to the amount of current passing through the wire and the intake air is therefore precisely measured.

Once the amount of air is known, the computer controlling the engine can add or subtract fuel as required to maintain the magic 14.7:1 air-fuel mixture resulting in the cleanest burn possible from an emissions (pollution) standpoint.

It does this by varying the "on time" of the fuel injectors. The injectors are pulsed on and off and the width of the pulse is lengthened or shortened as required. When you first start a typical engine, the pulse width is around 4 milliseconds but as soon as the engine "catches" the pulse width is shortened to about 2.2 milliseconds for idle. During operation, the measured air flow through the MAF will cause the computer to increase or decrease the pulse width as explained above.

MAF Operating Conditions

The Bosch MAF is more complex than the AC/Delco version. Both measure the air flow but the Bosch MAF has a circuit called the 'burn-off circuit' that cycles on for about 2 seconds when you shut the engine down. This circuit heats the wire to a high enough temperature to burn off any residue that may have collected on the wire during operation. If you are in a quiet area, you can hear the relays click on and then off on a 1985-1989 C4 as the burn-off cycle occurs.

There are two relays involved with the Bosch MAF: A power relay that passes current to the MAF wire during normal operation and the burn-off relay that provides the current for the cleaning cycle. Both are located on the firewall in the engine compartment, just behind the battery on the drivers side. Bad MAF power and burn-off relays can cause hard starting problems and should be changed periodically as preventative measure and any time you experience hard starting conditions.

The AC/Delco MAF has a power relay but no burn-off relay. For this reason, you should pay even closer attention to the condition of your air filter on a later model C4 than normal since a contaminated wire in a AC/Delco MAF is going to stay contaminated for the most part and cause false signals to be passed to the computer.

Also, the Bosch MAF outputs its information as a analog signal to the computer but the AC/Delco sends its signal as a digital component of varying frequency. For this reason, you cannot measure it's operation directly.

A scan tool is generally the best way to troubleshoot engine problems and with the 1994 and later Corvette, it is virtually mandatory. (An oscilloscope will also work on the AC/Delco MAF but a regular test meter will not).

MAF Problems

Faulty MAF sensors will normally light the check engine light on the drivers information center if the problem is constant and store a trouble code. If intermittent, a trouble code will still be stored as long as the battery is not disconnected.

Normally, the problem is a poor connection at the sensor and wiggling the wires, unplugging and reinserting the connector will often cure the problem.

A faulty MAF will normally cause a no start or difficult start condition and although you can eventually get the car into the "limp-home" mode in most cases, you need to attend to the problem ASAP.

AC/Delco sensors can become intermittent or give false readings if the wires become contaminated as explained above.

The MAF is a critical part of the emission control system and as such will cause the computer to react to problems very quickly, setting trouble codes and reducing performance in ways that cannot be ignored for long.

MAF Mods

The Bosch MAF is often modified by removing the two screens that are present in the front and rear of the cylinder. Removing these screens significantly increases the air flow through them and this results in more horsepower. Removing the screens is an old trick from the Corvette Challenge days in 1988 and 1989. It does work but is illegal in many states so be advised not to do anything that will get you arrested for a pollution violation.

The AC/Delco MAF is not readily modified. It is what it is but since it is a larger diameter than the Bosch, it responds well to changing the air filter to a free flowing type such as the K&N filter.

read related links and sub-links





the later 1992-96 corvette lt1 used a MAT SENSOR










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Staff member
wildcatlock said:
Ok guys, many of us have had a persistent code related to the maf. Either code 34,35, or 36. Some here have swapped anything and everything and still faught to fix these codes. Well I too had a persistent code. Mine was a code 34 - maf too high. Like most I tried everything....changed out and replaced the relays, tried a known good maf, went through the FSM checklist more times than I could count!!! Welll heres what I came up with. was searching around one day and came across a maf connector with pigtail. It was only 15 bucks and as I had already tried everything else and then some I figured why not. Swapped it out in about 45 minutes including soldering and heat shrink time. Old connector tested out good per FSM, but apparently the connections were worn or corroded to the point they wouldn't make good contact in the maf. After replacement code is gone and car runs better than ever.
So to all those out here who have the same persistent code and cant get it to stay cleared, you might give this a try. Our cars are getting old and wiring is getting brittle. Guess the connectors need a good looking over too, even if they test good. Hell for 15 bucks and about 45 minutes, if you've tried everything else, how can you go wrong.:cheers:
jktucker92 said:
There are books written out there that can give you the advantages / disadvantages of each system, but I'll try to keep it brief. In order to run as efficient as possible, you need to mix fuel and air at a specific mass ratio. 14.7:1 is the Stoichiometric ratio that is ideal, but richer mixtures can provide more power. With a MAF sensor, you measure the mass of the air flowing in the intake, which makes the calculation of how much fuel simple and accurate. The problem is the MAF sensors are more expensive than a simple pressure and speed sensor, especially early on. Also, the MAF sensors can be restrictive when you want to increase performance, so they are often removed in high performance applications. A speed density system uses the speed of the engine, manifold pressure, and temperature to calculate the mass of the air flowing into the intake and into the cylinder. This is pretty accurate, but not as accurate as a MAF sensor. As a result, the engines generally are tuned to run a little richer than the MAF systems to avoid damaging the engine by running too lean all the time.
Whether an engine is batch or sequential injected is a different, but related topic. In order to have sequential injection, the engine must be port injected, and the injector fires on each cylinder as each valve is opened. Batch systems can fire all the injectors on each revolution, or half one one revolution and the other half on the next. As it comes out of the injector, it's a fine mist, and the longer it is in the manifold, the more that mist becomes larger droplets, which burn less efficiently. Port injected systems are better than throttle body systems because their injectors are close to the ports and the fuel stays in the fine mist better.
The most efficient system is a MAF sequential injected system, which is why all new vehicles are MAF sequential injected systems.

the early corvettes like my 1985 came with a 160 baud processor speed, this was so slow that it took almost a full second to respond to sensor data,and make the next change required, making use of things like dry nitrous injection that in theory would rely on the engine sensing the overly lean fuel/air ratio a massive increase in oxygen to fuel ratio a sure way to burn pistons, simply because the early cpu processor speed was hopelessly slow. batch fire allowed the computer to control 4 injectors with a single control pulse ,effectively reducing the required processor speed by 75%
some of the early tpi injection intakes like the 1985 came with a 9th cold start injector whos only real function is to act as a choke and richen the fuel air ratio during cold engine starts
the function of the cold start injector on the 1985 tpi is to provide extra fuel, acting like a carb choke richening the fuel air ratio, if you just plug off the fuel rail and intake and remove the 9th injector it will still run OK just like a carburetor, without a choke will once its up to operating temperature, but it will be a P.I.T.A. to start on cool mornings and ALWAYS take a bit longer to start up, as would a carb engine without a choke because EFI doesn,t have the accelerator pump function where you can remove or bye-pass the symptoms by flooring the carb several times , to get the accelerator pump shot to richen the fuel air ratio.
but what you can do is upgrade the absurdly slow 160 baud processor this meant the computer control responding to sensor input,could ad or remove fuel only a couple times a second the CPU has to a more current version which controls ALL the injectors and RICHENS all the pulse durations to all the injectors under cold start conditions, thus eliminating the need for the 9th injector or installing its feed or connectors, but obviously the rear of the drivers side fuel rail needs to be plugged if the 9th injector feed is removes as will the injector mount hole in the intake manifold if its been removed
data is sent at 8192 baud by 1989, or 51 TIMES FASTER, and by todays standards the 1989 processor speeds a joke itself.as current speeds are hundreds of times faster, allowing individual cylinders to be tuned independently hundreds of times a second


85-89 High Performance Adjustable MAF Sensor


Re: MAF & code 34

the threads linked below may help, but its more than likely a bad temp sensor or the TPS OR IAC is out of adjustment. so the first thing Id suggest is to adjust the TPS and IAC per the linked info and check for loose connections and vacuum leaks, and loose plenum and intake runner gaskets
34 Mass Air Flow Circuit (1985-1990) Clean the throttle body. Check MAF connections. Replace MAF relay. Replace MAF Sensor. Possible ECM failure.
34 Manifold Absolute Pressure Low (1984) Check Vacuum hoses associated with MAP sensor. Check wiring and connections, particularly at ECM. Replace the sensor. Possible ECM failure.






http://www.mamotorworks.com/corvette-c4 ... 6-893.html
85-89 High Performance Adjustable MAF Sensor



Item Number: EH-521

heres a rather useful addition to the older c4 corvette tuning world, its a new (yes expensive)
but adjustable and thus some what tune-able MASS AIR FLOW SENSOR for the TPI corvettes
85-89 MAF TPI Systems

Below is a list of all the needed sensors to install a MAF TPI setup, and each of their functions.

Mass Air Flow (MAF) Sensor: This sensor is responsible for measuring air volume and density. It is located in the air duct, before the throttle body. All of the air that the engine consumes must first pass through the MAF sensor. At the center of the MAF, is a very thin wire whose resistance increases as it goes up in temperature. A constant voltage is applied to this wire. Air being drawn through the MAF has the effect of cooling this wire, which lowers its resistance, and increases current. As you might suspect, the more air is drawn through the MAF sensor, the greater the current flow. It is important to note that hot dry air is less dense and has less mass than cool moist air. As a result, hot dry air will cool the wire less than cool moist air.

A circuit mounted on the MAF sensor serves to convert the current flow into a variable frequency square wave on 1985 models, which is sent to the ecm. MAF units from 86-89 models output a simple analog signal instead of using frequency modulation like the 1985 units. The ecm will calculate the amount of fuel needed depending on the signal from the MAF sensor. It is very important that there are no air leaks (from a ripped air duct for example) between the MAF and the throttle body.
Oxygen Sensor: Responsible for determining the amount of oxygen in the exhaust manifold. Depending on how much oxygen is in the exhaust, the ecm can determine whether or not the air/fuel mixture is rich or lean. The signal sent to the ecm by the ecm varies between 0.0 and 1.0 volts. An ideal mixture (also known as a Stoichiometric mixture) of 14.7:1 is represented by .450 volts. If the oxygen sensor voltage is below .450, then the air/fuel mixture is lean. Anything over .450 means the mixture is rich. Since the sensor is essentially just a switching device, it will be fluctuating alot between lean and rich. This is normal, and an indication that the sensor is in working properly.

Keep in mind however that oxygen sensors (except wideband oxygen sensors), are not very accurate below or above .450 volts. Exhaust gas temperature will affect the oxygen sensor reading as well. The sensor will not read properly until exhaust gas temperature reaches approximately 600 degrees Farenheit. If you have headers installed, it is a good idea to use a heated oxygen sensor (3 wire) instead of the usual single wire sensor. Headers usually place the oxygen sensor further down the exhaust stream, where exhaust temperatures are cooler. A heated oxygen sensor will heat itself, allowing a more reliable sensor reading than a single wire sensor. If you have factory exhaust manifolds, then the single wire sensor is adequate.

Oxygen sensors are a regular maintenance item, and should be replaced every 30,000 miles. When an oxygen sensor goes bad, it tends to read lean, and will not fluctuate very much. The ecm will attempt to correct this false lean condition by richening the mixture. This will cause poor driveability, and high gas consumption.
Knock Sensor: Also known as detonation sensor, it is responsible for sensing spark knock. Basically, thats when the fuel mixture ignites before the spark plug fires. The piston is moving upwards as this premature combustion takes place. Since fuel is used to cool down the combustion chamber, a lean condition causes the temperature to rise, and ignites the fuel mixture prematurely. This is very abusive on the engine internals, and reduces the life of any engine. The more powerful the engine, the greater the potential for damage. Detonation can be cause by a variety of things. One of the more common causes on TPI retrofits where prom changes have been made to the fuel or spark tables is a lean condition. It isn't always loud enough to be heard, so just because you don't hear any pinging, doesn't mean its not happening.

Detonation will cause a vibration to travel through the engine block. The sensor listens for this vibration at a certain frequency, and sends a signal to the ecm when the frequency is heard. This frequency is different depending on engine size. To prevent possible engine damage, the spark timing needs to be retarded when detonation is present. The sensor itself does not pull the timing back however. The ecm is in charge of retarding the timing, and will do so according to a series of settings inside the prom. The knock sensor is located on the passenger side of the engine block on factory applications. It can however be relocated to the driver side of the block if needed (header clearance for example). They are different depending not only on the size of the engine, but also the ecm being used. It is important that the correct sensor is used to avoid problems. Although it is possible to run the car without one, I strongly suggest against this. I have had customers come to me looking for a $45 knock sensor after spending several hundred dollars and an extra month of work rebuilding a blown engine due to detonation.
Throttle Position Sensor (TPS): Responsible for reporting to the ecm the position of the throttle blades. The ecm will receive a signal which can vary from 0.0 to 5.0 volts. At idle, the TPS should be read .54 volts (factory specification) unless it has been set to a different value inside the prom. If it does not read .54 volts and the idle TPS voltage setting has not been modified in the prom, then it should be adjusted. Under full throttle, it should output close to 5.0 volts. Throughout its range of motion, the voltage should climb steadily, without any jumps or falls. If it is not steady or has some fluctuations as it is moved through its range of motion, it should be replaced. This sensor is located on the passenger side of the throttle body.

Coolant Temperature Sensor (CTS): This is basically a thermistor (means that it changes resistance with temperature) that supplies the ecm with the temperature of the engine coolant. This temperature reading is used for several important functions. The most notable is that the ecm adds extra fuel to an engine when its cold, and as the engine warms up, the extra fuel is reduced. This sensor mounts at the front of the intake manifold. The chart below shows the approximate resistance for this sensor in relation to temperature.

the later 1992-96 corvette lt1 used a MAT SENSOR




Intake Air Temperature Sensor (IAT): This sensor is also a thermistor (means that it changes resistance with temperature) that supplies the ecm with a temperature reading of the air being drawn into the engine. It is the same as the coolant temperature sensor on 86-92 models. The 1985 intake air temperature sensor used a different connector and cannot be used as a coolant temperature sensor because it had an exposed bulb. This sensor mounts underneath the plenum. The chart above shows the approximate resistance for this sensor in relation to temperature.

Idle Air Control (IAC) Valve: Although this is technically not a sensor at all, people often treat it as one. It is responsible for regulating the amount of airflow being admitted into the engine to adjust engine speed, particularly at idle and deceleration. The ecm controls the IAC at its discretion. The IAC works by moving a cone shaped pintle, which can extend and retract as needed to admit or block off incoming air. The valve moves the pintle in "steps". These steps are numbered and range from 0 to 160. A properly adjust throttle body should be idling when warm between 15-25 steps.

The IAC is used under a variety of conditions, not only at idle speed. The valve mounts on the bottom coolant plate of the throttle body.

Vehicle Speed Sensor (VSS): This is responsible for providing the ECM with the vehicle speed. It can be located either at the tailshaft of the transmission, or behind the speedometer on cars with a cable driven speedometer. It sends a 2k ppm (pulse per mile) square wave signal, and is needed for a variety of functions. It is absolutely critical for the ecm's learn mode, timing retard, emissions, torque converter lockup (automatic lockup transmissions only), idle speed control, and to avoid stalling on deceleration. It is possible to run without one. However, your car will NOT be street legal if you are required to retain emissions equipment, the ecm will not control the torque converter lockup, the ecm will not retard timing if you run into detonation, and it is possible to run into stalling /idle speed issues. In addition, the ecm will not adjust the fuel table properly as you drive (known as its "learning ability"). If the ecm does not know the vehicle speed it is assumed to be 0 mph.

If you still insist on not running a vss, I very highly suggest that the minimum vehicle speed for timing retard be brought down to 0 mph in the prom. The factory setting is 2 or 3 mph. If you don't bring this value down, and you do not run a vss, the ecm will NOT retard your timing under detonation.

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The Grumpy Grease Monkey mechanical engineer.
Staff member
war machine said:
Do these following checks before spending a lot of cash replacing parts.

First, make sure there are no cracks in the air duct and that it's tight.

If that is good, unplug the battery for a minimum of 10 seconds to clear the code. Next, unplug the MAF sensor and start the engine. Run it in park @ 2300 rpm's for a minute or until the service engine light appears.

If it's a code 33 after that test, you have a faulty MAF sensor or if the service engine light does not come back you might have a shorted ground 998 (dark green wire) coming from the MAF sensor or a faulty ECM.

Code 34 can also result from a dirty or misadjusted throttle body. Follow these instructions to do the minimum idle set procedure
> Code 33 (MAF) Mass Air Flow Sensor Problem

Code 33 (MAF) Mass Air Flow Sensor Problem

So i got the code 33 in my 87 Camaro 305 TPI. It only kicks on when the car in warm. So if i start it up in the morning take it to work 15 or so miles it is fine. shut it off and back on and the light come on. If i then drive it is has lost power and seems to idle funny. I changed the MAF but still does the same thing. Check the TPS and it is set fine. Also changed the Manifold Air Temp sensor cuz it was black. still no luck. Also the car gets really hot quick. I changed the thermo and it was a 87 degree day but i went on a 1hr ride came back to the park camaro and it was still hot. The radiator was flowing fluid and i aint really worry about that just a side note. But any other ideas on the MAF code 33 issue?

Re: Code 33 (MAF) Mass Air Flow Sensor Problem

The MAF burn-off circuit will not throw a code till the next start, because it is only used for a hot sec at power shut down (key off). Check the relay.

Re: Code 33 (MAF) Mass Air Flow Sensor Problem

, check the relay. I had the same issue before. I think I paid something like 6 or 7 bucks.

What was strange with my situation was, if I put the key in and prime the fuel system before starting(like I always do), it would set the code almost every time and the check engine light would come on. But if I would start the car right away without priming the fuel system, the light wouldn’t come on. I ended up replacing both the power and burn off replays.

Re: Code 33 (MAF) Mass Air Flow Sensor Problem

they are both new and same issue. Also this does it all day once the car is hot and it runs like crap. I am getting the code 33 not a burnoff problem code. Any ideas?

Re: Code 33 (MAF) Mass Air Flow Sensor Problem

As posted here at TGO:

"There is a code 33 troubleshooting chart here:

https://www.thirdgen.org/techbb2/show...hreadid=245300 (Source of MAF code 33?)

in short: Code 33 indicates:

ECM has seen flow in excess of 45 grams per second (above 2.2 volts) for one second when:

* Engine is first started
*TPS < 50% AND RPM < 2000"

"FYI: the 33 error code is set when the following 3 conditions are true at the same time:
1) engine under 2000 RPM,
2) MAF reading over 45 gms/sec, and
3) less than 25% throttle."

I would check/change TPS, MAF, and Ref pulse components, wiring, and connectors. Don't just buy new, find somebody with a known non code 33 set up, like yours, and swap parts one at a time till trouble moves. Also data log, then graph the area's of interest looking for spikes or opens. Remember that the best data logs only sample once every 1/10 of a sec, so a lot can be happening that can't be seen.

many times its the maf sensor connector, so be sure to clean the connector contacts before you test further,
its also very helpful if you can find someone who has the same year corvette who will let you swap maf (MASS AIR FLOW ) sensors for a couple minutes to test if the swap cures your issues (thus proving the maf sensors defective, ID also suggest swapping the air tem sensor in the plenum as they are cheap, easy to replace and sometimes the cause of that issue

war machine said:


Where it says to disconnect the distributor timing set connector, they mean this plug here...

For the Larger version click here.

Don't worry about the code 42. It will set when you disconnect that plug. Since the service manual instructions don't remind you to re-connect it I typed up my own instructions and goes as follows....

* If nobody has messed with this on your car before, the set screw will be covered by a pressed-in plug. It's located on the driver's side of the TB. Remove this plug if it's there.

* With the IAC connected and the ignition OFF, stick the paper clip into the diagnostic connector from A to B. This grounds the diagnostic lead.

* Turn the ignition to the ON position without starting the engine. Wait 30 seconds.

* Now, with the ignition still in the ON position, disconnect the IAC connector at the IAC. ! Improtant Disconnect the distributor set-timing connector.

* Start the engine and allow it to reach normal operating temperature. The idle speed will probably be really low, and you may have to coax the engine a bit with the gas pedal to keep it running for a while. Now remove the paper clip from the diagnostic connector.

* If your car is an automatic, set the parking brake and put the transmission in DRIVE. If your car is a manual, leave it in neutral.

* Adjust the idle speed screw to obtain 400 rpm in drive or 450 in neutral.

* Shut off the engine and re-connect the IAC.**There is no mention of re-connecting the distributor set-timing connector but this is when I re-connect that and disconnect the battery for 10 seconds to clear all codes.

The fact that the light went away might mean it's an intermittent. Check to make sure that the MAF sensor plus the MAF power and burnoff relay connections are tight.

Hope this helps,

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