How to know if a mass air flow sensor is bad

Written by dan ferrell
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How to know if a mass air flow sensor is bad
(Photo courtesy of Gillsensors at

There are several symptoms that can point to a bad mass air flow (MAF) sensor, including rough idle, excessive fuel consumption or not enough fuel reaching the injectors, engine stalling and even difficulty starting. However, many other components can be the source of these same problems. Follow these steps to determine the sensor's condition, whether you have a hot wire MAF-type sensor--on most Ford vehicles--or the hot film MAF type sensor--on most GM vehicles.

Skill level:
Moderately Easy

Things you need

  • Scan tool
  • Multimeter

Show MoreHide


  1. 1

    Retrieve the trouble codes from your car engine's computer using a scan tool. If you do not have one of this tools, most auto parts stores will retrieve this code for you free of charge. On Ford vehicles, a 26, 56, 66 or 76 code indicates a possible problem with this sensor; on GM vehicles, a 33, 34 or 36 code points to the same problem.

  2. 2

    Measure the hot wire MAF sensor's output voltage. With the engine running at idle, back-probe the brown wire with a white strip on the MAF sensor using a voltmeter. Your meter should read around 2.5 volts. Lightly tap the sensor with your finger; the voltage should hold steady. Pull on the throttle mechanism with your hand to increase engine rpm and watch the meter. The voltage should increase smoothly and relative to throttle opening. If your voltage readings are out of specifications, the sensor is not producing the appropriate signal.

  3. 3

    Check the self-cleaning circuit relay on your hot wire MAF sensor. The engine computer uses this circuit to clean the MAF wire or sensing element. If the relay that controls current to this circuit is broken, the car computer will not be able to clean the wire; eventually, dirt and other contaminants will interfere with the sensor operation. Consult your vehicle service manual to locate and test this relay.

  4. 4

    Measure the hot film MAF sensor's frequency. Hook the red probe of a multimeter capable of reading milliseconds to the signal wire of any fuel injector on the vehicle, and the black probe to a good ground--a bolt or metal bracket on the engine. Read the injector's pulse width output while cranking the engine and at idle. Compare these readings with the specifications provided in your vehicle service manual. If the pulse width is out of range, your MAF sensor is producing a bad signal or not at all.

Tips and warnings

  • Consult your vehicle service manual to determinate the type of sensor installed in your car and the sensor's and fuel injectors' output specifications. You can buy one at most auto parts stores or consult one for free at most public libraries.

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