Will P0100 fail an MOT?

Yes, in effect. P0100 turns on the engine management lamp, and an illuminated malfunction indicator lamp is a major defect under section 8.2.1.2 of the MOT inspection manual for petrol cars first used on or after 1 July 2003, and 8.2.2.2 for diesels from 1 July 2008. That single defect accounts for 371,859 recorded failures. Because a faulty airflow signal also upsets the mixture, the car may fail the emissions limits as well.

Can I drive with a P0100 code?

Usually yes, for short distances. The engine control unit falls back on an estimated airflow figure, so the engine keeps running, though with poor response and heavier fuel use. The risk is that some cars drop into reduced-power limp mode without warning, so treat it as a drive-to-the-garage code rather than one to live with.

What is the difference between P0100 and P0101?

P0100 means the signal is missing or electrically impossible, which points at wiring, the connector or a dead sensor. P0101 means the signal is present and within its electrical range but does not match the amount of air the engine should be drawing, which points at a dirty sensing element, an intake leak or a blocked air filter. P0100 is a circuit fault; P0101 is a plausibility fault.

Can I clean a mass airflow sensor instead of replacing it?

It is worth trying and often works, particularly if the car has an oiled performance air filter. Use a dedicated mass airflow cleaner, spray it onto the sensing element without touching it, and let it dry fully before refitting. Never use brake cleaner, a cloth or a brush, because the hot wire is thinner than a hair and breaks instantly.

Powertrain code · standard

P0100Airflow meter circuit fault

Standard definition: Mass or Volume Air Flow Circuit Malfunction

Last updated 27 July 2026

Quick answer

The engine control unit has lost a usable signal from the mass airflow meter, the sensor that measures how much air is going into the engine. The car will normally still run on a fallback estimate, but it will not run well and the engine light will be on.

How the car works this out

The airflow meter sits in the intake pipe between the air filter and the throttle, and reports air mass to the engine control unit as a voltage or a frequency. The unit checks continuously that the signal is present, electrically sane, and moving in step with engine speed and throttle angle. P0100 is the general circuit code, set when the signal is missing or so far adrift that the unit cannot even say whether it is reading too high or too low. In practice that points at wiring, a connector or a dead sensor rather than a drifting one.

What you might notice

Hesitation or a flat spot when you accelerate
Rough, hunting or unusually high idle
Noticeably worse fuel economy
Black smoke from a diesel under load
Stalling as you lift off or come to a stop
Reduced power or limp mode on some cars

What causes it

Most likely first. A code names the check that failed, not the part that failed it, so more than one of these can produce the same number.

1The airflow meter plug not fully latched, or corroded pins inside itCommon
How to check: Unplug it and look at the terminals for green corrosion or a pin pushed back in its housing, then refit until the locking tab clicks. It is the first thing to check because it costs nothing.
2A chafed, broken or water-damaged wire between the meter and the engine control unitCommon
How to check: With live data on a scan tool showing airflow, wiggle the loom along its length with the engine idling. A signal that drops out as you move it has found the fault for you.
3A contaminated hot-wire element, usually from an over-oiled aftermarket air filter or a filter fitted badly enough to let dust pastPossible
How to check: Look into the meter body for a dull or furred sensing element. Proper airflow meter cleaner is cheap and occasionally fixes it outright.
4A failed airflow meterPossible
5Split or disconnected intake ducting after the meter, letting the engine draw air that was never measuredPossible
How to check: Flex the concertina section of the intake pipe by hand with the engine off and look inside the folds, where splits hide.
6A poor earth or a missing supply voltage to the sensorLess common

What to do about it

1Check the intake pipework and the sensor plug before anything else. A pipe that has popped off its clip or a plug that was never clicked home after other work accounts for a good share of these, and both are free to put right.
2Have the live airflow reading compared against what the engine should be drawing. A garage can see in seconds whether the signal is absent, stuck or simply wrong.
3Try cleaning the element before buying a sensor, using airflow meter cleaner rather than brake cleaner or a cloth. The hot wire is fine enough to break if you touch it.
4Fit a good-quality replacement if the sensor has failed. Cheap copies of airflow meters are notorious for reading inaccurately from new, which leaves you with the same symptoms and no code to explain them.
5Do not clear the code and book the MOT for the morning after. Clearing wipes the readiness monitors, and the emissions check often cannot be completed until they have run again.

Can you keep driving?

Briefly, gently

The engine control unit substitutes a calculated airflow figure, so the car generally gets you home. Expect poor throttle response and worse economy, and be aware that some cars drop into limp mode with no warning, which is unwelcome pulling out of a junction. It will not pass an MOT with the lamp on.

Will it fail an MOT?

The lamp this code lights is the failure. An illuminated engine malfunction indicator lamp is a major defect for petrol cars first used on or after 1 July 2003 and diesels from 1 July 2008, recorded 371,859 times in the DVSA data behind this site. A wrong airflow signal also skews the mixture directly, so a petrol car can fail the CO and hydrocarbon limits and a diesel the smoke opacity limits on the same visit.

The light this turns on

Engine management lightThe engine management system has found a fault it cannot correct and has stored a code for it. The car will usually still drive, but an illuminated engine light is an automatic MOT failure on petrol cars first used from 1 July 2003 and diesels from 1 July 2008.

Common questions

Will P0100 fail an MOT?: Yes, in effect. P0100 turns on the engine management lamp, and an illuminated malfunction indicator lamp is a major defect under section 8.2.1.2 of the MOT inspection manual for petrol cars first used on or after 1 July 2003, and 8.2.2.2 for diesels from 1 July 2008. That single defect accounts for 371,859 recorded failures. Because a faulty airflow signal also upsets the mixture, the car may fail the emissions limits as well.
Can I drive with a P0100 code?: Usually yes, for short distances. The engine control unit falls back on an estimated airflow figure, so the engine keeps running, though with poor response and heavier fuel use. The risk is that some cars drop into reduced-power limp mode without warning, so treat it as a drive-to-the-garage code rather than one to live with.
What is the difference between P0100 and P0101?: P0100 means the signal is missing or electrically impossible, which points at wiring, the connector or a dead sensor. P0101 means the signal is present and within its electrical range but does not match the amount of air the engine should be drawing, which points at a dirty sensing element, an intake leak or a blocked air filter. P0100 is a circuit fault; P0101 is a plausibility fault.
Can I clean a mass airflow sensor instead of replacing it?: It is worth trying and often works, particularly if the car has an oiled performance air filter. Use a dedicated mass airflow cleaner, spray it onto the sensing element without touching it, and let it dry fully before refitting. Never use brake cleaner, a cloth or a brush, because the hot wire is thinner than a hair and breaks instantly.

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