What Is P0100?
P0100 is a circuit-level fault on the Mass Air Flow (MAF) sensor - the sensor that measures the quantity of air entering the engine. The ECU uses the MAF signal to calculate the correct fuel injection duration. P0100 means the ECU has detected the MAF signal is absent, implausible, or completely out of range rather than performing poorly within range (which would trigger P0101).
Without a functioning MAF signal, the ECU cannot accurately calculate fuelling and typically defaults to a backup fuel map based on throttle position and engine speed. The engine usually still runs but with noticeably worse performance, fuel economy, and emissions. The code points to a wiring or connector fault rather than a deteriorating sensor element.
Common Symptoms
- Engine management light on
- Poor throttle response and sluggish acceleration
- Rough idle
- Increased fuel consumption
- Possible black smoke (over-fuelling in fallback mode)
- Possible P0101, P0171 or P0172 stored alongside
Common Causes
OBD Fault Code Plain English Guide
- 100 most common UK fault codes in plain English
- A severity rating for every code so you know what can wait
- Causes and next steps listed for each fault
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How to Diagnose P0100
Inspect the MAF Connector and Wiring
Unplug the MAF sensor connector and check for corrosion, bent pins, moisture, or damaged wiring. Clean contacts with electrical contact cleaner and reconnect. Check the wiring back to the ECU for heat damage or chafing.
Check MAF Sensor Voltage
With the ignition on, probe the MAF sensor connector for supply voltage (typically 5V or 12V on the power pin) and confirm a clean earth. No supply voltage points to a wiring break or blown fuse.
Monitor MAF Live Data
Connect an OBD scanner and read MAF sensor value at idle. A healthy reading is typically 2–10 g/s at idle, rising proportionally with engine speed. A reading of 0 or a fixed implausible value confirms a circuit fault.
Swap or Substitute the MAF Sensor
If voltage and wiring checks out but the signal is absent, temporarily swap in a known-good MAF sensor. If the fault clears, the original sensor has failed.
Check for Fuses
Some vehicles fuse the MAF sensor supply separately. Check the fuse box layout for any MAF or sensor supply fuses.
Verdict
Start with the connector - corrosion and damaged pins are the most common cause of P0100. If the connector is clean and voltage is present but there is no signal, the sensor has failed and needs replacement. MAF sensors typically cost £40–£150 for quality aftermarket units.
P0102 and P0103 - The Same Sensor, Stuck At Opposite Ends
P0100, P0102 and P0103 are three ways of describing one component in trouble: the mass air flow sensor. P0100 is the general circuit fault, P0102 means the signal is stuck at the bottom of its range (low input), and P0103 means it is pegged at the top (high input). The direction is a wiring clue rather than a different diagnosis. A signal flatlined low usually means the sensor is unplugged, dead, or its signal wire is shorted to earth. A signal jammed high points at a short to voltage, or occasionally a sensor element so contaminated it reads nonsense.
Whichever variant you have, the diagnosis converges on the same three checks in this order: inspect the connector and wiring (unplugged and corroded connectors cause more MAF codes than dead sensors do), read the live grams-per-second figure at idle against what a warm engine of your size should draw, and only then try a known good sensor. The cleaning trick with MAF cleaner spray is always worth a pound-shop attempt before a £100 part - but if the code is P0102 with a completely flat signal, cleaning will not resurrect a sensor that is electrically dead.
