P0100 — Mass Air Flow Sensor Circuit Fault

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

Damaged MAF Sensor WiringThe wiring between the MAF sensor and ECU has broken, chafed, or corroded. Common on vehicles where the air intake loom runs near the battery or hot components.

Corroded or Damaged ConnectorThe MAF sensor connector is exposed to moisture and heat. Pin corrosion creates high resistance or open circuits, taking the signal outside the valid range.

Failed MAF SensorThe sensor hot-wire element has completely failed, producing no signal or a fixed out-of-range output rather than a live reading.

Air Intake Leak After MAFA large air leak after the MAF sensor does not directly cause P0100 but can alter the expected signal pattern and set multiple codes alongside it.

ECU Input FaultRare — the ECU MAF input circuit has failed. Only diagnose this after all wiring and sensor checks are complete.

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.

Frequently Asked Questions

P0100 is a circuit fault — the signal is absent or completely out of range, pointing to a wiring or connector problem. P0101 is a performance fault — the sensor is producing a signal but it does not match expected values, typically caused by a dirty or ageing sensor element.

The engine will typically run in a fallback mode using throttle position to estimate fuelling. Performance and economy will be poor. Avoid sustained high-load driving. Diagnose and repair within a few days.

A dirty MAF usually causes P0101 (performance/range fault) rather than P0100. P0100 typically requires a complete signal failure — absent voltage, broken wire, or open-circuit sensor element.

Cleaning with dedicated MAF cleaner spray fixes a contaminated element causing P0101 or P0171. It will not fix a broken wire or failed element causing P0100 — those require sensor replacement.

It depends on which warning light the code is triggering. Since 2018, any car presenting with an illuminated amber Engine Management Light (EML) at the MOT is a Major failure under DVSA rules — even if the car drives perfectly. A red warning light is always a Major or Dangerous failure depending on context. If clearing the fault makes the light go out and the code does not reappear during the pre-test drive, you will pass; if the code returns within minutes of clearing, the underlying fault must be fixed before MOT day. A tester is required to fail the car on the light being on, regardless of whether the underlying fault is something safety-critical or not. For codes that affect emissions specifically (catalyst, lambda, EGR), the car may also fail the actual emissions check. Fix the cause, clear the code, and drive the car for a few miles before the test.

P0100 — MAF Sensor Circuit Malfunction