Mass airflow (MAF) sensor codes

How its codes differ

• P0102 is a circuit-level code: the signal is lower than the sensor can produce in normal use, which points to wiring, the connector or the sensor's electronics.
• P0101 is a range or performance code: the signal is electrically plausible but does not agree with what the computer expects from engine speed and throttle, which points to contamination, an air leak near the sensor or drift.
• P0171 and P0174 can be caused by a MAF that under-reads: the computer under-fuels both banks. A lean code on both banks with no air leak is a reason to check the MAF.
• Rich codes (P0172, P0175) can come from a MAF reading higher than the real airflow; this is a lead to test rather than a common cause.

Codes

• P0101

  Airflow sensor reading doesn't match what the engine computer expects for the throttle opening

  Severity: Inspect soon

• P0102

  The airflow sensor's signal is lower than the engine computer will accept

  Severity: Inspect soon

• P0171

  Engine computer has reached its limit adding fuel on bank 1: the mixture is lean

  Severity: Monitor

• P0174

  Engine computer has reached its limit adding fuel on bank 2: the mixture is lean

  Severity: Monitor

• P0172

  Engine computer has reached its limit removing fuel on bank 1: the mixture is rich

  Severity: Monitor

• P0175

  Engine computer has reached its limit removing fuel on bank 2: the mixture is rich

  Severity: Monitor

Signs it's failing

• Check engine light with P0101, P0102, P0171 or P0174
• Hesitation or poor power
• Rough idle or stalling
• Poorer fuel economy

How to test it

1. Check the air filter and the intake duct for dirt, oil or a split between the sensor and the throttle body. (Low risk)

2. With a scan tool, compare the MAF reading at idle and at higher speed with the expected values for your engine, and watch fuel trims on both banks. (Use caution)

   Warning: Keep hands, hair, clothing and tools clear of the fan and belts while the engine is running, and do not run the engine in a closed garage.

3. If the sensor element is dirty, clean it only with a spray made for MAF sensors, with the engine off and the sensor dry before refitting. Never use carburettor, brake or household cleaners. (Low risk)

After the repair

• Recheck fuel trims on both banks at idle and at 2500 rpm: they should move back toward normal.
• Fit a clean air filter to protect the sensor.
• Clear codes only after the repair, then drive normally over several days with both city and highway driving so the readiness monitors can complete; clearing codes or disconnecting the battery resets them to not ready, which fails an emissions test.