The MAF sensor output voltage or frequency exceeds the maximum calibrated value. The ECU receives a signal indicating more airflow than is physically possible.
Common causes
Short to Vref (5 V) or B+ in the signal wire
Failed MAF sensor with output stuck high
Contamination on the sensing element causing an erroneously high reading
External electromagnetic interference
Symptoms
MIL illuminated
Excessively rich mixture (over-fuelling)
Black smoke on diesel
Poor fuel economy
Rough running
Severity
Moderate severity: an excessively rich mixture can damage the catalytic converter and increases emissions.
What to do
Measure MAF signal wire voltage at idle (should be approximately 1–2 V for analogue sensors)