For years I've been getting a check engine light on with a Code 4 from the diagnostic module.

(Code 4: Air injection system faulty hot film mass air flow sensor with hot wire.)

Pins 8 and 14 of the diagnostic module were checked and show no faults.

A few years back I replaced the air mass meter hoping the fault would go away... it didn't.

Otherwise, the car runs fine with good gas mileage. What could be causing this fault?

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95 E320 Cabriolet, 159K

The air injection ( in to the exhaust ) and mass air flow meter ( that measures air entering the intake ) are two completely separate systems.

I'm thinking that the code should read : < CODE 4 Air injection system faulty. ( Found on cars with a hot film style air flow meter or hot wire style flow meter ) >

The computer determines if the air injection system is working by turning on the air pump after the engine is warmed up and looking for the oxygen sensor to drop in voltage. You car should have a belt driven air pump under the alternator that uses a AC compressor type clutch.

If the 02 sensor dosent drop enough when air is injected, ie bad 02 sensor , then it will set that code.