1995 BMW 525 Mass Air Flow Sensor/ Oxygen Sensor

DCOYNE

MEMBER
1995 BMW 525
4 CYL
2WD
MANUAL
120,000 MILES

Over the past 5 years we have tried to smog the care with great difficulty. The oxygen sensor has been replace at least 4 times. Now the dealer(shop) is saying that the the Mass Air Flow Sensor is probably bad since replacing the oxygen sensor hasn't really worked. We get the "check engine" light on all the time. How should we start diagnosing and dertermining the cause of this problem. We want to start driving it again but it won't pass smog until the check engine light goes out.

1 Reply

DOCHAGERTY

MECHANIC
9,601 POSTS

If the check engine light is on then there is fault stored and it can be read out of the system with proprietary BMW software. If there is a problem with the mass air flow then it might show up there and could be out of range. Have you tried another sensor from another BMW to see if that changes the fuel trim? LTFT or long term fuel trim is the term used for engine management strategy to add or subtract fuel and to move short term fuel trim to the "middle" of what the engineers determine is the normal range for how much fuel to inject. If the MAF is out of range the fuel trim can be off by enough to cause the engine to turn on the check engine light and to fail a smog test.