BMW F30 lean mixture (P0171) and fuel trim
335i
F30 P0171 lean mixture, where is the fault usually found?
The lean mixture code and a high long-term fuel trim almost always point to a vacuum leak. On the N20 engine the cause was a torn rubber hose (crankcase breather); also check the ignition coils.
What fuel trim values are normal?
Long-term trim is normally around +5...-5 and short-term fluctuates around +10...-10. The long-term value is more important. A value of about 28 percent means the engine control is enriching strongly to compensate for the lean mixture.
How do you check the mass air flow sensor (MAF)?
Read the air mass value (g/s) with a diagnostic tool at idle and at different engine speeds. The signal voltage is about 0.5 V at idle and closer to 5 V at full throttle. If the short-term trim changes after the connector is unplugged, the sensor is probably faulty.
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