P0104

What the Code Means

P0104 flags an intermittent fault in the mass air flow (MAF) sensor circuit: the sensor signal is unstable or drops out temporarily. Normally the DME receives a continuous stream of mass air flow data and calculates fuel delivery from it. When the signal breaks up or jumps, the control unit loses its reference value and falls back on averaged parameters — hence the on-and-off nature of the symptoms.

Typical causes: a loose connection in the connector, damaged wiring, or an intermittent failure of the sensor itself.

Symptoms

The symptoms come and go, which tracks with the intermittent nature of the fault:

• intermittent loss of acceleration;
• rough idle;
• Check Engine light coming on;
• engine stalling.

Diagnosis and Repair

1. Inspect the MAF sensor connector and wiring for loose connections and damaged insulation.
2. Run a wiggle test: move the connector and harness with the engine running while watching the MAF signal on a scan tool — a drop or spike in the signal as you wiggle pinpoints the fault.
3. If you find damage, repair or replace the wiring.
4. If the intermittent fault persists after the wiring check, replace the MAF sensor — BMW part number 13627550076.
5. Clear the codes and take a test drive to confirm the repair.

A common diagnostic mistake is replacing the sensor without checking the connector and harness: with an open or corroded wire, a new sensor won't fix the problem.

Secondary Problems

• Intermittent faults cause unpredictable handling problems and can stall the engine in traffic — the risk while driving is significant.
• Over time the DME can store multiple fault codes and drop into limp mode with reduced power.

Don't put off diagnosis: the fault progresses from an unstable signal to a complete circuit failure.