P0102

What the code means

P0102 is a generic OBD-II code: the mass air flow (MAF) sensor signal is lower than expected. The sensor measures the amount of air entering the engine, and the control unit uses that reading to calculate fuel delivery.

There are three causes of a low signal: a faulty sensor, wiring problems (a short or an open circuit), or a clogged air filter restricting airflow.

When the signal drops out or becomes implausibly low, the control unit may substitute default values. This limp-home mode ignores actual airflow, so the mixture runs rich — more fuel is delivered than needed.

Symptoms

• Rough idle, up to and including engine stall;
• black smoke from the exhaust — a result of the rich mixture;
• sluggish acceleration, degraded performance.

Diagnosis and repair

Step 1. Air filter

A clogged filter restricts airflow and skews MAF readings low. Check the filter and replace it if it's dirty — this is the simplest place to start.

Step 2. Wiring

Inspect the MAF sensor wiring for shorts and open circuits. A damaged circuit produces a low or missing signal regardless of the sensor's condition.

Step 3. Output voltage check

Measure the MAF sensor output voltage. A deviation from the expected value confirms the fault.

Step 4. Sensor replacement

If the sensor is defective, replace it. The original BMW part number is 13627550076.

A common diagnostic mistake is to replace the sensor right away without checking the filter and wiring: the cause may lie there.

Secondary problems

Ignoring the code is risky because of the rich mixture that the substituted default values don't correct:

• spark plugs foul from excess fuel;
• the catalytic converter is gradually damaged by unburned fuel.

Severity is moderate, but don't put off diagnosis: catalytic converter degradation is irreversible.