P0266

What the code means

P0266 is a diagnostic trouble code that indicates a contribution/balance imbalance on cylinder 2. Like P0263, it signals that cylinder 2 is not working the same as the other cylinders, which causes rough running. The DME (engine control unit) calculates each cylinder's contribution from the crankshaft position sensor signal and sets the fault when the values deviate.

The most common causes are a faulty fuel injector (clogging, poor spray pattern, electrical failure) or a mechanical problem (compression, valvetrain, piston assembly). The code belongs to the fuel injection system, but it can also point to mechanical faults.

Symptoms

• Misfires, especially noticeable at idle or under load.
• Rough running (shaking), vibrations.
• Power loss, sluggish acceleration.
• Check Engine light on the instrument cluster.
• Possible surging while driving.

Diagnosis and repair

1. Read the fault codes with diagnostic equipment (ISTA, INPA) and check for related misfire codes.
2. Check the cylinder 2 fuel injector: coil resistance, control signal from the DME, no clogging. If needed, remove it and test it on a bench.
3. Check fuel delivery: rail pressure, pump output, fuel filter condition.
4. Assess the cylinder's mechanical condition: measure compression, check valve sealing, piston and ring condition.
5. Replace the faulty fuel injector once the failure is confirmed. Use original BMW parts with the correct part numbers.
6. After the repair, clear the adaptations and take a test drive to confirm the fault is gone.

Secondary problems

Running for a long time with P0266 and the associated misfires can overheat the catalytic converter because unburned fuel reaches the exhaust. That melts the honeycomb and leads to a costly converter replacement. Constant operation on a lean or rich mixture in one cylinder also speeds up wear on the piston rings and valvetrain, and can damage the spark plug and ignition coil. If symptoms appear, get the car diagnosed and the fault fixed as soon as possible.