BMW fault 2B6E - M ignition coil (cylinder 8)
P138C · Ion sensing coil fault
2B6E is stored by the DME when it detects a fault with the ion-sensing ignition coil on cylinder 8. Ion-sensing coils monitor combustion quality by measuring ion current in the cylinder after each spark event, allowing the DME to detect misfires and knock events directly.
The DME monitors the ion current feedback from cylinder 8 during combustion. At idle, it checks for combustion misses over at least 80 firing cycles. During overrun (deceleration with fuel cut), it monitors ignition timing offset tolerance over at least 200 cycles. If the signal consistently falls outside expected parameters, the fault is stored. Engine speed is limited and power is reduced as protection. The MIL illuminates after two consecutive driving cycles with the fault present.
- Module
- DME
- Severity
- Warning
- Likely cause
- Failed ion-sensing ignition coil on cylinder 8
- Common fix
- Replacing the ignition coil on cylinder 8
Symptoms
- MIL and EML warning lamps illuminated after two driving cycles with the fault.
- Engine speed limitation and reduced power.
- Rough running or perceptible misfire on cylinder 8.
- Uneven exhaust note.
Common causes
- Failed ion-sensing ignition coil on cylinder 8 where the ion current sensing circuit has degraded internally.
- Worn or fouled spark plug on cylinder 8 with excessive electrode gap or carbon fouling affecting the ion current signal.
- Damaged ignition coil connector or power supply wiring.
Diagnosis
- Read the fault memory and note whether 2B6E is isolated to cylinder 8 or whether multiple cylinders show similar faults. Multiple cylinder faults suggest a shared power supply issue rather than a single coil failure.
- Swap the ignition coil from cylinder 8 with a known-good cylinder. Clear the fault memory and run the engine. If the fault follows the coil to the new cylinder, the coil is defective.
- If the fault remains on cylinder 8 after the coil swap, inspect the spark plug for excessive wear, fouling, or incorrect gap. Also check the ignition coil connector and power supply wiring for damage or corrosion.
- If wiring and spark plug are confirmed good and the fault stays on cylinder 8 regardless of which coil is installed, the DME output stage for that cylinder may be faulty.
Resolution
Replace the defective ignition coil if the swap test confirmed it. Use OEM or OE-equivalent coils with ion-sensing capability, as aftermarket coils without ion sensing are not compatible. Replace the spark plug if worn. Clear the fault memory and verify the MIL extinguishes after two fault-free driving cycles.