P0300

What This Code Means

Code P0300 indicates that random or multiple cylinder misfires have been detected. The engine control unit (DME) monitors crankshaft speed fluctuations and sets the fault when the threshold is exceeded. On N55 engines, failed ignition coils from heat exposure or normal aging are a common cause. Spark plug or fuel delivery problems are also possible.

Symptoms

• Engine: rough, lumpy idle (shaking), power loss under acceleration.
• Behavior: may enter limp mode to protect the catalytic converter.
• Indicators: Check Engine Light (CEL) comes on.
• Fuel economy: increased fuel consumption from inefficient combustion.

Diagnosis and Repair

Step 1: Read the Codes

Connect ISTA or INPA and read the fault codes. Note any companion codes (for example, P0301–P0306 for specific cylinders).

Step 2: Check Plugs and Coils

Inspect the spark plugs for wear or deposits. Check the ignition coils — they're often the cause. Replacing them as a set is recommended.

Step 3: Replace Components

Use BMW genuine (OE) parts:

• Ignition coils: 12138616153
• Spark plugs: 12120037563

Step 4: Check the Injectors

If misfires persist, check the injectors for leaks or clogging. On the N55, injectors can leak and cause misfires.

Secondary Damage

Ignoring P0300 can lead to serious consequences:

• Catalytic converter: unburned fuel reaches the converter, causing overheating and failure. Replacing the catalytic converter is an expensive repair.
• Engine: fuel washes the oil film off the cylinder walls, accelerating wear on the pistons and piston rings and potentially causing cylinder bore scoring.

Fix the cause of the misfires as soon as possible to avoid costly repairs.