how to diagnose P0301 misfire code on BMW 3 Series E90/E91 (2005-2012)

Solution

1. Preparation

• Ensure the vehicle is parked on a level surface and the engine is cool.
• Disconnect the negative battery terminal to prevent electrical shorts.
• Gather necessary tools and replacement parts.

2. Replace Spark Plug

• Remove the engine cover using a socket set.
• Disconnect the ignition coil connector for cylinder 1.
• Remove the ignition coil by unscrewing it from the cylinder head.
• Remove the old spark plug using a spark plug socket.
• Install a new spark plug, tightening it to the manufacturer’s specified torque (typically around 13 lb-ft).
• Reinstall the ignition coil and reconnect the connector.

3. Test and Replace Ignition Coil (if necessary)

• If the ignition coil was suspected, test it using a multimeter to check for resistance; replace if it is out of specification.
• If swapping the ignition coil showed the misfire followed, replace the faulty coil with a new one.

4. Inspect and Replace Fuel Injector (if necessary)

• Disconnect the fuel injector connector for cylinder 1.
• Remove the fuel rail assembly to access the fuel injector.
• Replace the faulty fuel injector with a new one.
• Reassemble the fuel rail and reconnect the injector harness.

5. Compression Test (if necessary)

• Remove the spark plug for cylinder 1 again.
• Insert a compression tester into the spark plug hole and crank the engine to take a reading.
• Compare the reading to manufacturer specifications; replace the cylinder head gasket or repair internal components if compression is low.