how to diagnose a P0301 misfire code in my BMW 3 Series E90/E91?

Solution

1. Preparation

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

2. Inspect and Replace Ignition Coil

• Tools Required: Socket set, torque wrench.
• Sub-steps:
  1. Remove the engine cover to access the ignition coils.
  2. Disconnect the electrical connector from the ignition coil for cylinder 1.
  3. Unbolt the ignition coil using a socket wrench and remove it.
  4. Install the new ignition coil and secure it with the bolt, torquing to the manufacturer specification.
  5. Reconnect the electrical connector.

3. Inspect and Replace Spark Plug

• Tools Required: Socket set, spark plug socket, gap gauge.
• Sub-steps:
  1. Remove the ignition coil from cylinder 1 again to access the spark plug.
  2. Use a spark plug socket to remove the spark plug.
  3. Inspect the spark plug for wear or fouling; replace if necessary.
  4. Set the gap on the new spark plug according to specifications.
  5. Install the new spark plug and torque it to the manufacturer's specifications.

4. Test and Replace Fuel Injector (if necessary)

• Tools Required: Fuel injector tester (if applicable).
• Sub-steps:
  1. Disconnect the fuel injector connector for cylinder 1.
  2. Use a multimeter to verify the injector’s resistance. Compare with specifications.
  3. If faulty, remove the fuel rail to access the injector.
  4. Install the new fuel injector and reconnect the fuel rail and electrical connectors.

5. Final Assembly

• Reinstall the engine cover.
• Reconnect the negative battery terminal.