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

Solution

1. Preparation

• Ensure the vehicle is parked on a level surface and the ignition is turned off.
• Disconnect the battery to prevent any electrical short circuits.
• Gather necessary tools and replacement parts.

2. Inspect and Replace Ignition Components

1. Remove Engine Cover:

   • Use a socket set to remove the bolts securing the engine cover.
   • Carefully lift off the cover and set it aside.

2. Replace Spark Plug:

   • Remove the ignition coil from cylinder 1 by disconnecting the electrical connector and unscrewing the securing bolt.
   • Pull the coil straight out and set it aside.
   • Using a spark plug socket, remove the spark plug from cylinder 1.
   • Inspect the spark plug for signs of wear or damage. Replace if necessary.
   • Install the new spark plug, tightening to the manufacturer's torque specification (typically around 18 lb-ft for BMW).

3. Replace Ignition Coil:

   • Inspect the ignition coil for cracks or signs of wear.
   • If faulty, install a new ignition coil for cylinder 1, ensuring it is seated properly.
   • Reconnect the electrical connector securely.

3. Check and Replace Fuel Injector

1. Remove Fuel Injector:

   • Relieve fuel pressure by removing the fuel pump fuse and starting the engine until it stalls.
   • Disconnect the electrical connector and any retaining clips for the fuel injector on cylinder 1.
   • Remove the fuel rail if necessary, then carefully extract the fuel injector.

2. Inspect and Install New Injector:

   • Inspect the old injector for clogs or leaks. Replace if necessary.
   • Lubricate the O-rings of the new fuel injector with engine oil.
   • Install the new injector into the fuel rail or directly into the cylinder head, ensuring it is seated correctly.

4. Reassemble Components

• Reinstall the fuel rail (if removed) and reconnect all electrical connectors.
• Reattach the engine cover and secure it with the bolts previously removed.
• Reconnect the battery.