how to fix the check engine light code P0301 on my BMW 3 Series E90/E91 (2005-2012)?

Solution

1. Preparation

• Gather required tools and parts.
• Ensure the engine is cool and disconnect the battery to avoid electrical shorts.
• Raise the vehicle if necessary for better access.

2. Inspect and Replace Spark Plug

• Sub-steps:
  1. Remove the engine cover if applicable.
  2. Disconnect the ignition coil connector for cylinder 1.
  3. Remove the ignition coil using a socket set.
  4. Unscrew the spark plug using a spark plug socket.
  5. Inspect the spark plug for signs of wear or fouling; replace if necessary.
  6. Install a new spark plug, hand-tighten, then torque to manufacturer specifications (typically 18 ft-lbs).

3. Test and Replace Ignition Coil

• Sub-steps:
  1. Visually inspect the ignition coil for cracks or damage.
  2. If necessary, swap the ignition coil with another cylinder to see if the misfire follows.
  3. If the misfire code moves to that cylinder, replace the ignition coil with a new part.
  4. Reinstall the ignition coil and reconnect the electrical connector.

4. Inspect and Clean Fuel Injector

• Sub-steps:
  1. Disconnect the fuel injector connector for cylinder 1.
  2. Remove the fuel rail if necessary to access the injector.
  3. Inspect the injector for clogs or damage; clean or replace if needed.
  4. Reassemble the fuel rail and reconnect the injector connector.

5. Perform Compression Test

• Sub-steps:
  1. Remove the spark plug from cylinder 1.
  2. Insert the compression tester into the spark plug hole.
  3. Crank the engine and record the maximum compression reading.
  4. Compare the reading to the manufacturer's specifications (usually around 140-200 psi).
  5. If compression is low, further investigation is needed into potential internal engine issues.