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

Solution

1. Preparation

• Gather 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. Replace Spark Plug

• Remove the ignition coil from cylinder 1:
  • Unclip the electrical connector from the ignition coil.
  • Unscrew the coil using a socket set and remove it from the spark plug hole.
• Remove the spark plug:
  • Use a spark plug socket to carefully unscrew and remove the spark plug.
• Install the new spark plug:
  • Check the gap of the new spark plug with a feeler gauge; adjust if necessary.
  • Hand-tighten the plug, then use a torque wrench to tighten to manufacturer specifications (typically 15-20 Nm).
• Reinstall the ignition coil and connect the electrical connector.

3. Test/Replace Ignition Coil

• If the spark plug was not the issue, test the ignition coil:
  • Use a multimeter to measure the resistance of the ignition coil (check manufacturer specs).
• If the coil is faulty, replace it with a new one:
  • Follow the same steps as above for removal and installation.

4. Inspect/Replace Fuel Injector

• If misfire persists, inspect the fuel injector:
  • Remove the fuel rail (ensure to relieve fuel pressure first).
  • Examine the injector for clogs or leaks.
• If faulty, replace the fuel injector:
  • Install the new injector and ensure it is seated correctly in the fuel rail.

5. Perform Compression Test

• If electrical components are functioning properly, use a compression tester:
  • Remove the spark plug and install the compression tester.
  • Crank the engine and record the reading.
• Compare the reading with manufacturer specifications; if low, further investigation of engine internals may be required.