how to diagnose P0301 misfire code on my BMW 3 Series G20/G21 (2019-2024)

Solution

1. Preparation

• Gather necessary tools and parts.
• Disconnect the negative battery terminal to prevent electrical shorts.

2. Inspect Spark Plug

• Tools Required: Socket wrench, spark plug socket, torque wrench.
• Remove the spark plug from cylinder 1:
  1. Disconnect the ignition coil connector.
  2. Remove the ignition coil using a socket wrench.
  3. Extract the spark plug using a spark plug socket.
• Inspect the spark plug for signs of wear or fouling. Replace it if necessary. Install the new spark plug and torque it to the manufacturer’s specifications (typically around 25 Nm).

3. Check Ignition Coil

• Tools Required: Multimeter.
• Test the ignition coil:
  1. Measure the resistance across the primary and secondary terminals of the coil.
  2. Compare the readings to the specifications provided in the service manual. Replace if out of range.
• Reinstall the ignition coil and reconnect the connector.

4. Compression Test

• Tools Required: Compression gauge.
• Conduct a compression test:
  1. Remove the spark plug for cylinder 1.
  2. Insert the compression gauge and crank the engine.
  3. Record the compression reading and compare it to specifications (typically 140-200 psi).
• If the reading is low, perform a leak-down test to determine the source of the issue.

5. Inspect Fuel Injector

• Tools Required: No special tools required; access may necessitate removing engine covers.
• Check the fuel injector:
  1. Use a noid light to verify that the injector is receiving a signal.
  2. Listen for clicking sounds using a stethoscope while the engine is running to ensure it is operating correctly.
• Replace the injector if it is not functioning properly.

6. Inspect for Vacuum Leaks

• Tools Required: Smoke machine or carburetor cleaner.
• Check for vacuum leaks:
  1. Use a smoke machine to introduce smoke into the intake system and observe for leaks.
  2. Alternatively, spray carburetor cleaner around vacuum hoses and listen for RPM changes.
• Repair or replace any damaged hoses or gaskets found.