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

Solution

1. Preparation

• Gather all necessary tools and parts.
• Disconnect the battery to ensure safety during repairs.
• Allow the engine to cool completely before starting work.

2. Inspect and Replace Spark Plug

• Remove the ignition coil from cylinder 1 using a socket set.
• Remove the spark plug from cylinder 1 using a spark plug socket.
• Inspect the spark plug for wear; replace it if necessary.
• Install a new spark plug, ensuring it's gapped correctly according to the manufacturer's specifications (typically 0.028 - 0.032 inches).
• Reinstall the ignition coil securely.

3. Check and Replace Ignition Coil

• If the ignition coil appears faulty, disconnect the wiring harness and remove the ignition coil from cylinder 1.
• Install a new ignition coil, ensuring it is seated properly.
• Reconnect the wiring harness.

4. Inspect Fuel Injector

• Disconnect the fuel injector connector for cylinder 1.
• Remove the fuel injector using appropriate tools (may require a special puller).
• Inspect the injector for clogs or signs of failure; clean or replace as necessary.
• Reinstall the injector and reconnect the connector.

5. Perform Compression Test (if needed)

• Remove the spark plug and ignition coil from cylinder 1.
• Insert the compression gauge into the spark plug hole.
• Crank the engine for a few seconds and record the compression reading (should be within manufacturer specifications).
• If compression is low, further investigate potential causes like valve issues or piston damage.