what causes the check engine light to stay on in my BMW 3 Series E90/E91 (2005-2012)

Solution

1. Preparation

• Gather necessary tools and parts.
• Ensure the vehicle is parked on a flat surface, and the engine is cool.
• Disconnect the battery to avoid electrical shorts.

2. Addressing Error Codes

• Using an OBD-II scanner, note the error codes generated.
• Research the error codes to identify specific issues (e.g., P0420 for catalytic converter efficiency).

3. Inspect and Replace Faulty Components

• Ignition System:

  • Remove spark plug covers and inspect spark plugs for wear.
  • Replace spark plugs if worn or fouled.
  • Test ignition coils with a multimeter; replace any faulty coils.

• Fuel System:

  • Check fuel pressure using a fuel pressure gauge.
  • If pressure is low, inspect the fuel pump and replace if necessary.
  • Clean or replace clogged fuel injectors.

• Vacuum Leaks:

  • Inspect hoses for cracks or disconnections.
  • Replace any damaged hoses and ensure all connections are secure.

4. Reset the Check Engine Light

• After repairs, reconnect the battery.
• Use the OBD-II scanner to clear the error codes and reset the check engine light.
• Start the engine and allow it to reach operating temperature.