why does my BMW 3 Series E90/E91 (2005-2012) check engine light keep coming on

Solution

1. Preparation

• Gather necessary tools and parts.
• Ensure the vehicle is parked on a level surface and the engine is cool.
• Disconnect the battery's negative terminal to prevent electrical issues.

2. Address Error Codes

• Sub-steps:
  1. Use the OBD-II scanner to retrieve codes.
  2. Research and document the specific trouble codes (e.g., P0171 for lean condition).
  3. Clear the codes and take the vehicle for a short drive to see if they reappear.

3. Inspect and Replace Components

• Sub-steps:
  1. If the gas cap is faulty, replace it with a new OEM or compatible part.
  2. For vacuum leaks, inspect hoses and gaskets; replace any damaged components.
  3. If ignition system issues are indicated, replace spark plugs and coils as necessary.
     • Torque spark plugs to manufacturer specifications (typically around 15 lb-ft for BMW).

4. Fuel System Check

• Sub-steps:
  1. Attach a fuel pressure gauge to the fuel rail.
  2. Start the engine and monitor fuel pressure; compare to manufacturer specifications.
  3. If pressure is low, inspect and replace the fuel filter or fuel pump if needed.

5. Exhaust System Inspection

• Sub-steps:
  1. Visually inspect the exhaust for cracks or holes.
  2. Use a smoke test to identify any leaks in the exhaust system.
  3. Repair or replace any damaged sections of the exhaust.