P0420

What the code means

P0420 sets when the DME determines that catalytic converter efficiency (Bank 1) has dropped below threshold. The control unit compares the oxygen sensor readings before and after the catalytic converter: a healthy catalytic converter smooths out the signal fluctuations from the rear sensor, and if its response starts to look like the front sensor signal, the system concludes efficiency is lost.

On N55 engines, the common causes are oil consumption, misfires, and normal catalytic converter aging.

Symptoms

• Check Engine light is on.
• Possible sulfur smell from the exhaust.
• Worse fuel economy.
• Failure on an emissions test.

Diagnosis and repair

1. Before replacing the catalytic converter, diagnose the root cause: misfires and oil leaks are the typical culprits on the N55.
2. Make sure there are no air leaks in the exhaust system: leaks upstream of the rear sensor skew its readings and can trigger a false code.
3. If diagnosis confirms lost efficiency, replace the catalytic converter — OEM part number 18307560533.
4. After the repair, reset adaptations and take a test drive to verify the result.

Secondary issues

• If you ignore the code, the catalytic converter can clog, creating exhaust backpressure and reducing engine power.
• Damage to the oxygen sensors.

Severity is elevated: don't put off diagnosis.