P0420 BMW -

Question

My BMW Smog test message indicates

P0420 - Cat Efficiency below threshold (Bank I) and fail the Smog test.

What should be replaced?

Note
1. the car is a BMW740i (1997 model)
2. SMOG test is in California
3. Engine warning light is on for some time already, but passed SMOG test last year while the light was on

Thank you!
John

Answer 1

HI John,,

This usually means the bank 1 catalytic converter will need to be replaced. Once in a while an exhaust leak before the rear O2 sensor can cause this code and the rear O2 sensor itself can cause it, but most often a new converter is required.