Diagnostics Doctor
BMW Z3
Recurring cat code
Q I was hoping you could assist with an ongoing issue I’m having with my 2000 BMW Z3 1.9. I am getting a recurring P0420 Catalytic Convertor Conversion and also a P0136 Signal Oxygen Sensor after Catalytic Convertor code.
When I acquired the car in October 2018, it had the original BMW exhaust, but it had a fracture and corrosion just ahead of the cat plus it was looking ropey along its whole length. Whilst being stored in the garage over winter I decided to replace the whole exhaust from the manifold back, and fitted an IMASAF system ordered from Germany complete with catalyst - original from BMW dealer was over £2000. I refitted the original pre- and post-cat oxygen sensors.
Once driving the Z3, a few months later the EML would light after around 50-60 miles of A-road driving, always around the same part of the journey. On reading the codes it’s always P0420 and P0136. Unfortunately, I hadn’t really driven the car much on the original exhaust to see if the EML would have lit. The previous owner had spent a tidy sum getting vacuum leaks checked and had fitted: New narrow vacuum pipes around fuel regulator, plus new breather pipe at the back of the engine and also new MAF (non-OE), new plugs, new leads.
Using my Creator C310 code reader it showed normal fluctuation of the pre-CAT sensor and flat-line post-CAT - so assume cat is working fine. Only thing is that the post-cat sensor has a reading
