Code 'P0420'

I have a 2009 MINI One 1.4 that has an intermittent fault. It has irregular revs and either stalls or cuts out when at a junction. My local garage read off the fault codes and changed the camshaft sensor, advising that I may need to clean or replace the camshaft solenoids if the problem persists.

Could my problem be due to a fault with the stop-start system? I’ve changed the spark plugs and flushed out and replaced the oil. When I checked the fault codes, I found ‘P0420’, which I cleared and has not returned. The catalytic converter was replaced at the last MOT and has been a regular issue with the car when I checked its MOT history online. Gerry O’connor

The code ‘P0420’ refers to ‘Catalytic converter system, bank 1 – efficiency below threshold’. You do not mention if the 02 sensors were changed when the cat was replaced, but if this has not been done, it would be worth considering.

It is unlikely the stop-start system is causing the irregular revs, as this would cut the engine completely as opposed to causing the revs to fall and rise. I am assuming that when the engine cuts out at junctions it is not purely because the stop-start system is operating.

You do not mention the mileage of the MINI but it’s possible the engine has become coked up, particularly if it’s only used for local runs. If the problem was with the camshaft system, I would expect to see the camshaft sensor code reappear, but this is not a certainty. Also, depending on the mileage, you might have a stretched timing chain or, as mentioned by the garage, a sticking camshaft timing solenoid. Timing chain problems would be accompanied by a light rattling at start-up.