Awhile back my car engine light come on and I took your advice had it checked and reset the code (po420 catalyst system efficiency below threshold) now it came back on but just so happened it was having a service the next day, so had the sensor changed on the cat now 2weeks later the engine light is back on ? Any ideas ? Now cars going back in tomorrow for them to check over, but I'm getting fed up now of the same problem occurring

Any ideas ?

 

You dont say what car it is but have you had the downpipe cat removed ?? That will cause a P0420. If you still have the cat and assuming no intake or exhaust leaks then it would indicate your cat is getting tired. What were the emissions numbers from the last MOT ?? Have you run it with a misfire recently ?? Does it burn much oil ??

Depending what year it is the P0420 can be mapped out quite easily so that may be an option to you.

 

the car is a 55plate wrx and it's completely standard, and the car is running fine no miss fires and it doesn't use that much oil, well as my garage replaced the O2 sensor they are sending it to power station a tuning company in Tewksbury by me free of charge for it to get plugged into there equipment and see what the problem is hopefully they will find the problem