P0420 AGAIN!

This fault code seems to be cropping up more and more, by that I mean it comes back after the fix. But why?
Firstly lets understand the fault code and how its generated.
Modern cars have 2 O2 sensors, the first before the cat, monitors the oxygen content of the exhaust gas. It uses this to adapt the fuelling via the fuel trim to keep the mixture within acceptable limits. The 2nd sensor after the cat monitors the efficiency of the cat. If the switching ratio of the O2 sensor is similar then the cat isn't working. This ratio is monitored and once a set ratio is exceeded the fault code P0420 is set.
So why does this fault code come back?
There are 2 causes poor quality replacement parts, and not fixing underlying causes.
If you are presented with this code ensure before fitting a new cat the reason for the failure is correctly identified and fixed. Missfires will make short work of a new cat.
Once the cause is rectified ensure the replacement is of OEM quality, too often the returning code is down to the quality of the replacement parts.

Rally car trouble

This week we have a rally car to deal with. Once warm it cuts out, will not re-start until cold again. A simple temperature related problem so why has it proved so difficult to fix?
time to investigate.

Turns out a relay is failing.