Hi

just purchased a 54plate ST170 for the misses. It has the EML on and after reading code i have P0420. I know this is the cat but the cat is new and so are both o2 sensors. I've cleared the code then had a drive. No EML light as yet but i connected laptop when i returned and i now have the P0420 code pending. Any ideas on the possible cause????

That code can mean any of these are the cause

Damaged or faulty 02 sensor

Downstream 02 wiring damaged or sensor incorrectly fitted or both o2 sensors giving similar readings

Coolant temp sensor not working

Damaged or leaking cat backbox or pipe

Retarded spark timing

Leaking fuel injector or high fuel pressure

Oil contamination

Cylinder misfire

Simply put it needs thorogh diagnostics to find which Is at fault

Thanks for the reply. Lots to look at there then. think ill wait for the eml to come back on but i have ordered plugs, good quality leads and a good quality coil pack as the leads and coil have never been changed.

Thanks again for the info. I may run some live data while the misses takes it for a drive and see if all is as it should be.