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sean
sean, Car and LCV technician
Category: UK Car
Satisfied Customers: 3105
Experience:  UK City & Guilds qualified with 30 years experience
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Focus 1.8: got a p0420 code on my ford focus 1.8 2001 have

Customer Question

got a p0420 code on my ford focus 1.8 2001 have tried new cat plus o2 sensors throttle control valve goes of for about 100 miles but comes back on
Submitted: 2 years ago.
Category: UK Car
Expert:  sean replied 2 years ago.
Hello, I hope the following assists you.
P0420 is a catalyst efficiency problem. The usual reason for this code after the catalyst has been changed is an aftermarket catalyst been used. The aftermarket catalysts are cheaper for a reason, the surface area of the active material is smaller than in a genuine Ford catalyst. This means at higher gas flows when the engine speed is higher not enough of the exhaust gases can be converted to harmless gas, the downstream O2 sensor detects this which is when the MIL will come on again.
Check that the new catalyst hasn't become damaged by a misfire for example as this causes the catalyst to over heat.
I hope this helps but if I can advise further let me know and I'll reply as soon as I'm able to.
Best wishes,
Sean.