Hi all

Some time ago my Focus 1.6 LX developed a fault.

The amber engine warning light illuminated and my fault reader gave a P04020 error code. My local garage has replaced the cat, the forwardand rear lambda sensors, temperature sensor and the fuel regulator. These were replaced (on fords recomendation) separately but on each occasion the warning light again came on again after about 50 miles. The light is again on now, the error code is the same as on previous occasions but we are running out of options and parts to replace.

The following info came from my memoscan

Fault Code - P04020 Generic Catalyst efficiency Below Threshold (Bank 1)

Freeze Frame info

trouble code - P0420

Fuel SYS1 CL, faulty HO2S

Fuel SYS2 CL, faulty HO2S

calc laod 56%

coolant 112 deg C

ST FTRM 1 -3.1

LT FTRM 1 -6.2

MAP (Kpa) 0

engine RPM 2703

Speed 78Km/h

Any help with a diagnosis would be greatly appriciated

The first check is to make sure the second oxygen sensor is giving a steady reading around the mid value, say 0.5v. If this sensor is giving a fluctuating reading, i.e. switching between 0.1 and 0.8v then this proves the cat has failed.

The first sensor should always switch when in closed loop but will remain steady on over-run as it goes to open loop, usually at about 0.1v for a second or two.were genuin parts used as aftermarket ones are sometimes not up to the job.

also see here http://www.obd-codes.com/p0420