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Focus 2002: To Change Cat Or Not?


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Hi guys,

I have owned this car for a few months and have been getting the same problem where by the EML comes on after about 100 miles and stays on until reset. I'm suspecting its the cat but I don't want to start spending money changing bits unnecessarily, so I would be very interested to hear anybody elses thoughts...

  • Error code: P0420
  • Dashboard trick: DTC9318, battery and alternator have good voltages (also read that this is consistent with a knackered cat)
  • It looks like the front Lambda has been replaced fairly recently with a genuine ford part, maybe the previous owner was trying to fix this problem...before passing it onto gullible me!
  • Car is at 100,000 miles
  • Had an emisions test done which showed the lambda at 1.01 which is an MOT pass
  • Since I've had the car i've replaced: cam belt, speed sensor, air/fuel filters
  • Cleaned the idle control valve
  • Drives OK, MPG and power not as good as I would expect but don't really have much experience as my other car is a tuned up mini!
  • Makes a slightly unusual sound but not like its blowing, also I can cover the exhaust with a rag quite easily.
  • Also, the headlights flicker very slightly when the revs come down to idle, but otherwise I don't know of any other problems.

That's everything I can think of, thanks in advance if anybody can help.

Kind regards, Dan

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here :-

A code P0420 may mean that one or more of the following has happened: Leaded fuel was used where unleaded was called for A damaged or failed oxygen sensor (HO2S) Downstream oxygen sensor (HO2S) wiring damaged or connected improperly The engine coolant temperature sensor is not working properly Damaged or leaking exhaust manifold / catalytic converter / muffler / exhaust pipe Retarded spark timing The oxygen sensors in front and behind the converter are reporting too similar of readings Leaking fuel injector or high fuel pressure Cylinder misfire oil contamination

and :-

Some suggested steps for troubleshooting a P0420 error code include: Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required. Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady). Inspect the downstream heated oxygen sensor (HO2S), replace if necessary Replace the catalytic converter

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