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[colin33]

My "My Naff Code Reader" is telling me that I have DTE P0420 present (Catalyst efficiency above threshhold). It seems to come on intermittently and is happy to be cleared, but will come back on after 100 miles or so.

Does this mean the sensor is faulty or that there is something else wrong - engine mix, or catalyst itself?  Any suggestions appreciated.

Also when connecting up the start-up text shows
"Negative answer to service 0x30
Reason is 0x22
ConditionsNotCorrectOrRequestSequenceError"
Any ideas what this means?

Colin

[Proz]

This is a common problem with omegas .... it basically means the pre cat is begining to fail ... you can use your "My Naff Code Reader" to record the live data from your O2 sensors to rule them out and see if the car is going in to closed loop ...

[Proz]

I had this problem on mine a while ago albeit mine is petrol ... cleared the code and it would come back withing 100 miles .... if you can live with the light on then its not a problem ... i couldnt so bought a new cat .... i did try the fix on here somewhere which involved moving O2 sensors but it didnt help mine .... it shouldnt affect the car in terms of performance 

[colin33]

Thanks Proz.

What sort of reading should I expect from the O2 sensors?

[Proz]

Thanks Proz.

What sort of reading should I expect from the O2 sensors?

That is for someone a wee bit more clever than me ..... 
I think they should cycle between rich and lean then once warmed up go into closed loop ...
If you can copy and put the results up on here someone may be able to see if the O2 readings are ok .