Have just re-read this myself and it does sound like a load of old bull,
was cutting and pasting without paying too much attention its been a long day,enough of my excuses so will try to make the following more gramaticaly correct .
I have a problem with my emission light and upon interrorgating the vehicles system with an opcom reader,the fault P0420 (cat below efficiency) is displayed.
The vehicle performs quite reasonably when driven down motorways (a trip of approx 400miles ) during this time the emission light stays off.
Normally the fault reoccurs around town mostly when pulling away from traffic lights islands etc or when decelerating whilst exiting off the motorway,
since this has started to happen the car has what sounds like a resonating sound around 2000 revs which is more pronounced more pronounced when decelerating.
A new cat was fitted in the middle of last year as previous one had fell to bits,the actual fault started approx 3 months ago, since then i have had the rest of exhaust system renewed this year.
I noticed the other night the emission light came on as i was waiting for traffic lights to change, the tick over revs dropped slightly (or missed a beat ) emission light came on, also engine ticked over very slightly lumpy prior to this occuring.
I do have engine overrun when changing gears as well, its a 2.2se engine the question is, as i have had all the exhaust changed if the pre cat was loose would it give me the resonating noise that i hear and bring on the emissions light ?
As i am getting fed up trawling around garages being told different things etc the next question is would a faulty map sensor give me a very slight lumpy tick over ?? many thanks in advance...S