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    Re: Renault scenic 1.6 x reg Archived Message

    Posted by Michael Whitworth on February 3, 2008, 12:26 pm, in reply to "Renault scenic 1.6 x reg"

    Hi Les,

    Having read your post and the replies, you are probably none the wiser! As the replies certainly make little sense to me.

    First of all when a system flags a p0420 you will often not be able to confirm it by observing live data or carrying out scope or system tests unless the cat is totally shot.

    P0420 means the cat has failed its onboard cat efficiency test which will usually only be carried out on a long drive at steady cruise conditions. So chances are this will need a cat to stop it flagging this code again. (and not a cheap aftermarket one).

    Now on to the question about fuel trim, STFT is the systems immediate reaction the the fueling condition being reported by the lambda sensor. LTFT is the pcm long term adjustment to the fueling trend.

    This means if an engine suddenly starts overfueling the system will start to reduce fueling immediately by making the STFT negative. It will carry on reducing the fueling until it starts to see a lean condition reported by the lambda sensor. If it isnt able to make it lean it will usually stop at -25% if in your case the LTFT is at +4.7% this means the trend is lean but you have a local rich condition.

    You also report that the pre cat lambda is switching? so is STFT moving up and down around the 23% mark?

    I would think the p0420 is nothing to do with the fuel trim figures you report and maybe a good road test will allow the system to settle at a more reasonable +/- 5% STFT. If the -25% is still present look for an overfueling fault.

    HTH

    Mike

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