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Fault Code P0420

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The wife's Aygo has been reporting error code P0420 for a while now. It first happened when she was away down south and the garage she took it to told her it was the downstream lambda sensor and that she didn't need to worry about it, just get it looked at when she returned home.

Once she got back home I did a bit of research and ordered a new downstream lambda filter which I duly fitted. I reset the error code and all has been fine for a couple of months.

Yesterday the MIL came on - no noticeable difference in the running of the car - so when I got home I plugged the OBD2 reader in and checked the code: P0420.

Now I suppose a brand new lambda sensor could go bad after just a short time but it's unlikely.

I've reset the code and the MIL hasn't lit again - yet. I'll check the lambda sensor wiring at the weekend when I've got time to get underneath it, but assuming that's OK am I looking at a replacement catalytic converter?

Whilst Googling for information I came across a vague reference to a pipe located below the airbox that could trigger this fault code if it came loose or was leaking. I'm assuming this is something to do with the exhaust recycling system but there were no details and I wondered if anyone else knew what this is?

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I had this code keep popping up and replaced the rear lambda.

Had the code pop up a couple of times since but last time I cleared it sometime ago it hasn't come back, so to this day I still don't know what caused it.

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AFAIK the Aygo doesn't gave EGR.

But the pipe they refer to could be the one for the ventilation onf the sump.

It sits at the left of the airbox (when looking at the engine from the front), near the Oil filler.

It's a rubber pipe/hose that sits practically vertical.

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Thanks for the info, folks. So far it hasn't come back since resetting it yesterday so I'll monitor it.

I'll take a look at this pipe and see if I can identify it.

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  • 4 weeks later...

It has come back a couple of times and each time I've reset it. When I have a bit more time I'm going to take the new lambda sensor off again and check the wiring is OK.


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Nope. It went off for the MOT with the check engine light lit as I hadn't bought the ODB2 fault reader then.

They reset it for me and passed the car. Emissions were fine.

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