P0420 Catalyst System Efficiency Below Threshold (Bank 1)

IDI AMIN

Senior Member
#1
I could not believe the f**king management light came on again today after fitting a new front o2 sensor a few days ago with the same code p0420 does this mean maybe the cat is goosed or do i need to replace the other sensor too or do i need to change both anyone know any better suggestions please
 

Daz

Former Staff
Senior Member
#2
Its either gonna be the post cat sensor reading incorrectly or the cat is goosed.

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IDI AMIN

Senior Member
#3
Cheers daz i suspected that. i think i need to get the other sensor first i think its a bit cheaper than a cat .its possible i need a new cts sensor also engine seems to heat up a lot for what i think is a quite short journey this is on the list of problems on the obd website just need to replace them one at a time
 

Daz

Former Staff
Senior Member
#5
You'll probs find its the cat from my experience, the gases are flowing past the rear sensor to fast as they would if you had a 200 cell cat fitted hence why peeps with sports cats either put up with the light or get it mapped out.

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IDI AMIN

Senior Member
#6
Okay. the pre cat was removed a while back and i had the down pipe welded a couple of years back but not too well it was slightly out of position and doesn't hang properly on the exhaust rubbers one side sits higher and it started knocking on the floor a couple of weeks back when i fixed a slight leak at the joins for my mot emissions test. obviously disturbed the seating position but i have always thought of replacing the downpipe since that time but only want to buy parts when absolutely necessary.iwas looking on ebay over a year ago and they were about 70 odd quid need to have a fresh look to see whats on offer nowadays
But you are a regular mechanic so are more than likely correct
 
#9
@peng12 i now have replaced the 2 sensors and a new cat i do not have the p0420 code anymore i think it was down to the second sensor was past its sell by date .actually when i went to put new o2 sensor on i realised one of the wires to the sensor was broken and no matter how many times it was turned off with a code reader it would come back on within minutes of driving .it just took time to realise what it was. I suspected i would have to replace rear sensor because of age anyway .
I should have sorted it sooner as it was dragging my mpg down.Acceleration was a little restricted because of it ,when sensor was replaced you can notice straight away idle was a little jerky before and it accelerates much better, more responsive whereas before it would be a little hesitant
i would say it is one of your sensors causing this as with a new cat it was coming on until the 2nd sensor was changed
 
#10
Thanks mate I might just change that then first as its the cheaper option and take it from there. Although in saying that I might still need a new cat as I think the two have rusted together lol
 
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