P0420 Powertrain Catalyst system efficency below threshold (bank 1)

[dave1814]

Hi Guys,

Im currently getting the following error when I read the fault code off my clio 182...

P0420 Powertrain Catalyst system efficency below threshold (bank 1)

Ive been monitoring the O2 sensors and im not sure if they are functioning correctly.

From what ive read id expect the first sensor to be fluctuating from 0.1 - 0.6? and the second sensor to remain constant at about 0.4? However the first sensor seems to bounce all the way up to 0.9 sometimes and doesn't really have any pattern to it (or a very rough up and down pattern!) at one time going down the motorway the sensor was stuck at a constant 0.7.

Also most of the time the second sensor was constant at 0.4 however it did climb to 0.7 at one point and stuck there for a bit?!

Does this mean I should try replacing the pre-cat sensor?

The car does have a K-Tec exhaust system with sports cat.

thanks!
Dave

 

[TrackdayAl]

Upstream sensor should sweep between 0.1v and 0.9v yours sounds ok.

You need to look closer at the downstream one as this is what is flagging the code. The downstream one is in position to monitor the efficiency of the cat. The sensor should sit around 0.5v but if it mimics the upstream sensor then the ecu thinks the cat is not doing the correct job.

Read this should help more http://www.ukdiagnosticdirectory.com/obd-codes.html

Sorry I cannot help more as I normally only work on standard cars, and my car has had 2nd o2 sensor remapped from system.

 

[dave1814]

at the minute i think its mimicing the upstream sensor most of the time!

 

[TrackdayAl]

Well there is the reason for your code. If the cat is doing its job it should flat line. I am not sure how sports cats are meant to work.

I would keep an eye on it as normally will not pass MOT with that code stored.

 

[dave1814]

cheers mate.

just an update - it seems after the car gets warm the 2nd sensor goes a bit crazy, looks like this...

does this mean the cat aint working properly when warm?

thanks!
Dave

 

[TrackdayAl]

Would look like it, was this after a blip or anything like this?

Would really need to look over a longer timescale. If car is at idle does the 2nd one switch consistently?

 

[Dan519]

sounds to me like the pre cat sensor is old and slow

 

[dave1814]

the reading above was what I was getting for a 25 mile journey.

after the journey this is the reading i got when the car was left at idle for about 5 mins...

 

[dave1814]

just an update on this, i replaced my pre-cat sensor with a brand new one about 2 weeks ago and the light has come back on!!!! does this mean it was the post-cat sensor at fault?!

 

[TrackdayAl]

The Cat is fcuked

 

[dave1814]

i thought it could be that, but it passed the MOT emmissions fine? (the exhaust is a ktec with sports cat)