Lambda Sensor Replacement P0420

[MEP's Yaris GS]

Hi all. Long time no post.

Mum's Yaris having done 138,000 miles brought up the MIL the other week.

Having run my diagnostics on it, it shows 'P0420' 'Catalyst Efficiency Below Threshold (Bank 1)'. I suspect that this is actually a case of the Lambda sensor needing replacement, and not the CAT.

Am I correct in assuming that this is the Post CAT Sensor that is causing this fault code? Many thanks.

Jon.

[t-spiritpete]

Hi all. Long time no post.

Mum's Yaris having done 138,000 miles brought up the MIL the other week.

Having run my diagnostics on it, it shows 'P0420' 'Catalyst Efficiency Below Threshold (Bank 1)'. I suspect that this is actually a case of the Lambda sensor needing replacement, and not the CAT.

Am I correct in assuming that this is the Post CAT Sensor that is causing this fault code? Many thanks.

Jon.

Hi Jon this may help,

http://www.p0420.com/

Pete.

[Chris81]

Is your mums car still on original cat/exhaust? My friends Yaris 1.0 2003 had the same problem with the P0420 coming on so after an inspection found several small holes in the exhaust, a decent aftermarket cat was fitted (non toyota as they cost about a grand!). Passed MOT emissions fine then MIL came on again about 6 months later, P0420 again. I spoke to Toyota and they said it could be down to the aftermarket cat having different tolerances to the original system, or it could be the lambdas. They recommended changing the lambdas but at a hundred quid or so each (for the proper Denso ones) my friend decided not to bother as the car runs fine. The MIL remains on.

It depends how bothered you are about the MIL. If it's still passing emissions tests then I'd be tempted to leave it - is it really worth throwing hundreds of £££s on a 10 year car that's running fine apart from a stupid light? But if you're going to try and fix it I'd say check exhaust for rust holes/leaks as this may also throw up this code. The second o2/lambda sensor would be the first thing to replace, if it still the same after that then maybe go for the cat, but they aren't cheap and you may still have the damn P0420! Let us know how you get on.

[Golden bay NZ]

The oxygen sensors are expensive. Ive done some research and they can be cleaned as it is mainly carbon deposits that stop them working well.

Soaking them in petrol overnite will loosen the deposits.

Then they can be heated up with a blowtorch (not to red hot) and dipped in cold water

If you dont want to spend the money

I havent tried this yet but I found it on the net

[t-spiritpete]

Hi all. Long time no post.

Mum's Yaris having done 138,000 miles brought up the MIL the other week.

Having run my diagnostics on it, it shows 'P0420' 'Catalyst Efficiency Below Threshold (Bank 1)'. I suspect that this is actually a case of the Lambda sensor needing replacement, and not the CAT.

Am I correct in assuming that this is the Post CAT Sensor that is causing this fault code? Many thanks.

Jon.

Hello again Jon, as mum normally drives the car can I suggest you take the car out, get it fully warmed up and give it an Italian Tuneup .... that's a good fast drive in the lower gears for 20 minutes or so as this will get the cat and sensors really hot and may clear the fault, you may need to clear the original fault by pulling the EFI fuse or disconnecting the Battery neg for a few minutes. I've seen this done with success a few times when cars won't pass the MOT due to high emissions so give it a go as it will only cost you a drop of petrol ..... just don't get caught speeding etc ;).

Pete.

[MEP's Yaris GS]

Just thought I'd post a little update. The car is now on 151k and after having had a complete replacement of the exhaust (including CAT) back a few months ago the issue still persists? So I decided to replace the post CAT sensor.

Well when this issue first occoured last year I had purchased a replacement from a local motor factors, but upon going to fit this afternoon the plug is wrong - typical, and I can't even chop the plug off and rewire it as the wiring is a different colour completely.

So I've gone and ordered another for replacement tomorrow. According to parts catalogue at local motor factors (a different one this time) pre and post CAT sensors appear to be the same with the exception of wire length, so I've ordered the longer wired one so I can swap it with each sensor in turn if I don't get it correct with being post CAT.

I'm interested in possibly testing the existing sensors with a multi meter for their readings, has anyone any idea on what I should be looking for readings wise?

Also upon disconnection, if driven how quickly should the MIL come on, as I drove to order the replacement sensor with the post CAT sensor disconnected, a good 6 miles, and the MIL didn't come back on at any point.

Cheers,

Jon.