Dtc Code P0420 - Cat Or Lambda Sensor

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Hi Guys,

Wonder if you could help, I have got the EML now being displayed on the dash. Have code read the fault and it has come back with P0420 'Catalyst System Efficiency Below Threshold Bank 1'. I know that this code can be triggered by many things such as an air leak in the exhaust, lambda sensors (but usually P01?? code is usually triggered) I have read the freeze frame data but im wondering if there is anyone that could understand the data more than what I can. From what I've read up on, the SHORTFT and LONGFT should be as close to 0 as possible. However from the freeze frame data you can see that the SHORTFT has a positive reading and the LONGFT has a negative reading where its trying to compansate and correct the fuel mixture.

What I am wondering is, from this freeze frame data, could someone diagnose where for me to start replacing parts. I'm hoping that this is just a lamda sensor issue, I have already replaced the pre cat lamda sensor on Bank 1 (Denso part no DOX-0236). Without this help, my next step would have been to replace the lamda sensor by the main cat (not sure of the Denso part number for post cat sensor), so that bank 1 has had a replacement sensor before and after the cat, to see if this clears the code. I know that I might need to replace the cat, but at this moment in time I am trying the cheapest option first to resolve the issue. But all of your opinions are welcome.

Here's the freeze frame data:





Other data from code reader:




I hope someone on this site can point me in the right direction.

Many thanks,


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Hi Dean

You cannot really tell from the freeze frame data. You need to monitor the sensors whilst driving by data logging the information.

For that age of vehicle it could very well be the cat. I would take it along to an MOT testing centre, throw someone a few £, and ask them to check the emissions. That will tell you how well the cat is working.

The trims don't need to be at or near zero. The whole point of them is to compensate for manufacturing tolerances, wear, the environment, fuel being used etc.

Also have you checked for a exhaust leak?

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Hi check exhaust near sensor for possible leak,could be a pin hole leak.

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Thank you for getting back to me, my car has been into a friends garage before (last month) and their response was to first of all
replace the pre cat sensor, which I did and didnt resolve the problem. They checked for leaks and couldn't detect any leaks. I seem to be going round in circles, friends who I have asked haven't really been bothered to help and point me in any direction. I have asked to get my car in for a emissions test but wasn't getting anywhere, they didnt do a test when I specifically booked it in to be looked at. Anyone know how much I'm looking at for a new cat?

I think I might be better booking the car in at another garage to get it looked at properly.

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Just another thought, if I bite the bullet and end up getting a new cat, whats to say that the pre-cats aren't breaking up and if I fit a new cat, this could then get blocked by the pre-cats breaking up.

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My bet- main cat.

* Your o2 sensor is located behind it- other lambdas are before precats- it is unlikely they will detect faulty cat.

* 0420 is cat efficency below treshold- assuming your main cat is all good and if you install decat exhaust manifolds- you will not get this error (youre not supposed to get it anyway).

you can get new cats from ebay for approx 50-80 gbp or 2nd hand one from ideally 05 vehicle for....well around same price.

I had similar problem and it was main cat. I removed the underfloor cat pipe and rather large chunks of my main cat fell out of it- cut it out until i got new one- you can do the same and if you are annoyed by check engine light- disconnect battery for 3 min and connect it again- light wont come up for about a week.

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Hiya guys,

Sorry I meant to update this post but completely forgot, I have now resolved the P0420 EML problem I had. Turned out the main cat needed to be replaced.

Thank you for your advice. All very much appreciated.

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