P0420 ECU code - already had new cat' recently

8th October 2011, 01:07

Hi guys,

Had this exact problem on my old Saxo (now sitting in a scrapyard

). Engine light came on earlier, checked it with my £18 OBD scanner and code was P0420. Only bought the car two weeks or so ago and the full service history says the Cat' was replaced late last year.

On my old one the P0420 code eventually resulted in me replacing both the Lambda sensors and the Cat', which was done earlier this year.

Do you reckon it's worth taking off all of that from the old one (it's the exact same spec') and sticking it all straight on the new one; or do you reckon that as the Cat' has been replaced so recently that it's one of the Lambda's that's faulty (as I suspect and hope)?

Am the ol' two zero years of age tomorrow/today(!) but might spend some of it down the scrapyard, but not to worry, I like cars

Any advice would be appreciated guys.

8th October 2011, 01:10

Try a new lambda, only takes 5 mins to unscrew and plug a new one in.

Remmeber to reset the ECU.

8th October 2011, 01:46

Yeah thinking that's deffo the best option at the moment... there's no evidence in the history of the Lambdas being done, but if it really is the Cat', they obviously got very ripped off.

Also I always make sure I reset the ECU just to make sure that there's no other faults that might crop up unnoticed!

We shall see but it's unlucky that it's my bday today and I'm back to Uni tomorrow!

13th October 2011, 11:32

K guys,

It's just had an MOT and failed on emissions believe it or not.

So, the cat is barely a year old, do I assume it's a dud one and order another, or, do I order a new post cat Lambda (I have the relatively new pre-cat sensor from the old car, but the other sensor and the cat itself had been hacked off

), or both and be done with it?

If I get the two it should still be less than a ton. Is this the correct cat' for my car (2002/52 1.1i Desire petrol) (post-cat): http://www.ebay.co.uk/itm/NEW-Plug-O...ht_7832wt_1037?

This is a bit disappointing - it's never failed an MOT and to fail on something as shit as this!

13th October 2011, 13:54

Try eurocarparts.com 25% off you might get a good deal

13th October 2011, 23:21

Right, bare with me, it's a long one...

What makes this a lot harder is the fact that I am doing this away from home as I'm at Uni and leaving it all up to my Dad, which I'd rather not, he's done too much for me with my cars already.

Anyways, the info for the only refusal on the MOT paper is "Exhaust emissions carbon monoxide content after 2nd fast idle excessive (7.3.D.2b)"

That's the only thing it failed on, oh and got an advisory on a tyre, but that's no problem to sort.

As that's high CO, I believe, from what I've seen, it could well be the Lambda's that need replacing. Some background would help here: the last service was around 43000 miles, it has never failed MOT's before (never even had any advisories), the entire exhaust was changed exactly one year ago (Cat, down pipe and back box).

Such a recent/regular servicing (current mileage 49,478) suggests to me that the engine is running adequately. Such a recent overhaul of the exhaust suggests to me replacing it is unlikely to fix it. So, there has been no history of the Lambda's being replaced, and as this seems to be one of the symptoms for the exact reason for failure above would you agree that my best bet is to replace the Lambdas?

I will use the pre-cat one from the old car as it is still quite new and buy a new post-cat as this seems to be the most important - don't have it anyway because some nutjob hacked it all off when they wanted the 'precious metals' from the cat in the old car!

Apart from this, I was also looking at perhaps new spark plugs - which fix do you think is best: Lambda's or plugs, if any?

Shit 'appens eh?

Any form of advice is appreciated here guys

13th October 2011, 23:38

Personally, I would start by giving the car a good service: plugs, air filter etc. Check the exhaust for any leaks too.

If you feel a lambda sensor is at fault, you want to replace the pre CAT sensor only. The post CAT sensor doesn't alter the emissions in any way - it's only used to check the 'health' of the CAT and put the EML on if the CAT is 'out of spec'.

Unless a good quality CAT was fitted, you can't really rule that out either. Cheap ones from a motor factors sometimes only last an MOT or two.

14th October 2011, 00:39

Cheers LeeumH,

The exhaust was done by Britannia Tyres in Nuneaton according to the service history, if it matters, and I don't have the receipt with me but the total came to something like pretty much £200, for the three things above and labour, presumably.

I'd rather not service it atm as everything else seems fine to me and I know I haven't had the car very long but based on my old one it all feels and looks good/better. I also would rather slam the problem precisely without having to spend too much money at the moment - another reason why I don't really want a service yet, apart from which one is not due for at least another 5,500 miles.

But you're sure the pre-cat Lambda is the one that controls the ECU mixture settings? If so that's a babe because we have the virtually new pre-cat from the old car - it might be free to fix!

Although would you recommend replacing the downpipe one as well at all? I guess it agrees with the MOT so it must be working somewhere!

14th October 2011, 16:37

For the price of them, it's probably worth renewing the plugs and air filter, just in case these are causing the problem. These are changed as part of a major service, so might not have been done for some time.

Quote:

Originally Posted by mikejr

But you're sure the pre-cat Lambda is the one that controls the ECU mixture settings?

Yes mate. Don't waste your time renewing the post CAT sensor - it won't alter the emissions in any way.

14th October 2011, 17:47

Cool, can't really see exactly what was done on the service history anyway so I guess it's not an issue (except for the money). Just rather get it done before the 20th when the MOT runs out!

Think im gonna try for the pre-cat sensor and some spark plugs as they're currently the cheapest option (especially with a free Lambda

) and the most likely solution... hopefully.

What does a high CO (and only CO, as opposed to HC or CO2, which are fine) level on fast idle normally mean though, baring in mind the history of the car is mint?

Do you reckon it might have been because it wasn't running hot when they did the test?

Hopefully speak to my Dad later and I'll let you know how it goes.

14th October 2011, 22:55

IIRC, high CO is usually caused by overfueling.

Give what you've suggested a try, and make sure the CAT is nice and hot before the retest. Failing that, you'll need to dig deeper. Could be anything from the CAT itself to a dodgy coolant temp sensor.

16th October 2011, 22:02

Right just came off Skype and my Dad (bless him) done some work this weekend and changed the plugs and coils. Said two of the plugs broke up as he took them out and reckoned they had never been changed.

He said he couldn't do the pre-cat Lambda as it was stuck solid in the thread. So that will have to wait.

So, chances of it passing on the 2nd fast idle CO test with new plugs and coils now?

I am thinking if all else fails I will just bite the bullet and reinvest in a new Cat.

Cheers.

16th October 2011, 22:19

Good advice given above but don`t rule out your injectors mine nearly failed the emissions because of blocked injectors.After a dose of injector cleaner it passed.Just a heads up that`s all mate.

16th October 2011, 22:27

Quote:

Originally Posted by stevo67

Good advice given above but don`t rule out your injectors mine nearly failed the emissions because of blocked injectors.After a dose of injector cleaner it passed.Just a heads up that`s all mate.

Will consider that too but if it were blocked injectors wouldn't it be running a little unusually anyway?

16th October 2011, 23:01

Quote:

Originally Posted by mikejr

Will consider that too but if it were blocked injectors wouldn't it be running a little unusually anyway?

With my blocked injectors the problem comes & goes sometimes the car will start & run perfect from cold other times it`ll be a complete pig.

16th October 2011, 23:06

Quote:

Originally Posted by stevo67

With my blocked injectors the problem comes & goes sometimes the car will start & run perfect from cold other times it`ll be a complete pig.

Cool, will watch out for that then too!

But rather betting on the plugs at the moment... what money would you put on them passing it (baring in mind what it failed on was something so minuscule)?

16th October 2011, 23:18

Sounds like you've possibly found your problem. Check the air filter too, as that may be in similar condition to the plugs.

Lambda will probably come out with some heat. Either warm the car or use a blowtorch. Chop the connector off the end of the wire and use a big 22mm ring spanner on it.

18th October 2011, 23:57

Right,

Plugs replaced, and now the pre-cat Lambda too, as well as a bit of a thrashing up the motorway. Everything necessary hopefully. My Dad's a legend!

(Why do things like this always happen whilst I'm at uni?

)

If this doesn't do it then I guess it will just have to be a new Cat' - but the bummer there is that will have to wait till Christmas, and then I'll have to SORN it until then and it's all a bit crap...

So, getting retested tomorrow... Fingers crossed!!!

19th October 2011, 14:12

Problem. Solved.

Just checked the MOT status online because I couldn't wait to hear any news and she passed. Win.

Thank you Dad!

So, for anyone else with this problem the answer is simple - try a new pre-cat Lambda and/or plugs.

Let's hope for everyone's sake no nutjobs collide into the side of this one and it doesn't end up on it's side sliding down the road at 40mph (two separate incidents lol)!

Anyways thanks for the help guys - hopefully won't be needing any MOT related advice for a year if not a long while!