[AstraGuru]

I could do with a little help on an issue :confused:

I keep getting the P0420 code and an engine management light on. The car is a 2003 Astra SXi 1.6 16valve (Z16XE)

Here's a little history to help, three months ago I was forced to replace the CAT as it had collapsed at 88k. Not wanting to fork out for a OEM CAT I brought an aftermarket CAT for £258 which is really cheap. All was good for two months then one evening the light came on and I checked the code which was P0420.

I checked that everything was in order, no air leaks pre CAT or hoses leaking. All was fine.

So I decided that the Lambda sensors were knackered and replaced them with good quality NGK replacements and reset the light. Two days later on the M25 at cruising speed the light has come back on.

Got a mate to do a four gas analysis of the emissions and although not MOT failing (0.2%) I was getting a 0.166% CO2 at idle, now bearing in mind at the last MOT the idle CO2 was at 0.015% that is a marked difference....

Checked on diagnostic - EGR valve which is six months old and fine, MAP sensor functioning, Coolant temp sensor.


So before I go and blow £600 quid at Vauxhalls on another CAT and get done for GBH for hurling the aftermarket CAT at the part-factors has anyone had any bad experiences of aftermarket CAT going wrong fast as I suspect the CAT is the problem and although not an MOT fail probably triggering the light due to tighter manufacturers tolerance being exceeded?

I thought all UK sold CATs were now type approved to stop bad quality emissons control parts?

Any help or advice would be kindly appreciated - Dave :)

[Hometune]

All you say is correct. Even though a cat is now (supposedly) type approved, many aftermarket ones will not meet the tolerances set by the manufacturer. As you have found, it is well within MoT emissions limits which is usual. Before you knock out the parts people do some checks for confirmation.
1. In live data menu on your scanner watch the number one oxygen (lambda) sensor and it must switch once a second between 0.1 and 0.8v approx.
2. If it does now watch the second sensor. With the engine hot hold at 3,000 revs for 3 min, and the sensor should be fixed at 0.6v approx. If it is moving like the first one, this confirms a faulty cat.
3. If the second sensor reads either 0.1v steady or 0.8v steady, the fault is elsewhere.
4. The feedback voltage from the EGR valve must be less than 1.20v which you can see in live data. This is the ONLY correct way to test these.

There have been many cases like yours posted on here. Try a company called Autovaux (Google it) as they supply genuine and afermarket Vauxhall parts to trade and public cheaper than dealers.

[AstraGuru]

Thanks for the info Hometune :cool:

Right well, sensor 1 (pre CAT) is switching as described so this seems to be functioning as normal.

Sensor number 2 (post CAT) however is far from stable and is switching like almost like sensor number 1 so I would imagine it is receiving much the same input as the first sensor ergo the CAT is near dead :( .....crazy, i never got a light on when my OEM CAT was falling apart and yet this intact aftermarket CAT is still not as good!

EGR voltage was well under 1.2v so EGR is functioning well, which I pretty much expected since it was replaced at the end of last year after giving the car a Kangarooing effect when crawling in traffic and on inspection was well clogged.

Is there a specific MAP voltage I should be looking for? This changes in line with changes in throttle position so I am assuming that the MAP is ok. My mate reckons their pretty reliable sensors and far more reliable than a MAF.

I guess I don't want to run the risk of damaging an expensive OEM CAT if there is an underlying cause. I have now also sourced a CAT luckily from a Vauxhall dealer who wanted to shift it from stock for a shad over £500 not bad when they normally sell for £700 odd...

EXPENSIVE issue which could have been avoided if I hadn't rushed to buy an aftermarket CAT in the first place. Avoid aftermarket CAT's will be my opinion moving forward!

[Hometune]

The MAP is unlikely to be the fault. At idle the map will be steady, blip the throttle and it will go up then as the revs drop the map will drop too then stabilise again. Yours sounds ok.
From your description of the second sensor switching it does point to the CAT.

One of my customers (a Grumbleweed for any who remember them on the telly) has a Saab Griffin, V6 turbo and that did the same last month. New ones are a fortune so he found an exhaust manufacturer who made him a cat using an old Aston Martin one and fitted it in a stainless steel casing for £300. After 5 miles the code P0420 came back. So he went back and had another cat fitted behind the first - 2 in line - and for now the light is staying out.