Xsara 2.0i 16V 136 HP - P0420 trouble code help

[pcspinheiro]

Dear all,

my Xsara is at it again... This time it spat out a P0420 trouble code (Catalytic converter low efficiency) as I was driving on the highway, 110 Km/h (which I don't do so often I should add, only small town rides, about 5 km). I cleared it once, then drove on the highway again and it came back. No other code present. I googled it around and found a very informative webpage about correctly diagnosing this fault (leaving it here for future reference):

http://www.easterncatalytic.com/educati ... o42o-code/" onclick="window.open(this.href);return false;

So, I did as suggested and measured the front and rear lambda sensor data on a warm engine while revving it to about 3000 rpm (using a lexia 3). I'm trying to attach the jpeg of the data but I'm getting "Sorry, the board attachment quota has been reached."... Anyway, basically the rear sensor was oscillating pretty much in the same way as the front one, indeed suggesting the cat.... HOWEVER, I have noticed for a while now that when down shifting to 3rd or 2nd gears there is a strange sound coming from what seems like the exhaust system. Could a cracked exhaust be letting in too much O2 and causing the rear lambda to behave this way and triggering a cat fault error? Mind you, I don't know if there is a crack or just something loose but I don't have the time right now (or weather, since it's -4C and snowing like crazy) to look into it. I just wanted your knowledgeable input before going out selling a kidney to get a new cat

Best wishes,
Paulo.

[pcspinheiro]

:: BUMP ::

Nobody???

[Mandrake]

Unfortunately if the downstream sensor is closely mirroring the upstream sensor switching when the exhaust is hot then as far as I know that means the catalytic converter efficiency is very low, eg the cat is dead.

An exhaust leak upstream of one of the oxygen sensors would cause it to read too lean but it couldn't cause the downstream sensor to oscillate in tandem with the upstream sensor.

A faulty cat is not something to be diagnosed lightly though, given the cost! The best online resource I've found for cat diagnosis is Scanner Danners youtube channel - he has many videos covering catalytic converter testing, this one is probably a good start:

" onclick="window.open(this.href);return false;


If you've exceeded the board attachment limit (as I have) you'll need to upload the images to one of the free image hosting sites and link to it. I use Dropbox but there are many other sites that will work. You can imbed an image directly within a post using the IMG tag as long as the URL is a direct link to the image not a link to a web page.

[pcspinheiro]

Thanks Mandrake, very valuable input! I guess I could do the enrichment test as well with some propane... However, I seem to have read somewhere that a leaky exhaust could also cause problems due to insufficient pressure, leading to cat malfunction. That American loud bloke, Scotty Kilmer, suggests a cat can be cleaned... but I'm a bit unsure about his methods as they seem like "just do it"

Here is the O2 sensor data that I mentioned:

http://l.bitcasa.com/w318nwuQ" onclick="window.open(this.href);return false;

I guess it all points to a bad cat but I'll check the possible exhaust leak before making any move...

[Mandrake]

Thanks Mandrake, very valuable input! I guess I could do the enrichment test as well with some propane...

I think the oxygen storage test is more to identify a "marginal" cat - one where the downstream sensor does not switch rapidly and would otherwise be deemed "ok", but whose catalysing ability may be severely compromised and intermittently trip a fault code. By measuring how much oxygen it can store you can see how close to failing it is. A long storage time is good, (several seconds) a short storage time (fraction of a second) is nearing the end of its life. In the case where the downstream sensor mirrors the upstream sensor I don't think the oxygen storage test is required as the result is already a clear failure.

However, I seem to have read somewhere that a leaky exhaust could also cause problems due to insufficient pressure, leading to cat malfunction.

I did hear Paul (ScannerDanner) say in his video to make sure upstream exhaust leaks have been ruled out first although he didn't say why, so I guess that could interfere with the operation of the cat. A cat will only catalyse if the exhaust stream is near stoichiometric most of the time - an air leak would cause the gas going into the cat to be constantly lean, so part of the 3 way conversion process would not work. So definitely check for upstream exhaust air leaks first.

That American loud bloke, Scotty Kilmer, suggests a cat can be cleaned... but I'm a bit unsure about his methods as they seem like "just do it"

Yeah I think I've seen that before, I'm very dubious about cleaning out a cat in the way he depicts!

Here is the O2 sensor data that I mentioned:

http://l.bitcasa.com/w318nwuQ" onclick="window.open(this.href);return false;

I guess it all points to a bad cat but I'll check the possible exhaust leak before making any move...

That does look bad alright, the downstream oxygen sensor is almost mirroring the upstream sensor - compare that to Paul's videos and it does seem like the cat has failed unfortunately.

The question is why - you need to be sure that the engine is running properly, at the correct mixture, and not misfiring (even under load) before replacing the cat, or the replacement could be damaged too. Have you done any recent repairs to the ignition, or to correct any running problems ?

[Ben82]

EricTheCarGuy tried Scotty's method, and posted up a video about it here: " onclick="window.open(this.href);return false; in short, it didn't make a difference.

If possible, it's worth taking the car for a drive from cold with the lexia connected, hopefully you can add more measurements to it like whether it's closed or open loop, and engine rpm, vehicle speed etc

[pcspinheiro]

Thanks again guys!

There has been no repair (made by me at least) to correct any engine malfunction. I've owned the car for over 1 year now and the cat problem only appeared a couple of weeks ago. Then again, it seems that it is only diagnosed when keeping a steady highway speed, and that rarely ever happens as the car drives some 6 Km/day to take the kids to daycare... When I firs got it it was riddled with small things, the most annoying of which was a broken resistor on the cooling system that caused the engine to slightly overheat (but within reason) until the fan kicked in at full speed. It was basically without the lower fan speed and that often triggered an error related to the injection system, don't remember which. This error would go away on its own once the engine cooled and, since I replaced the resistor, never came back. It was, in my opinion, triggered by the sudden current draw of the fan kicking in a full speed, causing some engine judder.

The only other thing is that it idles a little low and tends to stall if one is not careful with the clutch, but it is otherwise responsive. I have not been able to do anything about that...

In any case I will repeat the measurements with some more parameters and make sure the engine is really up to temp. (i.e., rev it a little longer before looking at sensor data). I have always felt it uses too much fuel but that could be the misses... I will also try to "invent" some time (you sure need to be a time wizard with 2 small ones...) to take it to a diy garage and lift it to check for leaks/cracks... It sound too much a coincidence that this problem would arise when I also started hearing this strange, kind of knocking/popping sound when downshifting move "vigorously"

I will report back any findings! Hopefully they may also help others.

best wishes,
Paulo.

Could the lack of proper heating up in the short daily trips let to too much dirt accumulating in it over time?
Other question, can the lexia plot the fuel trims? I could not really identify that parameter to diagnose/graph...