Lambda light on but cars fine, help!!

[ANDTWENTY]

Hi, my 1996 Volvo V40 1.8 petrol has just had the lambda light come on, it comes on after about 5 mins of driving but the car seems absolutely fine!? No reduction in mpg of power etc. Over the weekend I did a bit of welding but had the battery disconnected the whole time, I also disconnected the injectors while the battery was off but put it all back and started fine. Went to fill up with fuel and it came on.

I'm in the process of converting it to LPG but all I have done is put the tank in and make some brackets, I haven't touched any of the engine apart from unplug the injectors. I need to sort it before getting the lpg kit on. Please someone shed some light.

I did try disconnecting the battery for a while but did not sort it.

[canis]

Okay, bit of a guess but I might as well relate my experience and if it matches yours, then here's how i solved it.

In my case I noticed it only came on during long periods (about two minutes or so) of decelleration, for example on a long hill. Or ticking over at traffic lights. If I got on the motorway immediately after starting and cruised, the lambda didn't come on. Once on, it didn't go off again no matter what style of driving. And the only way to clear it was to sit with the ignition off for about five minutes.

If your symptoms EXACTLY match these, then it's the throttle position sensor. Like a dirty volume knob on an old radio, the carbon track wears at common points (e.g. tick-over) so gives a false signal. Replacement is the only option, but you can get a cheapie from ebay - they're fitted to dozens of cars by a plethora of manufacturers. This is one case where paying the volvo price is unnecessary.

[ANDTWENTY]

Thanks for the reply! I had an issue with this a while back cause the car not to idle sonic replaced it with a cheap one, a few months later I had problems with the car jolting and jerking and the light was on and tracked it down to be a crap part and replaced it again with a better one. It seems very quick to be that again but I don't have any symptoms of bad idle or jolting driving bit you say you didn't either? I'll give it a check all the same

[ANDTWENTY]

So I took the car in for an MOT today as it was due, the emissions test came back with high CO, so chances are the lambda is reading right and there is an issue causing my light. Any ideas what could cause the high CO without impacting the driving? The lambda sensor is only 6 months or so old and I believe there is only the one before the cat which would mean its not just that its the cat gone, although speaking to the garage I may have to replace this.

[canis]

From 1999 onwards these cars had two lambda sensors, one before the cat and one after. The one before the cat is used to adjust fuelling. The one after it is used to assess whether the cat is working. And that's why it's become law, I reckon, to stop people running de-cat.

Now, whether that's the cause of your high CO, I don't know. I'm not entirely sure how a cat works, to be entirely honest. I'm guessing it's no longer performing it's chemical function and not scrubbing the air, or something? And the fact it's failing at this task is being detected perhaps by your second lambda sensor and putting the light on. But this is speculation, I'll admit.

[ANDTWENTY]

Originally Posted by canis

From 1999 onwards these cars had two lambda sensors, one before the cat and one after. The one before the cat is used to adjust fuelling. The one after it is used to assess whether the cat is working. And that's why it's become law, I reckon, to stop people running de-cat.

Now, whether that's the cause of your high CO, I don't know. I'm not entirely sure how a cat works, to be entirely honest. I'm guessing it's no longer performing it's chemical function and not scrubbing the air, or something? And the fact it's failing at this task is being detected perhaps by your second lambda sensor and putting the light on. But this is speculation, I'll admit.

Thanks for that, so mine being a 97 would only have 1 which is what I thought and its pre cat. This figures with a rich fuelling then and a bad cat, the lambda light is on to say the mixture is out and the cat isn't doing its job of removing the harmful gasses as seen from the emissions test in the high CO.

I kind of recall a petrol smell when I pulled the injectors out, may check if they're leaking, any other ideas?

[aldred309uk]

Originally Posted by canis

From 1999 onwards these cars had two lambda sensors, one before the cat and one after. The one before the cat is used to adjust fuelling. The one after it is used to assess whether the cat is working. And that's why it's become law, I reckon, to stop people running de-cat.

Now, whether that's the cause of your high CO, I don't know. I'm not entirely sure how a cat works, to be entirely honest. I'm guessing it's no longer performing it's chemical function and not scrubbing the air, or something? And the fact it's failing at this task is being detected perhaps by your second lambda sensor and putting the light on. But this is speculation, I'll admit.

Sorry to ask this here but the citroen forum isn't very good and you sound like the man who would know the answer.

My 1.1 saxo desire has high emissions trouble, started a few years ago when the front of the exhaust failed and fell off and had to replace the front part of the exhaust which is a cat straight off the manifold and a down pipe to the bottom of the engine. Theres an o2 sensor on the manifold and one in the front section just after the cat.

The cat was replaced and the whole remaining exhaust was changed too. After a while my engine management light came on and it was due to inefficient catalyst efficiency. I reset the light and put it in for MoT and it only just passed on emissions. The next year it failed on emissions so I replaced the cat and it passed with flying colours.

Similar time to the first replacement later and my engine light came on again. I managed to get the light off with some Tesco 99 and an Italian tune up but it has since come back on again. I thought the garage had put a cheap cat on the car but since it came back on a similar amount of time after another cat I started to wonder if it was a damaged o2 sensor and was looking to get some but they're expensive and makes the car less economically viable.

I appreciate this is the wrong forum but the correct forum is useless and this s40 section is the best forum I've been in. Thanks for your help!!!

[canis]

Originally Posted by ANDTWENTY

Thanks for that, so mine being a 97 would only have 1 which is what I thought and its pre cat. This figures with a rich fuelling then and a bad cat, the lambda light is on to say the mixture is out and the cat isn't doing its job of removing the harmful gasses as seen from the emissions test in the high CO.

I kind of recall a petrol smell when I pulled the injectors out, may check if they're leaking, any other ideas?

Ah, interesting. Not many of the phase 1 models still around, even mine is parked on the garden, but there's a lot to like about them and in my opinion they are in many ways superior to the other marques.

There are many things which can cause over-fuelling. The obvious answer is to get the OBD codes read, although in this regard the phase 1 is a bit of a nuisance - it only partially adheres to OBD standards. And you need the technology. A PC can do it, just barely, although my attempts were rather hit-n-miss.

Off the top of my head, I would suspect the lambda sensor, the MAP and MAF sensors also. Without accurate diagnosis you'll be guessing.

I wish I could help more.

[canis]

Originally Posted by aldred309uk

Sorry to ask this here but the citroen forum isn't very good and you sound like the man who would know the answer.

<snip>

I appreciate this is the wrong forum but the correct forum is useless and this s40 section is the best forum I've been in. Thanks for your help!!!

I'm sorry if I've given the wrong impression! I'm nowhere near knowledgeable on this subject - I only know what i've read, desperately trying to scrape my cars through their MOTs each year like yourself.

Are you absolutely certain the cat was changed for a new one? It's doubtful it would fail so quickly, unless it was burnt out by some other problem (unburnt fuel?) prematurely. They're supposed to last for years.

I'm sure there are forum users who know more about this than me. I would've thought they'd have something to say on this thread by now, I sure hope they do.

I would hazard that the lambda sensor and cat have similar life expectancy. They work by chemical reaction, and don't last forever, so if it's on it's original sensor it's entirely likely it's at the end of it's life. They don't entirely stop working, they emit (or affect in some way) a pulse of electricity according to gases present, but as they age the range between high and low becomes thinner. I've often wondered if their life could be extended crudely by amplifying this effect, in a smiliar way to how LAN repeaters extend range, but i've never put any experiments into action. After my kitten relieved itself on my ancient oscillloscope, such experiments are rendered beyond my technology. I went through all this with my old Sierra.

As in the previous post, the best bet is to get codes read if you can. Otherwise, it's down to buying sensors and replacing them until the problem is fixed. Which sucks, but it's the only option really available to a home mechanic stabbing in the dark.

Again, sorry I can't help more.

[KBB]

Originally Posted by ANDTWENTY

So I took the car in for an MOT today as it was due, the emissions test came back with high CO, so chances are the lambda is reading right and there is an issue causing my light. Any ideas what could cause the high CO without impacting the driving? The lambda sensor is only 6 months or so old and I believe there is only the one before the cat which would mean its not just that its the cat gone, although speaking to the garage I may have to replace this.

Can you post the results of all the gas analysis readings ie:

CO %
O2 %
CO2 %
HC ppm
Lambda Numeric Value

There should be Two sets of values one for idle and one for Fast Idle (part load)