Error code advice - pre-cat lambda sensor

miniyazz · 27 Mar 2014 at 12:35

I'm considering a new (old) car, namely a Jag S-type 4.2L n/a

So torn between spending (much) more and getting a better one or spending less and getting one needing a bit of work. The cheapskate in me says the latter; realistically the difference in cost is a couple of grand.

Now the car I'm looking at has a few cosmetic issues none of which are particularly major (bits of missing trim, that sort of thing) but also has a check engine light and a check of the fault codes have revealed an issue with the bank 1 sensor 1 O2 sensor. The car also has an irregular idle. It seems to drive fine without any obvious gearbox issues.

So I suppose my question is, how much is it likely to set me back to sort out the pre-cat O2 sensor? My local garage muttered something about £2-300 and advised me to steer clear

rodenal · 27 Mar 2014 at 12:54

Assuming it is just the lambda and nothing else then the cost of buying a new sensor, some swearing as you try to break the old one free and inevitable burning yourself on the manifold after all this fails and you end up heating it up to release the o2 sensor.

If it's more than the sensor, then who knows.

I wouldn't touch an early S type to be honest, they've dated horribly inside and aren't exactly the last word in reliability (though i'm sure you'd get by fine if you're hands on)

miniyazz · 27 Mar 2014 at 14:04

Yeah well that's the question really, whether the lambda is the cause or just a symptom. Is it feasible for a pre-cat lambda to be causing a dodgy idle?

It's a 2006 so should be more reliable than the earlier models. I'm not under any illusions that it'll be as cheap to run as a golf though.

As a side note, the garage confused me by suggesting that there would be 6 lambda sensors on a car like this - one on the manifold, one pre-cat and one post-cat (x2 as two banks). Is that right? I thought there would just be four..

Dogbreath · 27 Mar 2014 at 15:05

miniyazz said:
Yeah well that's the question really, whether the lambda is the cause or just a symptom. Is it feasible for a pre-cat lambda to be causing a dodgy idle?

Yes, it's possible. It's also possible for a misfire (caused by something else) to throw an error code as the oxygen sensor will be seeing a constantly lean mixture, no matter how much fuel is added.

Further investigation would be needed to separate cause and effect.

Born_2_Kill_83 · 27 Mar 2014 at 16:29

Might be worth checking the fuel trims also, this can tell you if the car is running lean or rich, which will also tell you if it's just a faulty O2 sensor, air leak, injector etc. The pre-cat lambdas always cause the problem you've described, 9/10 chance this is all it'll be.

With Jag's always spend more, get the papers so you know the money has been spent, potential neglected ones will almost certainly be a money pit. Soon as you fix a problem another will occur, you want to avoid buying a Jag as a project car, unless of course you know a lot about them, which you don't.

As for the price of an O2, I think the pre-cats are the same an the X-type and my XJ, which means their cheap, check Brit parts you'll find them on there for £60-90

miniyazz · 27 Mar 2014 at 18:30

Alright thanks. The other question is whether the lambda sensor causing a dodgy idle could cause problems with the cat - if there's a misfire this is obviously a concern - but there wasn't a code for an impending error relating to misfiring, which I thought came up pretty quickly with a misfire.

Born_2_Kill_83 · 27 Mar 2014 at 20:01

Running the car rich or lean with damage the cats in the end, but this will take a long time so nothing really to worry about on this car.

Do you have a misfire or a dodgy idle? Normally a Lambda that has failed or not working correctly will cause the car to tremble when idling, can easily be felt from inside the car, it just isn't smooth, a misfire is a lot worse and feels like the car is going to stall.

miniyazz · 27 Mar 2014 at 21:05

TBH it wasn't even that noticeable from inside the car, it was simply that I could see the idle speed fluctuating on the rev counter between ~6-900rpm. Apparently it's been on for about a year because they got it checked out and were told it was nothing more serious than a dodgy sensor and because the car still drove fine

This business about having 6 lambda sensors is confusing me though, why would there be three sensors per bank? Or was my mechanic talking rubbish?

Born_2_Kill_83 · 27 Mar 2014 at 21:58

Nahh there's 2 Lambdas before the cats and 2 after the cats on each bank (4 total), the ones after the cats rarely fail and wouldn't cause a idling problem, just high emissions which btw your car will also have.

miniyazz · 27 Mar 2014 at 22:47

Born_2_Kill_83 said:
Nahh there's 2 Lambdas before the cats and 2 after the cats on each bank (4 total), the ones after the cats rarely fail and wouldn't cause a idling problem, just high emissions which btw your car will also have.

But post-cat O2 sensors failing wouldn't cause high emissions. All they do is monitor the cat efficiency (and trip if they think the cat is faulty), the car doesn't act on any data from them except to trip a fault code.

Born_2_Kill_83 · 28 Mar 2014 at 09:11

I'm pretty sure a failing post-cat lambda will also send a signal to the ECU altering the air or fuel mixture, but I'm no expert, and you don’t have to worry about this problem anyway.

Made your mind up whether you’re going for the car?

miniyazz · 28 Mar 2014 at 13:16

I'm going to have another look at it. Realistically it's unlikely to cost me the difference between it and a car potentially without these issues and once you factor in the spec which is quite rare with e.g. adaptive cruise control I think I probably will go for it yes but will probably try to knock a little more off.