Lambda sensor / cat converter

nosedive

Hi all, I'm looking for a little advice - I have tried a search of previous threads but need some help regarding the o2 sensor on a 2002 audi, 1.6 petrol.

Story is: I have a bluetooth OBD tool and was looking at my o2 sensors after check engine light came on - voltages for both sensors were varying between 0.1 and 1 but my mechanic (an indy with a basic enough diagnostic) reckoned the problem was the heated, downstream sensor. This weekend he replaced that sensor (second hand) but the problem remains only now my OBD shows no voltage for the downstream sensor - so it must be dead?

Anyway, if the original sensor was ok he thinks the problem might lie with the catalytic converter and I'm wondering how we might check this before spending a lot of cash replacing it. I can't remember now if, when reading the voltages before any work was done, whether readings from both sensors were the same or different.

The problem has lead to a couple of instances of the engine dying/struggling for a few seconds on the motorway - as if there was a small amount of dirty fuel or a blockage that cleared itself but i presume that was just the engine computer not knowing what mixture to feed into the engine??

Any help appreciated fellas - NCT in a couple of weeks and I'd like to have it sorted by then - with a bit of luck it's the last time I'll be putting her through - 182k on the clock now!

kingtut

Bring it to a garage that can do an emissions test ?
I did that with mine and they found out that the cause was the catalytic converter, got a new one and the problem was solved.

nosedive

.....but wouldn't rich running from a faulty sensor give high emissions readings anyway?

I suppose the real question I have is, if the voltage on a lambda sensor varies as it should, could the sensor still be faulty?

Technoprisoner

why did you look at the lambda sensors? was the fault code showing a lambda problem?

nosedive

Technoprisoner wrote: »

why did you look at the lambda sensors? was the fault code showing a lambda problem?

Whatever diagnostic my man had lead him to believe it was the downstream sensor but wasn't 100%. I'm wondering now if.i.took out a good sensor for no reason and the problem is the cat?

nosedive

Technoprisoner wrote: »

why did you look at the lambda sensors? was the fault code showing a lambda problem?

Whatever diagnostic my man had lead him to believe it was the downstream sensor but wasn't 100%. I'm wondering now if.i.took out a good sensor for no reason and the problem is the cat?

crosstownk

Lambda sensor voltage fluctuating is normal and it is quite possible that there was nothing wrong with your original sensor.

As asked previously, what prompted you monitor both probes? What did the fault code suggest the problem was?

George Dalton

If the post car sensor was fluctuating in a similar way to the pre cat sensor then that usually means the cat isn't doing its job. The post cat sensor voltage should be reasonably steady. So in this case it sounds like there was nothing wrong with the original sensor and the problem is the cat.

nosedive

@crosstownk; as far as i know the on board system will illuminate the check engine light for a failed sensor but does not differentiate between sensor one or two so both need to be checked.

@George Dalton; thanks for that response it seems to have conformed what i was thinking. it seems the problem is indeed the cat (possibly a sensor as well but sounds like bad news and i need a new cat).

Technoprisoner

the engine management light will come on for a lot of sensors on the car. Did the fault code come up for emissions?