Catalytic Converter - new lambda sensor needed?

Horizon81

Not my car but a friends... The engine managament light came on around September last year. Local independent garage plugged it into laptop and diagnosed it as emissions after the catalytic converter. They said exhaust was rusting badly, particularly with little holes just before the cat which were the likely cause. The car was 7 years old so friend proceeded with the new cat. All was well with the new cat fitted, management light went out and stayed out.... until last week. Garage have run diagnostic again and say it is the same fault code coming up. They say that they will try to get another cat under warranty from their supplier (because, in their eyes, the new cat has failed) but, I was reading the literature that came with the replacement cat and it said 'A new Lambda sensor must be fitted when a new cat is fitted' regardless of the cause of why the original cat failed. I've mentioned this to the garage but they say it isn't necessary. I just fear that it's the original Lambda that's faulty, and killing the cats (!) and that another replacement cat could get fitted and the same problem would occur in a few months time.

So, to all petrolheads on here, if you were changing your cat, would you also insist on a new Lambda sensor being fitted?

hubert_cumberdale

i wouldnt imagine it would damage unless the cat was so blocked up that it increased the ambient temperature of it so much that it damaged the lamda sensors probe. I have seen cats glow red from being blocked up but the car didnt rev much over 200rpm

maybe they damaged the lamda sensors wire as they become quite fragile over time
or perhaps when they removed it from the old rusty pipe it damaged it

colino

Get a second opinion from a local friendly independent. Misinterpretation (or deliberate misinterpretation) of code readers/scans is a handy way of jacking up bills. The leaky exhaust may have thrown up the cat code (P0420?) and the cat may not have needed replacing at all.
Also, don't agree with always changing sensor with cat. Cleaning up an oem sensor and refitting is preferential to fitting a new, aftermarket item.

bondy01

What car is it? May be worth posting on their ownersclub forum. I have an 8 year old Rav 4 and have replaced 2 sensors (so far!) The heater circuit generally seems to go open circuit. Do you know anyone with a code reader , handy little tool and not too dear either.

tbourner

Take the sensor out and stick it in half a lemon overnight, then rinse and refit. Worked for mine. You'd have to reset the ECU as well after fitting a new O2 sensor or cat.

hubert_cumberdale

if the car has two lamda sensors leaky cats can throw up faulty sensor codes.

basically the sensor monitors the amount of oxygen before and after the cat so if there is a leak somewhere they wont be getting true readings and cause problems but this doesnt explain why the fault has come back if the cat has been replaced

epninety

I imagine the supplier says that to cover themselves, since a lot will be fitted by home mechanics without much idea how it all works together.

An air bleed in the exhaust before the cat may well cause the cat to run very hot, but it's hard to guess if thats damaged the lambda sensor. I've seen several aftermarket cats that failed before the following MoT, but 4 months seems a bit short even then.

I wouldn't insist on the lambda being changed, but it is trivially simple to clean and/or test it. Of course, with garage labour rates and the design of many cars, the replacement part is a small section of the overall cost, so it's simpler and cheaper to just change it.

Horizon81

epninety - what makes have you seen fail? This one was made by 'Fuel Parts' - is that a reputable aftermarket manufacturer?

Also - how easy is it to remove the sensor? Can't say I'd ever spotted any wires coming from the cat to the engine!

hubert_cumberdale

the sensors are usually about 6 inches away from the cat on cars i work on

vauxhalls, hondas, bmws

some new cars now have a couple of cats as well some nearer the exhaust manifold

tbourner

You might be able to see it sticking out of the downpipe if you lift the bonnet. Probably has 2 bolts holding it to the exhaust downpipe and a small cable coming out and going to a clip connector.

Lum

Sounds like you have two lambda sendors. The one before the cat is the important one as the output from this is used to tweak the air/fuel ratio and keep the car running efficiently. If your engine is running badly this can damage the cat, e.g. clogging it up with soot.

The one after the cat exists only to nag you if your cat is faulty, a requirement in certain US states. It can usually be replaced with a 10p resistor as emissions testing in the UK is done with a probe not by relying on the car's on board diagnostics like they do in the states.