Is a car question OT?

Doubt it. The engine will just keep going and if the cat's not working (or not there) it really makes no difference to the engine. This depends, of course, on that dash light only meaning the cat has failed, and not something else in control of mixture.

Think about it, the cat's output all goes out into the air. It just means more emissions.

NT

Not for me but for a friend of a friend (I'm a SNDB)

Apart from being illegal, being likely to fail a roadside check and being certain to fail an MOT, is any further damage likely to be done by driving a car with a non-working catalytic converter? The dashboard light says that emission control has failed but the owner would rather just keep driving until the MOT test which is due in a couple of months.

Personally I would have the error code diagnosed as it might be just a case of a failed/dis-connected sensor or something like that

Or someone has nicked his catalytic converter. There has been a spate of it around Middlebrough recently.

Aye, though I suspect the OP might notice a small increase in exhaust noise as I doubt any tea leaves would join the down pipe back to the rest of the exhuast system when knicking the cat...

Could just be a knackered/disconnected/dirty conections sensor.

Is he sure it's the cat that's failed? Has he read the fault codes?

Unless it's very very recent, not that many cars have post-cat lambdas, which are really about the only thing that'll infallibly tell the ECU and fault codes that the cat has failed.

If it IS the cat, and there's no post-cat lambda, then a failed cat is unlikely to tell the ECU to slap a light up. It'll probably be fine, but it MAY clog or break up and block the exhaust flow, reducing power drastically and overheating the exhaust. A fire might result.

If it is the cat, and there's a post-cat lambda, then the ECU might put the car into a limp-home mode.

If it isn't the cat, then all bets are off. It might run very rich, drinking fuel and diluting oil. It might be the first warning of another issue.

If he's guessing it's "just" the cat, he's probably wrong.

Will he be leaving other problems - brakes, f'rinstance - until the MOT, under this widespread belief that the MOT is all the maintenance a car needs?

In article , Nthkentman writes

While it may fail a roadside emissions test, the existing MOT is still valid and the vehicle is unlikely to be unroadworthy.

My only concern would be that failure of the lambda sensor could result in over rich running and excess hydrocarbons in the exhaust, leading to damage to the (expensive) CAT. I doubt very much that the light indicates a failure of the CAT itself or that it is monitored in any way.

A post to uk.rec.cars.maintenance may be useful too.

This was my thinking but there are other, different, well-thought-out opinions in this thread that I'm taking seriously too.

Nick

The error code is deffo for the emission control but the duff sensor is worth looking into.

Nick

Haven't heard so much about roadside emissions stops for the last few years, but they do include the ability to place an immediate prohibition order on any vehicle.

Does the fault code give any more detail than that? Because that's incredibly wide ranging. If that's all it comes back with, then the cat is about the lowest on the list of likely causes.

This happened to my son while he was parked up in Detroit. Fortunately, it was Detroit so getting a replacement wasn't too difficult or expensive.

Nick

Maybe he even got his own one back again?

It's a nearly three-year-old Vauxhall Corsa so I presume, unless the law changed (I'm a SNDB, remember) it's due for its first MOT very soon. I'm surprised that, with all the give-aways and deals that come with new cars that it is not still under warranty but I'm told that it isn't so there we go.

According to the handbook, that light flashing indicates a failing emissions control system; constantly on indicates it has failed. I'm getting the impression that there's more to emissions control than just the cat, right?

Of course not (he says, hopefully).

Thanks for all the comments: I'll see that the owner gets to read them.

Nick

That thought did cross my mind!

Only what I mentioned upthread. Back in the days when all you needed was a good set of spanners and a feeler gauge, I used to enjoy looking after my own bikes and cars but because I don't drive any more, I've lost interest in the maintenance thing since the car has become smarter than I am.

As I mentioned earlier, I'll see that the owner gets to read all this.

Thanks,

Nick

the ECU will possibly try and compensate and that could mean extra fuel consumption, but I doubt it would damage anything

sounds to me like a dodgy lambda sensor.

Could be no more than a disconnected wire corroded terminal or a failed sensor.

I've never dealt with that fault but it is probably little more than under the car, inspect and juggle, and at worst unscrew and replace sensor and reconnect.

I've just thought of something though. a vague memory says that cat converters get poisoned by working at wrong part of the curve, so if it ain't fixed you MIGHT end up with having to get a new cat even though existing one is actually OK.

Some older cars did, though mostly Japanese IIRC.

It's getting worse though, my bog standard Focus has two cats and four sensors, one pre and one post for each cat...

Lee