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1994 Corolla Mot Failure Co Emissions Help!

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Hello all,

1994 Toyota Corolla 1.6 petrol, 160,000 miles.

Failed MOT today:

1. Anti-roll bar has excessive play in one rubber mount and ball joint in linkeage. Not worried as would think quite cheap and easy to fix.

2. Suspension component mounting in two areas of outer sill excessively corroded. Not worried as they quoted £60 + VAT for welding to repair.

3. Exhaust emissions carbon monoxide after 2nd fast (and first) idle excessive.

This I am worried about. The garage (where car MOTed) said they couldn't tell me what was wrong then but it could be the CAT or the lamda sensor or just fixed by putting additive in the fuel.

HC and lambda readings were all okay as was the idle CO emission.

I've read that some garages make a fortune by fitting new CATs when they are not needed and I gather they are expensive and the car is not worth anything.

I have dealt with the garage before and have found them to be trustworthy and fair but don't want to have a CAT fitted if not necessary even if the garage genuinely believes that is the cause, but turns out not to be.

I noticed it failed on same thing two years ago not in my ownership but was fixed although no mention of CAT or detail of how it was fixed on bill I have and no mention of anything obvious like sensor or spark plugs or air filter as parts, so I suspect it may have been some adjustable setting or something. It didn't fail on that last year either.

Is it worth me changing the air filter and spark plugs and filling with redex and taking for a good burn to clear any crap out of the system. Neither have been changed in the two years I've had the car and I don't know before then they were last changed.

Any comments or advice would be welcome.

Many thanks,


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Finding a garage you can trust is a good thing, but you now seems to have doubts.

A quick look on the web shows the cat for your car seems to be from £50 + fitting, though expect its like always, you get what you pay for.

Though you do not mention any ecu error codes, it might be the lambda, some like on my old Avensis can be a stupid price , I got a second hand one from the scrap yard for £40.

For a new one I would avoid anything but a genuine Toyota one, unless the garage guarantees it.

What you can do is take your car to your local council mot centre, there is one in most large towns, they have to mot all the taxis etc so are well aquainted with Toyotas.

They charge full price for the mot £54 or so, but they cannot do any repairs to the car or recommend any garage to you, they can only test it, so you get at totally unbiased report.

Something I learnt a couple of years ago with my old Avensis, it failed the emmissions test, sugar, I thought ! - but no, he then put it though the manual emissions test and it passed.Never knew about that !

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Think its what this leaflet refers to as the Extended test, but don't really know anything more than what the tester told me.

Think its also got a lot to do with the engine being fully hot so everything is working at optimal, thats why many folk say take it for a 30min run before going to the tester.


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I'd go for the cheapest first;

How many miles have you done in two years of ownership.

I'd change the air filter and spark plugs- they should cheap from your local motor factors.

why not give it it a good blast with some fuel additive what have you got to loose.

as oldcodger mentioned, give your local scrap yards a call for a lamda sensor.

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Thank you both.

I've done about 20,000 miles in two years.

I haven't had the plugs or air filter changed and don't know how long ago they were changed before that so they are probably over due.

But, I would have thought if these were in bad condition I would be experiencing bad running which I'm not.

I agree about going for cheaper options first and will get the spark plugs and air filter changed and also go for this I've seen:

Wynn's Clean Burn Emissions Reducer

There is also redex which I presume cleans the injectors as well, not sure what the wynns stuff actually does.

Might be also worth filling up with Shell v-power too before going on a good run.

Couple of other things:

I noticed a while ago the air filter box sensor was actually out of the box so put it back in. I wonder whether that was taken out two years ago to pass the CO test and why it passed last year.

The car overheated about six months ago due to lack of coolant (no leak found) but topped it up and appeared to be no lasting damage, but wonder whether that has had an affect on it.



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Thank you oldcodger.

I've checked the air filter. It looks quite clean on top but on the underside there is a noticeable amount of dirt on it so hopefully that's the problem.

I've always been a bit sceptical of additives so not sure whether to try that as well yet. They do seem to get very mixed views with some people swearing by them and others saying no difference at all.

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Be very careful about using Redex or other additives, if your car has done 160k, they clean engines yes,but they clean all of the interior of the engine, including the skirt area of the piston, on an old engine this is not a good idea as the carbon formed in this area acts as a sliding plug around the piston and above the rings, removal of this could expose the wear in the bore and piston rings and may even introduce the dreaded piston slap! Ideally the only area you would want to clean would be the piston crown and as far as I lnow the only way to do that is to perform a strip and decoke, and again on an old engine that may induce other unwanted side effects. Go for plugs and airfilter change and see if that has any affect before you get in too deep.

Good luck

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Too late as I've already put some in and given it a good blast down the motorway at high revs.

Just hope it doesn't damage the engine.

At least I know not to use it again.

Changed air filter and spark plugs and is currently in garage undergoing the work and hopefully will pass the retest.


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Hello All,

Toyota finally passed the MOT.

After changing the Oil and filter and with my changed air filter and spark plugs they redid the emissions test and found the CO was WORSE than before!

They found the lambdor sensor wasn't sealed properly, so they removed it, rethreaded it and replaced it. Got through the test.

Not sure how that would work. Maybe extra air being sucked in around sensor is picked up by it and it thinks there is too much oxygen in exhaust and increases petrol injected into cylinders. Yet CO isn't indicative of unburnt fuel, but extra fuel may have been burnt, and spark plugs weren't black. CO indicates not enough air getting in. Maybe lamdba sensor sensed too much air and reduced air going to cylinders (can it do that?)? I'm confused!

I do know one thing. When the lambda sensor failed on my Jag, there was too much petrol being pumped into the engine, it ran rough and there was black smoke from the exhaust.

Cheers, Simon

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