Does the Cat need to work in order to pass the MOT?

[PhilMcConnachie]

A garage mechanic told me that my catalytic convertor was broken and would need to be replaced to pass the MOT which is coming up soon. However I've read the odd post on here suggesting that the cat does not need to work if you are running on LPG. My car - 2004 Landcruiser - switches to LPG automatically therefore I would expect it to be tested on LPG (unless the mechanic pressed the button to switch it to petrol).

Is the above assumption correct because I don't really want to replace the cat if I don't need to?

Thanks

Phil

[Gilbertd]

If the car is presented for MoT running on gas, it should be tested to the non-CAT standards. See http://www.motuk.co.uk/images/730_newnoncat.pdf . It might be worth printing it off and showing it to the tester if he tries to test to the petrol standard.

[rossko]

The MoT rules change in January. It seems that if a cat was fitted originally, it will need to be fitted for an MoT. I'd say you need the cat to be present.

However, the running-on-LPG emissions testing remains laxer than on (modern) running-on-petrol. You don't need a functional cat to acheive the LPG standard.

I'm not a lawyer or tester, but so far as I can see if the cat is physically present and it's tested on LPG there shouldn't be a problem.

I would question why the man thinks the cat is bust though? Might be symptom due to some other cause (busted lambda sensor?), or just a rattle

[Gilbertd]

I've looked through the new regulations and, as far as I can see, it will only affect cars that have had the cats actually removed. If the cat is still there, even if it has had the innards removed so looks like it is still there, it should be OK.

[Tubbs]

What a lot of the lads over on the other forum i frequent have done is either remove the cat and re welded the plate back on on the side facing the floor. Or they have protected them with body armour over the cat so the mot inspector cant see the cat at all. If you run an off roader then they expect it to have a few modifications, so it doesnt really look out of place having plates over the exhaust boxes.

[PhilMcConnachie]

The MoT rules change in January. It seems that if a cat was fitted originally, it will need to be fitted for an MoT. I'd say you need the cat to be present.

However, the running-on-LPG emissions testing remains laxer than on (modern) running-on-petrol. You don't need a functional cat to acheive the LPG standard.

I'm not a lawyer or tester, but so far as I can see if the cat is physically present and it's tested on LPG there shouldn't be a problem.

I would question why the man thinks the cat is bust though? Might be symptom due to some other cause (busted lambda sensor?), or just a rattle

Thanks. yeh the Cat has a rattle, maybe caused by off-road use by the previous owner. So I guess if I present it and tell the MOT tester to do the MOT running on gas then I should be okay......

[Gilbertd]

So I guess if I present it and tell the MOT tester to do the MOT running on gas then I should be okay......

No, it must be presented for test running on gas. You don't tell him how to do the test, you tell him what fuel it is running on. If he tests it to the cat test standard, then you can point out that he's testing to the wrong standard.

[dannyf]

I have a sport cat on a Vband flange it just swaps out no probs at all
mind you it passed with out the cat fitted

[Dualtech]

I removed the cat on a granda cosworth my 'better half' had, then removed the guts out of it with a pry bar. (best not to breath in the dust though!!) Refitted the cat box, all is well. Its on lpg so passes the emmisions test, it still looks like it has a cat so inspector is happy, got rid of the rattle and it went a bit better to boot!

[PhilMcConnachie]

I can present it running on gas but if he doesn't test it for a few hours it will start on petrol before switching to gas which presumably gives a @!## reading on the emissions if the cat isn't working. That's why I want to tell him. Is doing the MOT with a broken cat somehow a bit dodgy? (I don't see why if the emissions are lower on lpg). If not, I may as well be open with him .

Maybe they run the emissions tests over say 5 minutes or more which will give the LPG the chance to kick in therefore no problem.

[mat_fenwick]

From the MOT manual:

The test should be conducted with the engine warm. Testing a cold engine could lead to an unjustified failure.

Running on petrol, during the warm up period the emissions are likely to fail due to the cold start enrichment. So you should be OK. Worth mentioning though, in case you have to rev it to get it to switch.

[PhilMcConnachie]

Thanks Matt, good info.

[john_d]

any mot station will make sure the engine is up to temp. before testing emissions.

[Gilbertd]

I've always found that if you take a car for test at a place where you leave it and go back later to collect it, it will fail. Take it somewhere that does it while you wait, and there's a better chance that it will pass as they've got to show you the faults rather than tell you about them later.

[dannyf]

I've always found that if you take a car for test at a place where you leave it and go back later to collect it, it will fail. Take it somewhere that does it while you wait, and there's a better chance that it will pass as they've got to show you the faults rather than tell you about them later.

yep
my car failed when it went in on petrol
I asked them if they had it upto temp
nope
one blast up an down the road 10 mins latter it passed its test no probs at all

[reggieroo]

I will have to take my L reg Range Rover classic for MOT soon, it only runs on gas & has had the cat removed for a tubular manifold & full stainless side exit exhaust. I was told that it will be fine going through an MOT without the cats as it runs on gas, is this not the case now then?

[Sir Henry]

The MOT tester's 'bible' states that all vehicles have to be

a) up to full operating temperature before any emissions test is carried out (there's a box on the form for the temperature reading)
and
b) tested on the fuel it is running on when presented for the test.

Unfortunately many don't bother running the engine long enough for it to warm up properly and rarely bother to check if it's switched over to gas before putting it on the Sun machine. That's why it's best to book the car in for the test on a day when you can drive it there so as to arrive just in time for the test so that you know it will be running on gas.

One of my cars is an old dirty diesel which I run on filtered WVO. Even so the emissions can be borderline if the engine isn't up to full temperature so I give it a good highish speed blast and get to the MOT station no more than 5 minutes before the time it's booked in for. Then I leave it running through all the pre-emissions tests just to be certain. To date it's passed with a good margin to spare.

[Gilbertd]

Nope. An L plate will be a '93 so would have had cats originally so should have them now, even though they are not tested during the emissions test. In reality, you may be OK but you may have to have a pair of cats (or empty cat boxes) to fit for the test.

[PhilMcConnachie]

I've just had exactly this situation - my MOT was last week and I know my cat is not working. I had a word with my mechanic who deals with the MOT man and he had a word with the MOT man in advance, just to check out what the requirement is. The MOT man was confident that the Cat was not a requirement if it was running on gas. My specific worry was whether the engine would have cooled down by the time of the MOT in which case it would start on petrol and fail. Luckily i was first on the ramp at 9am so the engine was warm and it passed the MOT.

Sir Henry's advice above about driving the car to the MOT station is therefore good advice.

I wonder what would have happened if the MOT was at 3pm and the engine was cold.

[Gilbertd]

If the test had been done correctly, it would have been warmed up before being tested so would have switched over. If it was tested on petrol and failed on emisions, it's because it was not tested correctly because it wasn't tested with a hot engine. In the same way that the regulations are quite specific, if the car was originally fitted with a cat, it will fail the test if it has been removed. This is irrespective of what fuel it is running on and whether or not it fails the emissions test. In fact, I would say that any tester that is doing the test by the book, wouldn't even test the emissions if there is no cat as it has already failed.