[doovd]

> Everything tested during an MOT (except emissions, which I've never seen fail)

Don't think this is true, I was looking at the MOT record of a car a few days ago and emissions failure was definitely stated as MOT failure reason.

[dazc]

I drive a 12 year old car which has been brilliant mechanically but on the past two MOTs it has failed the emissions test but passed the second time with the aid of a fuel additive. My understanding is that this is now the defualt procedure with local garages.

[Scoundreller]

Pro tip for the emissions test: make sure the catalytic converter has had a chance to get red hot (by taking the test after a high speed drive).

[klyrs]

Ah, goodhart's law strikes again. Car won't pass emissions? Drive it harder before the test!

[rootusrootus]

I'm under the impression that many emissions testing programs (at least in the US) have discarded their more sophisticated testing mechanisms and now just rely on the ECU saying everything is okay. Since most cars on the road are new enough to have that ability. Oregon definitely ditched the dynos, and while they use a sniffer for really old cars, I wouldn't be surprised if they ditch that before much longer.

They're getting a lot more strict on ECU testing, too. Used to be you just needed to have no check emissions codes being thrown. Now they're more thorough, for most ECUs they can check to also verify no modifications to the emissions settings (can't just tell the computer it has no cat so it doesn't throw codes).

[KingMachiavelli]

In my state they will just do engine light & ECU test for new cars. On older cars (>=10 years) when they do the IM240 test on a dynamo but if you look at the emissions limits they are all 100x what a normal car produces. Here's the GPM values (reading/limit) from my last test on a 2011 Honda Pilot:

Hydrocarbons 0.023/1.2 (1:50) CO 0.3/15 (1:50) CO2 502/NA (guess this one has no limit?) NOx 0.081/2 (1:25)

Maybe some states apply newer, stricter limits to older cars retroactively but in my experience as long as a car is operating normally it is impossible to fail emissions. I wouldn't be surprised if the ECU threw a code before the dynamo test caught something. IMO the most practical check they actually do is the presence of a functioning gas cap.

[Scoundreller]

> IMO the most practical check they actually do is the presence of a functioning gas cap.

Even that throws a code these days. 14y.o. BMW has a pump that pressurizes the fuel system to check for leaks and it’s usually the pump that breaks and throws a code.

17y.o. Corolla threw a fuel system code but it was the gas cap.

Only thing is that this pressurization check system only runs when the weather is warm enough, so resetting a code usually lets you pass e-testing as “Not Ready” because months can go by. Maybe there’s differences in summer/winter e-testing criteria.

[rootusrootus]

I don't know if it happens this way any more, but my last Subaru would do the pressurization test about 30 minutes after the car was turned off. Kinda weirded me out the first time I went into the garage and heard what sounded like something in the rear of the car inflating. Turns out that is exactly what it was doing.

[multjoy]

Mine failed an emissions test (11yr old diesel mondeo with 170000 on the clock), but passed when I took it for a thrash round the M25 and a load of additive in the fuel tank.

The next year emissions weren't an issue, mainly because the only journey it did was a 250 mile run up the A1/A66/A6 and back again on a weekly basis.