MoT test instruments: how accurate are they?

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My car recently passed all emissions tests apart from the lambda reading. λ upper limit for a pass was 1.030 and the reading for my car was apparently 1.031. Only 1 part in over 1000 over the limit !!! Can the test centre's instruments really be that accurate (0.1%)?
 
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I suggest no because the formula to calculate lambda uses the measured values from the sensors and when you take a look at typical emission tester specs the sensors are not sufficiently accurate to achieve 3 dp.

I guess by displaying to 3dp and having a 2 dp fail limit of 1.03 it allows a reading of say 1.028 to pass.

If the reading was only displayed to only 2 dp would you round up the calculated figure lets say 1.0286234 to 1.03 and fail?

It's a complicated calculation....

https://www.bridgeanalyzers.com/air-fuel-ratio/

Take it to another test station and maybe get a pass this year or bite the bullet and find the problem.
 
I suggest no because the formula to calculate lambda uses the measured values from the sensors and when you take a look at typical emission tester specs the sensors are not sufficiently accurate to achieve 3 dp.
Thanks. It's what I suspected. It seems rather draconian to fail an MoT test on the basis of such a tiny reading excess. I'm surprised there's no specified allowance for instrument tolerances (at least, none shown on the test report sheet).
In the event the garage did an oil change and the car then passed the emission tests.
 
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Probably one for Mottie but get the exhaust melt down hot and try again.
John :)
Correct. Or, test again without the engine speed connected and hold it at whatever revs it passes at. Some testers have been known to stick another car on and take the readings from that. Not me of course......
It's what I suspected. It seems rather draconian to fail an MoT test on the basis of such a tiny reading excess.
Tester has no say in it - the machine fails it and an emission failure certificate is issued which must be kept for 90 days.
 
I believe the exhaust gas analyser is digitally connected to the MOT system now (but could be wrong).

They can't Fudge readings.

Take it for an "Italian Tune-up" (good roasting down the bypass).
 
I believe the exhaust gas analyser is digitally connected to the MOT system now (but could be wrong).
At the moment, only new brake testers from October last year (or secondhand ones installed after last October as well as stations that undergo a change of ownership). Decelerometers from Feb this year. Emission testers will soon follow along with number plate recognition software so no more writing out tickets for friends who keep British registered cars in Spain for example............
 
Correct. Or, test again without the engine speed connected and hold it at whatever revs it passes at. Some testers have been known to stick another car on and take the readings from that. Not me of course......

Tester has no say in it - the machine fails it and an emission failure certificate is issued which must be kept for 90 days.
I used to put a kink in the gas sampling pipe - or tread on it years ago when you had to rev the nuts off diesels 7 times :eek: :mrgreen:
If you wanted a 'donor car :whistle:' Honda was the make of choice.....
I don't think that would work with todays gear though.
John :)
 
I probably along with many others remember when a car didn't need an engine in it to pass an MOT ...... or I am making that up?
 
I think you've always needed an engine. Even the very first tests needed a brake check and the car had to be driveable to carry that out - no roller brake testers in those days.
 
I guess it's a bit like being "a little bit pregnant" or breaking the law "in a certain way"?

There has to be a cut off. I'd thrash the car for a while and retest
 
Take it for an "Italian Tune-up" (good roasting down the bypass).

Did exactly that with my van on Friday. Some years earlier, the van had only just scraped through the emissions and the tester's advice to me was to take it for a run immediately before the MOT and "drive it like you stole it". I now do this every year giving it a good rant in low gears and watch the black soot coming out the back in my mirror. Then a blast down the M5 and after all this, no more black soot under hard acceleration.

Still seems like sacrilidge though, as I hate thrashing an engine and normally drive like Reggie Molehusband. Suppose once a year won't hurt anything. Anyway, it worked as it's the 13th MOT under my ownership, and 13th first time pass. Amazing for an old Fiat, but I do look after it and strongly believe in preventive maintenance.
 
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Good going, 13 out of 13 .

Yes, was bracing myself for rear suspension arm bushes as it's started clonking slightly from the rear recently. These Scudos tend to need them at this age and mileage, but no mention so they're obviously not that bad.

Also, always take my cars for an MOT on a Friday afternoon as i reckon the testers will be in a better frame of mind. Wouldn't ever book it in for first thing on a Monday morning.
 
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