It isn't just our type of cars that have wider limits for the lambda readings. Quite a few of the newer BMW's have a much higher limit due to their design. Watched my local tester selecting various menus to allow a Late 3 series to be tested.
The DVSA document that has information that sort of explains the reasons on pages 4- 6. To me it seems a bit daft but my reading of it is that the lambda result should be between 0.97 & 1.03 but if it fails then check the list and if the vehicle/engine is listed then do another test that has a wider tolerance. It sort of means that 0.97-1.03 is a default pass but there are exceptions (possibly all K Series Caterhams). The test kit must use the default rather than automatically look up the the wider band based on the VIN.
"The emissions limits to be met are specified for both the fast and normal idle tests.
At fast idle, CO must be at or less than 0.2%, HC at or less than 200 parts per million
(ppm), and the lambda value(1) must be between 0.97 and 1.03. At normal idle, CO
must be at or less than 0.3%.
Where these limits are not met, a check will be made to identify vehicles (e.g. those
without a catalyst) which should only be required to meet the non-catalyst emissions
check requirements. If the vehicle does not fall into this category then the analyser
will automatically schedule an extended emissions test using the values quoted in
Table 1 or Table 2 and the listings in the Annex in this booklet.
Extended emissions test
As indicated above, an extended emissions test will be applied where the limits are
not met under the basic emissions test and where the vehicle has been identified as
needing to meet the “catalyst” test requirements. These will include petrol-fuelled
cars fitted with advanced emissions control systems (e.g. vehicles with catalytic
converters) first used on or after 1 August 1995, (2) petrol fuelled cars first used on or
after 1 August 1992 which are listed in the Annex to this booklet and large passenger
cars and light goods vehicles first used on or after 1 August 1994 and listed in the
Annex to this booklet.
As in the basic test procedure, emissions are assessed during two separate tests.
The “fast-idle” test in this case involves running the engine at the speed specified in
the Annex to this publication (typically 2500 - 3000 rpm), and the emissions of CO,
HC and Lambda values are checked. A check of CO emissions at ‘normal idle’ speed
is also carried out.
The analyser will require the tester to complete each stage of the test before
proceeding to the next. In addition, where a failure is recorded during the fast idle
test, the analyser will automatically schedule a repeat test.
(2) Passenger cars first used before 1 August 1995 and not mentioned in the Annex to this publication are tested
to non-catalyst standards.
This is to ensure, as far as practicable, that the catalytic converter has reached its
normal operating temperature.
Before starting the emissions test, the Tester will check the engine oil temperature to
ensure that the engine is at the manufacturer’s stated operating temperature. This
will require an oil temperature probe to be inserted into the dipstick tube.
Once the preliminary checks have been carried out and the Tester has identified the
vehicle model in the Annex to this booklet (stored on the emissions analyser
database) the Tester will begin the fast idle emissions test.
This test requires the engine speed to be increased to that specified in the Annex.
Once the engine speed has stabilised between the upper and lower limits, the
emissions will be measured for a continuous 30-second period. At the end of this
period, the analyser will automatically compare the results of the test with those
specified in this booklet. Provided the results are equal to or below the specified
limits, the vehicle will have passed this section of the test and the analyser will
automatically proceed to the normal idle test. As indicated above where a failure is
recorded during the fast idle test, the analyser will automatically schedule a repeat
The “normal idle” test is the same as that described above for the basic emissions
The vehicle must pass both the fast and the normal idle emissions tests to secure a
The test kit must use the default rather than automatically look up the the wider band based on the VIN.
Yes, I suspect that's exactly what happens. I've asked DVSA to clarify. I'll post up their reply.
Maybe it's set up that way because the basic test is simpler (and cheaper?) to apply than the extended test?
It could be done that way to encourage people to get their cars as clean as possible. For example I think my lambda reading is on the high side so I may have a pinhole leak in the system. My 2019 reading was 1.001 but that was with a plastering of Gun Gum in several places after a noticed a cracked weld at the exit of the cat a couple of days before the test. I had it re-welded shortly after the test. This year with no Gun Gum the reading is 1.050.
Or to avoid confusion find an MoT tester who applies common sense..........nudge, nudge, wink, wink.
Well, I asked DVSA, and this is what they said:
On all petrol engine vehicles first used on or after 1 August 1992, the exhaust gas analyser will conduct a basic emissions test (BET). The BET applies default limits of 0.2% CO, 200ppm HC and a lambda of 0.97 – 1.03. The majority of vehicles will pass the BET and that provides a speedy and efficient emissions test process. If a vehicle fails to meet the BET requirements, it goes on to have an emissions test specific to the engine age and type, which commonly means testing to vehicle specific limits, which in this case meant testing to broader Lambda limits.
Whilst this emissions test process requires two emissions tests to be carried out on a small number of vehicles, it provides a quicker process for the vast majority.
This is what we'd assumed, I think. The moral of the tale is to be sure to take the relevant page of the DVSA Emissions Standards with you, just in case!
We have a 1992 Rover Mini that has a 'cat' but is on an SU carb. There is a list of anomalies in the MOT testers appendix for this type of vehicle. The Mini is on this list and requires to pass the 'pre cat' emission test.
My MOT tester states 'if it has a choke then it is pre cat limits'.
Sorry I do not have the location of this appendix to hand but hope this helps a little in understanding the complexities of this legislation.
R.I.P. SLR No.27.
Good to to know the reasons. I had a tester a few years ago say I was just failing but he had another bigger test he would try and it passed with ease. It made little sense at the time but it is now clear. The tester this year seemed a bit stumped until I showed him the full limits and then he offered a very weak explanation of why there is a softer set of limits so it is definitely worth having the paperwork and knowing why. It's probably something the testers have to learn but might not remember the reasons as older engines become rarer.
It would also be worth taking along the two flowcharts (Charts 1 and 2) from Section 8.2.1 of the MOT inspection manual. As you'd expect, they match exactly what the DVSA person said to me (#17).