This is probably the one area of the MOT test that generates the most phone calls to us for parts and technical advice.
Please note that this is our own interpretation of current VOSA MOT regulations with regard to emissions testing and should not be considered as gospel. The only relevant interpretation is that of the tester examining your vehicle, so it would make sense to validate any of the following information with your local testing station before making any decisions or purchasing parts.
Using a gas analyser whilst the engine is running, the current MOT emissions test measures the CO (carbon monoxide), HC (hydrocarbon) and the O2 (oxygen) content of the exhaust gas. The O2 test is more commonly known as the lambda test. It is so called because a lambda value of 1 is the name given to the ideal air/fuel ratio, also known as the stoichiometric ratio, of 14.7 to 1
With regard to the FTO however, there are potentially two levels of MOT emissions test, with differing requirements, depending upon the date of first registration. That is to say, the date of first registration in Japan and not the UK.
Date of first registration before 31/07/1995. Theoretically a vehicle of this age should be subject to what is known as a ‘non-cat’ test where the maximum CO level is 3.5% and maximum HC level is 1200ppm (parts per million). There is no O2 test requirement, as this test pre-supposes that the vehicle is not equipped with a catalytic converter or the accompanying lambda sensor. This test is carried out at ‘natural idle’ only.
Date of first registration 01/08/1995 to 31/08/2002. A vehicle of this age will be subjected to a ‘cat test’ with maxima of CO 0.30% and HC 200ppm, plus a lambda value of 0.97 to 1.03. This test is carried out both at ‘fast idle ‘ 2500 to 3000 rpm, and natural idle, normally considered to be between 450 and 1500 rpm. Realistically both 4 and 6 cylinder versions of the FTO should idle at around 800rpm when up to normal temperature.
Areas of MOT failure
All FTOs, 4 cylinder or V6 are equipped with a cat (catalytic converter) as original equipment and any model, if it is in good condition, would fly through the earlier test and should be able to pass the later more stringent emissions test quite comfortably. An FTO failing the earlier ‘non-cat’ test would indicate some pretty serious engine maladies and it would almost certainly be running like a bag of hammers and/or consuming prodigious amounts of fuel. Failure of the later test however is not uncommon, but before we get into the technicalities let’s go back to basics.
Firstly, an engine and exhaust system that is not up to optimum temperature is much more likely to fail the emissions test than a one that is mad hot. It is imperative that during the emissions test, the catalytic converter is as hot as possible. Similarly an infrequently or badly serviced FTO will be far more likely to fail than it’s properly serviced counterpart. Do yourself a favour and get the car serviced before the MOT. Even without considering expensive component failure such as O2 sensors and cats, your humble ropey spark plugs, air filters that pose a health hazard and even ancient engine oil will all reduce the chances of passing first time. From our experience, paricularly with high mileage vehicles, a dose of decent quality fuel system cleaner (eg. Wurth or DAE) prior to the test can help in marginal cases, but this should be considered an addition to and not as a substitute for correct servicing.
It is far too complex a subject to go into all of the combinations that could lead to emissions test failure, but to summarise from our experience, the problems are likely to be excessive CO and/or HC readings indicating an excessively rich mixture. This can be as a result of poor servicing mentioned above or component failure, but areas to investigate would be lambda sensor, exhaust system and even engine coolant temperature sensor. Ropey signals from sensors to the engine ECU can play havoc with the mixture and the resultant emission readings.
Incidentally, don’t assume that a lambda fail on the MOT test is automatically a sensor fault. Exhaust leaks, both upstream and downstream from the sensor can materially affect O2 readings, which in turn could lead to an unacceptable lambda value from an otherwise good sensor.
Similarly do not assume that high CO levels always mean a faulty catalytic converter. A popular if not particularly eco-friendly modification by some FTO owners in search of increased power is to remove the standard catalytic converter. We have seen emission readings as low as 0.60% CO from a de-catted but otherwise healthy FTO GPX; so you can see that, even in this state the car would pass the CO requirements of earlier test quite comfortably and wouldn’t be a million miles away from the later test. Following this vein of logic, CO readings of 2 or 3 % almost certainly indicates underlying problems other than just a b*ggered cat.
Boring but important
Although we have been very careful in the compilation of the advice pages of our website and the content is thoroughly checked by our panel of specialists, we cannot be held responsible for any loss or damage caused if you decide to follow the advice contained herein. Please bear in mind that a job which may well be a piece of cake on a vehicle lift could easily turn into a total nightmare when lying on your back in the front street in the regulation puddle. Don't tackle any work unless you are confident that you fully understand the complexity of the job you are undertaking. Make sure that you have the correct tools to hand, always wear the appropriate protection and never, ever work under an unsupported vehicle. If you are not entirely confident that you can complete the job, then it should be entrusted to a specialist.
If you feel that there are any errors on any aspect of this or other pages of our website, or if you simply wish to comment, please call Malcolm on 0191 586 7724 or send us an email