According to recent Vehicle Operator and Standard Agency (VOSA) figures on average 9.7% of petrol cars fail to meet MOT emissions standards, compared to 3.9% of diesels.
This is mainly because the MOT emissions test is far more stringent for petrol cars than diesels; petrol cars are checked for the levels of various gasses and hydrocarbons in the exhaust emissions, while diesels are only tested for smoke levels.
The only vehicles that don’t have an emissions test as part of the MOT are:
- vehicles with less than 4 wheels
- vehicles with 2-stroke engines
- hybrid vehicles
- quadricycles
Also if you have a classic car (petrol vehicles used before August 1975 and diesel vehicles used before August 1979) then only a visual check is made, and you are only likely to fail if exhaust smoke is “excessive” in the opinion of the tester.
Your vehicle can also be tested during a roadside check if you’re unlucky enough to be selected for one of these, which are usually aimed at vans and light commercial vehicles rather than passenger cars. If it fails the check you will have 10 days to fix the problem or face prosecution. If it’s seriously in breach of emission standards you will forced to stop using the vehicle immediately, and could also be prosecuted.
The gases checked during the MOT emissions test are:
- Carbon Dioxide (CO2) – this is an indication of the completeness of the burn of the fuel in the engine block. The more complete the burn, the higher the CO2 reading.
- Carbon Monoxide (CO) – this is an indication of the air-to-fuel ratio; too much fuel in the exhaust gases results in a high CO reading. This can be caused by a faulty catalytic converter.
- Hydrocarbons (HC) – indicates unburned fuel exiting the exhaust; the more incomplete the combustion, the higher the HC reading. This is often caused by worn valve seats.
- Oxygen (O2) – another indication of incomplete combustion, or possibly an air leak in the induction system, or an incorrect air-to-fuel ratio.
- Oxides of Nitrogen (NO2) – could variously be incorrect combustion temperature, low compression pressure, retarded ignition timing or a too-rich mixture.
Common Causes
The emission control system is largely responsible for the “breathing” of the engine, which needs to maintain the correct mixture of fuel and air in order to burn the fuel correctly (and completely) inside the cylinders.
If emissions are high them this is often due to a reduction in air flow, which in turn could be one of many problems from a simple blocked air filter to a fault in the Engine Management Unit (the car’s “electronic brain”).
Engine damage is a very common cause of poor exhaust emissions, especially if the damage causes excessive oil consumption. This can be caused by a blown cylinder head gasket, worn cylinder rings or damaged pistons.
There’s also the dreaded catalytic converter – if this is not working effectively then it can cause an increase in dangerous emissions to be released from your vehicle.
This might help…
If you have concerns that your car may fail its MOT due to emissions you could try Cataclean, a fuel additive which is claimed to reduce emissions by up to 60% by flushing the system and removing any pollutant build-up in the catalytic converter. At under £20 from car accessory shops it may help your car get through the MOT, however the cause of the raised emissions will still need to be looked at.