Emission Related Problems

  • MOT failure?
  • Using too much fuel?
  • Check Engine Light On?
  • Cat or Catalytic Converter Failure?


Check Engine Light

Most people have no idea what the check engine light is for or why it comes on. The answer is quite simple really if anything is not functioning properly that may cause an increase in vehicle emissions, the check engine light will come on to alert the driver that there is a problem.

Some cars have more sophisticated engine management systems than others, so when the check engine light comes on the cars performance may be further restricted by going in to "Limp Mode" Limp mode frequently causes the engine to be difficult to start and normally restricts the speed that the car can driven at, sometimes this can be 30 or even 50 miles per hour. The real idea behind this is to force you to get the problem fixed, however if there is a problem that causes excess fuel to leave the engine unburned due to say a misfire, it also protects the catalytic converter or Cat, Cats are easily damaged by prolonged exposure to excessive fuel passing through the exhaust system. So don't put off getting your car fixed if the check engine light comes on or you may need to replace the cat as well as fix the actual problem.


Catalytic Converter

We see so many cars that have had the catalytic converter replaced, sometimes they are changed each year due to MOT failure! catalytic converters do have a limited lifetime but are frequently changed unnecessarily. The problem is that exhaust emissions are checked at the end of the exhaust, this only tells you what is coming out of the exhaust not what is going into it at the other end - from the engine. The catalytic converter can only convert a limited amount of excess fuel and undesirable chemicals, if the engine is outputting more than the cat can cope with you will still have high emissions, changing the catalytic converter wont change that (it may for a short period, until it gets destroyed by the problem)

So before changing your catalytic converter ensure that it has actually been tested, many garages have no idea how this is done and change it just to eliminate the catalytic converter as a cause of the problem. Fortunately all cars with catalytic converters have a lambda or O2 sensor so it is easy for anyone with a serial code reader to get accurate readings of the gasses before they enter the catalytic converter and with a gas analyzer a reading of the gasses that come out of the catalytic converter, it does not take much work to see from those two results if the catalytic converter is working properly or not.

The following symptoms NORMALLY indicate that there is a fault elsewhere that is causing a problem

  • Excess Fuel Consumption
  • Strong Smell from under the car
  • Water coming out of the exhaust

The smell normally indicates that the catalytic converter is being overworked due to too rich a mixture, lots of water coming out of the exhaust shows the catalytic converter is working, it is again being overworked and one of the products that excess fuel going into the catalytic converter produces is water, hence all that extra water (there will often be some water especially if the car is frequently used for short trips, but it should go after a few minutes at motorway speeds.

If your catalytic converter is definitely not working make sure you repair the cause of the problem before it is changed, otherwise you will be changing it again very soon!

We have developed our own method of testing catalytic converters that has over a considerable period of time achieved a 100% success rate, so you can rest assured that we will only recommend catalytic converter replacement if it is actually not working, from our experience over 90% of catalytic converters are replaced either unnecessarily or without fixing the cause of the failure.