What gases are cars tested for in a MOT?

Answer 1

Two contents of the exhaust are of primary concern.

The first is CO (Carbon-Monoxide), which is a colourless, odourless gas that is highly toxic, even in low concentrations.

The second is not a gas as such but a class of gasses called "Hydro-carbons," which are basically unburned fuel. These are a major contributor to Photo-chemical Smog, and are also toxic.

Both the above are generally produced when the engine fails to fully burn all the fuel, and are generally a sign that the engine is not running correctly.

The third gas is Oxygen, as in theory there should be no oxygen in the exhaust, as it will have all be used up burning the fuel. This is not so important as oxygen in the exhaust is not in it self harmful, just indicative of a fault.

The exact reading (expressed as a percentage for hydrocarbons and oxygen and as PPM <Parts-Per-Million> for hydrocarbons) varies depending on the age the car was first used.

For very old cars (Pre war) there is no test as such, all that is required is that the vehicle is not "Visibly Excessively smoking" as when the car was made the technology at the time was not capable of making the car meet any tougher test than this, nor was the danger pollutants as well understood as it is today, and also there were far fewer cars- the number of cars on he road then probably produced less pollution (in total, all added up) than todays cleaner cars- so it did not matter as much that they be clean.

The above applies to petrol vehicles, diesel vehicles are tested in a way not directly involving gasses.