MOT gas test - andy sampson
Hi Chaps!!,

Can anyone explain to me the MOT gas test and how the results can used, i.e. High CO must be as a result of poor cumbustion, what about hydro-carbons???,

Can someone go through this and explain each gas test and the possible reasons (generally) for high/low readings.

Many thanks (yet again)

Re: MOT gas test - mike harvey
this can be a quite complex subject, but here goes. CO, Carbon Monoxide, is produced as a product of combustion when the fuel air ratio is too rich. A correct ratio, 14.7:1 by mass, would allow extra oxygen into the process, and the extra oxygen will combine with the CO giving CO2. This reading is usually ignored on a gas analyser, though you will be taxed on it soon as it is a greenhouse gas. HC, or hydrocarbons are unburned fuel. This results from things such as incomplete combustion because of perhaps a misfire, incorrect ignition timing or too richer mixture- too much fuel and not enough oxygen. You can see that with a high HC reading, often a high CO reading will accompany it. Adjust the mixture, and the CO should fall too. It is important when setting up the CO on servicing, it is important that all other components are in good condition and adjusted correctly. The fuel is the last thing to adjust. NOX is oxides of nitrogen, caused by high combustion temperatures of modern cars. Exhaust gas recirculation can be used to cool down the charge and reduce the emission to an acceptable level. A CAT will help too. Oxygen is measured to see if the CAT is working effectively, and the fuiel air ratio is correct to start with.
It's not always that straight forward though, as many a mechanic will tell you.
Hope that's helped.
Re: MOT gas test - Andrew Hamilton
I had problems with high CO in an old metro(No catalyst) I purchased a Gunsons Gastester from Halfords for £100. Results same as MOT. Easy to use once you understand its operational principles. Worth every penny!

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