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Emission Faults (1)

One of the most frequent questions we are asked in the technical department concerns MOT emission failure faults and their diagnosis. So for this and the next technical tips we will attempt to give an overview of the most common faults you may come across. With this issue we will start by analysing the gasses from a typical petrol engine exhaust. As we will be talking in general terms we will attempt to avoid specific emission readings and thus this should be taken as a guide not a manual.

With any emission related fault you should always start by going back to basics and checking the simple, often over looked parts, first. Plugs, leads, ignition system patterns & voltages, filters, P.C.Vís, carbon filters/canisters (see future tech tips) etc. All too often, a simple plug misfire, or even plug gap, fault caused a high HC or low CO fault that has been the subject of many hours of head scratching.

In view of the latest interest that the trading standards are taking with the motor vehicle aftermarket, donít take the customers word for it, check these items yourself, replace them if youíre unsure, get to the simple faults first time around.

The engine condition, mileage and the use to which the vehicle has been put should also be taken into consideration. The most troublesome vehicles tend to be big-engine low mileage, shopping cars, which never get to full operating temperature. These can and will display all types of elusive problems not only idle/emission related. Renewing emission related parts with this type of vehicle might temporarily reduce emissions to below the legal limit, but the fix will only be temporary and the customer should, at least, be made aware of this.

The minimum test equipment required to diagnose emission-related faults is an accurate four-gas tester, the type used for MOTís is fine. Ensure that it is regularly serviced / calibrated. Right, now weíre up the first rung of the ladder lets look at what the four gasses/readings mean.

Carbon Dioxide (CO2)

Carbon dioxide is produced as a result of the combustion process. The more complete the combustion, the higher the level of CO2 produced.

Too much fuel or too little oxygen will result in incomplete burning and thus a lower than optimum CO2 reading. The, often quoted, figure for complete combustion is 14.7 parts of air to 1 part of fuel, this is by weight not volume i.e. 14.7 Kg of air to 1 Kg of fuel. This is the Stoichiometry (chemically) calculated correct ratio, but it should be noted that this might vary slightly with different fuels. There are running conditions for engines for which this target figure would not be ideal. Acceleration, for example needs a richer than ideal mixture, light throttle cruise mixture is often weaker to improve fuel consumption. These conditions will lower the CO2 levels and will result in an increase of other pollutants. CO2 is not regarded as a harmful gas. The CO2 level at this 14.7:1 ratio is normally around 13.5%

These other pollutants are the ones that are usually measured on todayís exhaust gas testers. Carbon Dioxide readings, although very popular on older machines, rarely feature on modern gas testers.

Carbon Monoxide (CO)

Carbon monoxide is produced as result of incomplete combustion as a result of too much fuel or too little oxygen. Hence the level of CO is strongly dependent upon the air: fuel ratio. Too much fuel results in a high CO reading. Incorrect or poor mixture distribution in the cylinder can also cause a high CO reading. It also interesting to note that an excessively lean mixture can cause an increased CO reading, this is due to low combustion temperatures preventing the post combustion oxidation of CO to CO2 in the exhaust system. The CO level at the 14.7:1 air fuel ratio is normally just under 1%.

CO is probably the most damaging of exhaust pollutants emitted. It is a colourless, odourless gas which when inhaled prevents the blood from carrying oxygen around the body. A concentration of just 0.3% CO in the air that we breathe would cause death in 30 minutes. When you are testing/working on a vehicle try using the gas tester sniffer to measure the CO level around the rear of the car. Now start worrying about your health.


Hydrocarbons in the exhaust system are once again a result of incomplete combustion. It is a measure of unburnt or partially unburnt fuel. It could be translated as a measure of the efficiency of combustion. Under ideal conditions, which are not currently technically possible, this hydrogen combines with the atmospheric oxygen and is emitted as water or water vapour and the carbon combines with the oxygen to form harmless carbon dioxide.

Increased HC levels are not purely dependent on air: fuel ratios, as the CO level, but many other factors, which have an influence, should be taken into consideration. The ignition system plays a big part in HC levels; a modern trend in ignition timing is to advance the ignition to increase engine output. Advancing ignition timing results in lower exhaust gas temperatures, which reduce the after-burning effect of HCís. Retarding the ignition timing reduces the amount of time available to completely burn the fuel and also results in increased HC levels.

Other factors, which increase HC levels, include; ignition inefficiencies i.e. Misfires, low spark voltage or duration, mechanical condition i.e. Excessive blow-by or low compressions, sticking valve, engine breather faults. Any change from standard spec. I.e. High lift/duration cams, big carburettors, a blocked or restricted exhaust, will all cause an increase in HC levels.

The air: fuel ratio for the lowest HC reading is slightly weaker than 14.7:1 at around 15:1. The actual HC reading at this level varies car to car.

Exhaust gasses contain varied hydrocarbons; namely; saturated, unsaturated and aromatic. Saturated hydrocarbons are odourless, and cause drowsiness, headaches, eye and nose irritations. Unsaturated hydrocarbons are a sweet smelling gas which as well as causing the same physical symptoms as above they are an essential factor in the formation of smog and ozone. Aromatic hydrocarbons have a characteristic pungent smell and are in part carcinogenic (cancer forming) and part nerve toxin with a slight narcotic effect even at low concentrations.

Oxygen (O2)

Oxygen in the exhaust is once again the result of incomplete combustion. In ideal conditions, as above, there should be no unburned oxygen in the exhaust, all unburned oxygen should combine with unburned hydrocarbons to form carbon dioxide (CO2). In practice this is not yet technically achievable.

An over-rich mixture will result in a low or zero oxygen emission; an excessively weak mixture will result in a high oxygen reading.

The same factors that affected the HC readings will also affect the O2 readings. I.e. low combustion efficiency; engine breather faults etc. but also intake air leaks allowing un-metered air into the engine will cause a high O2 reading. A post combustion air leak i.e. exhaust air leak, will cause a high O2 reading but will dilute the other (CO2, Co and HC) readings and cause a lower than normal reading.

At the correct 14.7:1 air: fuel ratio you should normally expect an O2 reading of less than 0.5%. Oxygen, on itís own, is not at all harmful.

Oxides of Nitrogen (NOX)

During the combustion process the nitrogen in the air drawn into the engine combines with oxygen to form nitrogen monoxide (NO) and nitrogen dioxide (NO2), which are generally grouped together as oxides of nitrogen (NOX).

Oxides of nitrogen are a feature of the combustion process and generally increase in line with combustion temperatures. High compression ratios, slightly weak mixtures, advanced ignition timing all increase the NO2 emissions. A reduction of NO2 emissions can be achieved with lower compression ratios, a slightly rich mixture and retarded ignition timing. If you have been paying attention you might realise that these are completely the opposite of that required reducing other emissions. The catalytic converter is the biggest advance in reducing NO2 emissions, Exhaust gas re-circulation valves also reduce the NO2 emissions, and this is due to a reduction of the combustion temperature.

The peak NO2 emissions usually occur at slightly weaker than the ideal 14.7:1 air: fuel ratio.

NO is a colourless gas, it is considered a severe hemotoxin which combines with the blood to cause a rapid paralysis. NO2 is a reddish brown gas with a sharp pungent odour. It causes respiratory irritation and damage to lung tissues. Oxides of nitrogen also combine with unsaturated hydrocarbons to form smog and ozone.

There are other gasses/pollutants in exhaust gasses, including lead compounds, but they are in even smaller percentages and not effected by tuning/maintenance settings.

Engine Design

Engine design plays a major part in exhaust emission. The most critical factors are; compression ratios, combustion chamber shape, valve timing and intake/exhaust design. Manufactures have placed great importance on reducing emissions to meet both political and legal requirements. It must be remembered that every engine represents a compromise between power, economy, noise and emissions. Various devices have been introduced over the years to improve this compromise and reduce emissions. Catalytic converters obviously have played a major part in reducing all emissions.

We will be looking at the topic of reduction of emissions and expected emission level in the next technical tips. That is, after reading the above, you are still interested in fixing cars, after all I did once meet a mechanic who made it past his 30th birthday.


Emission faults (2)