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The importance of a 4-gas emissions test

By autotech-nic on March 31, 2020

By Stuart Still, EEC Technical Co-ordinator

One of the most important diagnostic tools in the engine repair sector is the 4-gas analyser, although it is an essential tool to carry out a MOT it is rarely used as a diagnostic tool.

I am often surprised how little information is retrieved by the technician after completing an emissions test using this equipment. The usual statement to the customer is, “Your car has passed/failed the MOT on the emissions”. The problem is that the MOT only requires values of CO, HC plus a lambda reading.

For example, if the reading of CO was 0.35, HC at 123ppm and a lambda reading of 0.99 at 2,500 rpm, this would constitute an emissions failure. You could conclude that the catalytic converter needs replacing because it is not working effectively, as the CO is too high. This situation is often resolved by fitting a new catalytic converter! This could reduce the CO to 0.2, as the new catalytic converter will be working at 100%, for a few weeks. This would enable the vehicle to pass the emissions section of the MOT with the HC at 123 PPM being within the 200 PPM parameters.

This is only a short-term fix as the CO and HC values will reduce for only a short period of time.

There is a problem that needs to be corrected before the new catalytic converter is damaged and locating and solving these problems are fairly simple and straightforward. You need a report from your 4-Gas analyser showing the values of CO, CO2, HC, O2 and a lambda reading.

The two values that are missing from the MOT test are CO2 and O2, these are extremely important in assisting a diagnosis of any emissions fault.

You need to add the two missing values of: CO2 @ 13.6 and O2 @ 0.56 to the MOT test report of CO @ 0.35 and HC @ 123PPM with the lambda reading @ 0.99. It is now possible to diagnose the fault!

The O2 value should be less than 0.2, but as you see it is too high at 0.56! This is a result of a small hole, damaged gasket, failed exhaust manifold gasket, even a lose manifold stud, and could result in air being sucked into the exhaust system before the first lambda sensor.

This information is fed back to the ECU, assuming that the engine is running lean due to the extra O2, it will adjust the fuel mixture to the correct ratio of 1 part petrol to 14.7 parts air (lambda) in turn making the mixture richer, resulting in an excess of HC and CO thus causing damage to the catalytic converter, and a possible failure of the converter within its warranty period.

The air leak needs to be found and repaired immediately, the best way of locating any air leak is by using a Smoke machine which will help locate any air leak.

EEC recommends that you print and analyses a four-gas report when carrying out a MOT emissions test. Repeating the process when fitting a new catalytic converter is an ideal opportunity to locate several emission faults, you know that the catalytic converter is working at 100% therefore the problems must be elsewhere. These could include over-fueling, the air filter, exhaust back pressure, ignition problems etc.

Note: Always check the 4-gas values after replacing a failed coil pack as it can be the cause of a catalytic converter failure.

The ideal values are:

CO<0.2% CO2 >13.5%, O2 <0.2% HC <15PPM Lambda between 0.99 & 1.01 @ 2500 rpm.


At the beginning of the year, I presented an emissions training course for a Midlands free fit/repair chain and, as a result, they purchased a handheld 4-gas analyser for each of their sites – since then, their diagnosis of catalytic converter failure has halved.  

EEC have embarked on a full emissions training program to include catalytic converters, lambda sensors, exhaust systems, DPF’s and how to read and understand a 4-gas analyser value report.

These training programs can be arranged throughout the day or evening.

For more information, please contact Stuart Still at EEC, email



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