The Power of Four

The Power of Four

European Exhaust and Catalyst’s (EEC) Stuart Still talks through a recent case, which he feels demonstrates the importance of the four gas test when identifying an emission-based problem.

Last week I received a call from Steve Johnson, a Citroen specialist in Grimsby, and he told me that he had a 2012 Peugeot 208 in for a service, and that the engine management light was on, with fault code PO420 on display.

Steve had checked the cat and found that the internal core had collapsed. I suggested that he fit a new catalytic converter, then carry out a four gas emissions test and send me the four values. This way I could diagnose where the problem was that had damaged the catalytic converter. This was duly done, and the emissions reading was: CO 0.50, CO2 12.4, HC 450PPM, O2 0.4 and the lambda was at 1.12, showing that there was an issue within the ignition process due to a very high HC value – e.g. faulty spark plugs, injector etc.

The injectors were found to be at fault, so these were replaced, along with the spark plugs. A second four gas test was carried out with a reading of: CO 0.11, CO2 8.3, HC 68PPM, O2 0.4 and a lambda of 1.11, showing that the major problem had been rectified. Although these readings would pass an MOT emissions test, I explained that there was still an underlying issue with these readings.

The readings pointed to a back pressure issue, which is usually an exhaust problem, such as a damaged silence e.g. a hole, degraded/rust which would result in a low back pressure, or a blocked silencer, where the internals had collapsed, stopping a free flow of gasses through the exhaust system. Both high and low back pressure would be in the same four gas test result range.

The MOT’s emissions test of CO <0.2 and an HC value of <200 PPM does not help diagnose an emissions problem; you need to use the four gasses, as the MOT values are quite wide. For example, the HC pass is less than 200PPM – that is unburnt fuel at the tail pipe!

If the HC exceeds 35PPM, this is high enough to damage the catalytic converter. A new exhaust system was fitted, then the third four gas emissions test was carried out with a perfect reading: CO0.01, CO2 17.1, HC 3PPM and O2 0.02 and a lambda of 1.

Steve’s Crypton four gas analyser has been mothballed for a number of years but now he has brought it back into service as he can see the benefits. EEC has embarked on a full emissions training program to include catalytic converters, lambda sensors, exhaust systems, DPFs and how to read and understand a four gas analyser value report.

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

For more information, please contact: Duncan Richards (, Ben Kendrick (, Stuart Still (, or 

Related posts