On my many visits to garages (independents, MOT centres, national chain or main dealers) I have been surprised how little information is retrieved by the technician after completing an emissions test using a Four Gas Analyser. 

The usual statement to the customer is: “Your car has passed or failed the MOT on the emissions.”

The problem is that the MOT only requires a report on the CO, HC plus a lambda reading. Therefore if the reading of CO was .3, HC 123ppm, and a lambda reading of .99 at 2000 rpm, that would constitute a fail. You could conclude that the catalytic converter needs replacing because it is not working effectively, as the HC and CO are too high. This situation is usually resolved by fitting a new Cat as this would reduce the HC and CO, enabling the vehicle to pass the emissions section of the MOT.

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 Cat is
damaged but locating and solving these problems are fairly simple and straightforward.

You need a report from your 4 Gas Analyser showing the 4 gas values of CO, CO2, HC, O2, and a lambda reading. The two values that are missing from the example test earlier are CO2 at 14.1 and O2 at 1.21, which are extremely important to interpret your diagnosis.

The O2 value should be less than .2. As you can see there is a high reading of O2 – this is a result of a small hole, damaged gasket or a failed seal between or around the lambda sensor and cylinder head.

As a result the lambda sensor is picking up a high volume of O2, this information is fed back to the ECU, assuming that the engine is running lean and it will adjust the mixture making it richer. This will result in an excess of HC, causing damage to the catalytic converter, and a possible failure of the converter within its warranty period.

I would recommend that you print and analyse a four gas report when carrying out an MOT emissions test, and repeat the process after fitting a new catalytic converter as it is an ideal opportunity to locate and report a number of emission faults.

These could include over-fuelling, air filter, exhaust, catalytic converter and ignition problems. As you know, the catalytic converter is working at 100% therefore the problems must be elsewhere.

The ideal values are: CO: <.2%, CO2: >13.5%, O2: <.2%, HC: <15PPM, Lambda: Btw.99 & 1.01 @ 2000 rpm EEC have embarked on a full emissions training program to include catalytic converters, lambda sensors, exhaust systems, and how to read and understand 4 gas analyser values. These training programs can be arranged through the day or in the evening.