Diagnosing a catalytic converter fault

Diagnosing a catalytic converter fault

Technical training coordinator for EEC, Stuart Still, comes to the aid of a customer faced with the prospect of a catalytic converter whose nine lives were up. A killer coil turned out to be the main culprit.

A great deal of my time is spent fault finding and diagnosing emission faults, some of which potentially could result in a damaged catalytic converter or DPF, even though the faulty component was unrelated and would not normally be associated with a damaged cat.

One of the most effective emissions diagnostic tools is the 4-gas analyser; understanding and reading the four gas values correctly will enable you to locate and rectify many possible faults. Some of these failed components, especially ignition related ones, could have inadvertently damaged the catalytic converter, even though you would not initially have identified the link, but are often interpreted as an ineffective cat.

It is extremely important that when a faulty component has been located, repaired, or replaced, you check that damage has not been caused to any other related component.

To enforce this scenario I was recently contacted by a customer who explained that the Saab 9.5 he was working on had failed its MOT due to the emissions, he diagnosed the problem was the cat, as it was way below its threshold, the 4 gas values backed this up with readings of:

CO: 1.2 per cent

CO2: 14.3 per cent


O2: 1.2 per cent

Lambda: 1.04

These are typical values of a damaged cat that is not working efficiently.

He could not understand this as the car belonged to one of his loyal customers, which is serviced regularly with him, he had also replaced the catalytic converter just five months previously, this was due to external impact damage, which caused the monolith to collapse.

He was so convinced it was the cat, that he agreed for me to visit him the following day to assist him with his diagnosis, this would give me an opportunity to give the car a once over before he replaced the cat.

It is extremely unusual for a cat to fail within a two year period, the ones that do fail are usually associated with an emissions failure or a related faulty component, e.g. EGR valve, air mass meter, injectors etc.

Arrangements were made for me to visit the garage and examine the Saab, the first thing I checked was the coil pack, knowing that they are a constant problem on Saab 9.5s; my concerns were correct, as a new coil pack had been fitted, the invoice in the service book, showed that it had been fitted three months ago, so backing up my theory. My customer agreed that this could have been the source of the problem. We replaced the damaged cat, followed up by a 4 gas emissions test, the results were perfect:

CO: <0.20

CO2: > 13.5

HC: <15PPM

O2: < 0.20

Lambda: 1

I explained that the damaged cat was a direct result of the failed coil pack, and that it was good practice to carry out a 4 gas emissions test directly after replacing any failed ignition component, especially coil packs.

Recently, I was talking to two major auto electrical manufacturers who explained the UK market for coil pack is immense. There is an opportunity for us all, to be more effective when repairing customers’ vehicles, by reporting possible damaged related component as a consequent of the original problem.

Carrying out this practice will not only reduce catalytic converter problems, it will also help with continued customer satisfaction by being proactive, thorough and conscientious.

I am convinced that failed coil packs are resulting in a large number of cat warranties, to help reduce this industry problem we have compiled a warning flyer advising that an emissions test is done after replacing a coil pack or ignition related components.

Want to know more? For more information click here.

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