THERE ARE MANY CAUSES OF CAT FAILURE. IT IS CRITICAL TO DETERMINE THE CAUSE OF MOT TEST FAILURE BEFORE REPLACING THE CAT.

Replacing the CAT before rectifying the original fault jeopardises the service life of the replacement CAT.

• Physical damage

Major knocks to the CAT can result in internal damage to the ceramic monolith. The ceramic brick can powder and blow through to the rest of the exhaust system and may even block the rear silencer. This appears to be the most common cause of CAT problems today. Metal’Cat with metallic monoliths do not suffer from this problem.

• Check

Look for dents and physical damage to the CAT casing and listen for rattling noises.

Ceramic monoliths are fragile. They can crack and powder.

• Lead poisoning

To prevent poisoning of the catalyst, it is necessary to use only unleaded fuel. Lead rapidly forms a thin film which prevents the catalytic process from taking place. One or two tanks of leaded fuel will deactivate the CAT. This may even occur during the inadvertent use of petrol from storage tanks poisoned by lead.

• Check

The level of poisoning, and therefore the efficiency of the catalyst, is directly proportional to the quantity of lead deposited upon the monolith. The presence of lead can only be determined by a sensitive lead detection kit. TIMAX lead detection test kits are extremely sensitive and can identify quantities as small as 5 -10 micrograms of lead in exhaust soot deposits. TIMAX test kits are available in packs of 10 or 50 to suit user requirements.

In both the above cases the CAT should be replaced.

• ECU/Lambda sensor failure

A malfunction in the engine management system or Lambda sensor can result in a MoT emissions test failure.

In these circumstances replacing the CAT will not rectify the fault and would result in premature failure of the replacement CAT.

• Check

Use ECU/Lambda diagnostic equipment to identify the faulty component.

• Leakage

Any change in the air/fuel mixture that is outside the control of the sophisticated engine management system will lead to a reduction in efficiency of the CAT and longer term damage. There may be leaks from broken welds, loose pipe connections and damaged or sub-standard gaskets.

• Check

Make a visual inspection of joints, welds and pipes upstream of the CAT for air leakages.

• Engine misfire or bump starting

Engine misfiring or bump starting can result in unburnt fuel being released into the CAT which then burns and causes sintering or meltdown of the ceramic monolith. However, jump starting using jump leads is permissible.

Check the monolith for damage caused by overheating/misfire. Faulty fuel injection or incorrect ignition timing could be the cause. Ceramic monoliths carry a higher risk of melt-down than Arvin Replacement Products Ltd. metallic monoliths which conduct heat better than ceramics, thus helping to eliminate overheating.

• Ageing

CAT ageing can be caused by fuel additives that are not CAT friendly, loose particles of metal from the engine, and general wear and tear. These result in a progressive reduction in the operating efficiency of the CAT and consequently a reduction in its ability to catalyse all the harmful pollutants.

• Check

Regular vehicle servicing and maintenance, including the checking of the CAT housing for leaks, is essential to ensure optimum performance and longevity.

Failures due to the above reasons will generally occur later in the vehicle’s life and will require a replacement CAT.