That is because the cat is functioning most of the time but if you look at the readings you will probably see it is right on the limit. Mine usually seems to come on after a spirited run when slowing down with no throttle. In this situation little or no petrol is being injected and the engine is working only as a pump to pump "cold" air through to the cat. This would cool the cat below operating temperature and as soon as throttle is reapplied the cat would show a malfunction. Other circumstances may create the same problem. The pre-cat sonda lambda can also be replaced with a faster sensor but I have reliably been informed that this does not seem to make much difference.

The ECU is programed to ignore the apparent malfunction for a certain period after the car has been started to allow the cat to reach operating temperature.

If you need to take the car for an emissions test as part of the registration renewal process make sure the car is good and hot before they run the test, particularly if you have a sports exhaust system fitted. Many sports exhaust systems have a 200 cell cat fitted which may be high flow but marginal in catalytic function. Now many sports systems are being fitted with 400 cell cats which function better. Mine has been changed to a 400 cell cat but it has not entirely resolved the problem.

Changing the sensors may help but it is probable that the system is right on the borderline in any case.

Last edited by Gambalunga; 20/12/13 08:06 AM.
Peter

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