Fault Criteria, Catalyst Damaging Misfire

Misfiring could be due to the fuel-air mixture that should have been burnt in the engine being burnt in the three way catalytic converter instead.

The number of misfires that cause an overheated and consequently damaged three way catalytic converter depends on engine load and speed.

An engine load and speed dependent matrix in the engine control module's memory specifies how many misfires per 200 engine revolutions will damage the three way catalytic converter.

When the conditions for diagnosis are fulfilled, the combustion signals from the ignition discharge module are evaluated. The fault criteria are satisfied when the number of misfires per 200 engine revolutions exceeds the matrix value.

Conditions For Diagnosis
^ Engine running
^ Ignition timing synchronized
^ No fuel-shut-off