DTC ECM-3503

Condition
Diagnostic trouble code (DTC) ECM-3503 is stored if the control module registers that the engine is misfiring to such an extent that there is a danger of damage to the three-way catalytic converter (TWC) (the extent varies with load and engine speed (RPM)).

ECM-3503 is always stored together with a diagnostic trouble code (DTC) which indicates the cylinder which is misfiring (ECM-35X3).

Substitute value
- three-way catalytic converter (TWC) diagnosis shut off
- leakage diagnosis shut off
- rear heated oxygen sensor control shut off.

Possible source
- defective spark plugs
- blocked/leaking injectors
- uneven compression
- air leakage
- faulty fuel pressure
- fuel stoppage
- water in spark plug wells
- contaminated or faulty fuel
- repeated cold starting where the engine coolant temperature (ECT) has not reached normal operating temperature between starts.

Fault symptom[s]
- engine misfires
- engine difficult to start
- poor performance
- uneven idle
- the engine does not run on all cylinders.