As in the cases where similar conditions were found to exist in the 3.1L, 3.4L, and the 3.8L engines, it becomes apparent that if the condition is found in the 3.5L and/or the 3.9L engines and is not corrected then any replacement converter will suffer the same contamination and deactivation fate as the OEM unit that is being replaced.
The unrepaired coolant contamination condition will result in a P0420 DTC very soon after installation of a replacement converter.
The affected vehicles are at an age and mileage where a higher than normal frequency of MIL on conditions are being experienced by vehicle owners. The incidence of improper diagnosis and replacement only of the catalytic converter could very well lead to a higher than normal “warranty claim” incidence (P0420) for those converter part numbers designed for replacement on the affected vehicles.
Recently we have begun to field Tech Line calls from Police and Taxi repair facilities with high mileage and extreme service vehicles that are experiencing a MIL on condition with the original converter and a MIL on condition once a replacement converter has been installed. We are recommending that facilities use their service provider to access the above mentioned GM TSB in an attempt to help them complete a proper repair and satisfy their customer’s needs.
As always, the technician must check to see if there are or may be other TSB’s available that address other related conditions with these same GM engines.
The following GM Bulletin pertains to several possible coolant leaks on this vehicle, one of which is through the cylinder head gasket.
As most technicians are aware, Dexcool (the pink colored GM antifreeze) does not contain silicates and therefore does not pose the same threat to oxygen sensors and catalyst substrates as its familiar green counterpart does. While these vehicles were designed to use Dexcool, many consumers have decided (against the recommendation of GM) to change to the less-expensive green anti-freeze which does contain damaging silicates which would be hazardous to a catalyst if consumed through a cylinder head gasket.
Regardless of the coolant being used, these leaks can still be the root cause of catalyst damage due to improper engine coolant temperature.
First, a severely overheated engine will consume more oil due to wear and exaggerated tolerances due to heat and friction. This type of damage will usually not remedy itself.
Second, when the coolant temperature sensor is not immersed in engine coolant due to a low fluid level, it generally will not report the high temperatures in the engine itself, but rather, report cooler temperatures to the PCM due to the lack of fluid necessary to transfer this thermal information to the sensing bulb of the sensor. This can cause the computer to switch out of closed loop and ultimately richen the air-fuel ratio since the PCM believes the engine is running below normal operating temperature.
The excess fuel in the richer air-fuel mixture will in turn damage the catalytic converter substrate very quickly.