One or both secondary air injection check valves may become restricted causing poor air flow resulting in illumination of the Check Engine Light. It is often recommend to replace both check valves even if a fault code is set for only one bank of cylinders. Replacing only one valve may result in a return trip to the repair shop.
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Repairpal verified 2004 Pontiac GTO problems
A coolant leak may develop from the water pump. This can result in an engine overheating condition. A leaking water pump will require replacement.