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.
Check Engine Light Due to Restricted Secondary Air Injection Check Valve on GMC Yukon Denali
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A Loose or worn gas cap may cause Check Engine Light to illuminate.
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