Just the Facts: Water Intrusion and Premature Coil Failure (UF303) Coils and Water Don't Mix! On some GM models, water intrusion will cause premature failure of the coil and spark plugs— it's not a coil problem! Signs of Premature Failure If a customer complains of a rough idle and/or the MIL/SES light flashes on after a rain shower or car wash, this may indicate water intrusion. Check for possible DTC codes P0300 through P0306 and inspect the coils and spark plugs. If upon inspection of the coils, a white chalk-like build-up on the inside and/or outside of the spark plug boots is detected, the coils have been damaged and must be replaced—Standard hi-quality UF303 coils should be installed. It is also advised that the spark plugs be replaced. The Remedy Water can seep past the engine cam cover and collect below the coils in the spark plug wells of the cylinder head resulting in a possible misfire. To prevent this condition from reoccurring, the AIP seal must be replaced with a replacement seal that redirects the water away from the engine cam cover. Vehicles Involved 4.2 Inline 6 cylinder LL8 Engine (VIN code S – PRO LL8) 2004-2007 Buick Rainier 2002-2007 Chevrolet TrailBlazer 2002-2007 GMC Envoy 2002-2004 Oldsmobile Bravada 2005-2007 Saab 9-7X View our 21st Century Tune-Up® video video Back to Just the Facts