Possible Causes & Corrections

WARNING: This page does not describe the selected car, but rather 19 other vehicles, including the 2003 Hummer H2, 2003 GMC Yukon XL, 2003 GMC Yukon, 2003 GMC Sierra, and 2003 GMC Savana Special. However, it is still accessible from the selected car via links, so may be relevant.

Check the following items:

Search for bulletins.
Verify that the PCM grounds are clean, tight, and in the proper locations.
If there are no engine mechanical faults, fill the fuel tank with a known high quality fuel that meets the vehicles minimum octane requirements. Road test the vehicle and re-evaluate the vehicles performance.
Check fuel pressure. See FUEL SYSTEM DIAGNOSIS under FUEL SYSTEMS in BASIC DIAGNOSTIC PROCEDURES - 4.8L, 5.3L & 6.0L "C", "G", "H", "K", "N", "S" & "T" SERIES - FLEX FUEL & GASOLINE article.
Inspect for a restricted fuel filter.
Inspect for a contaminated fuel condition.
Inspect for conditions that can cause engine to run lean: Water intrusion in HO2S connector, exhaust leak between HO2S and engine, vacuum leaks, incorrect fuel pressure, restricted fuel injectors, inaccurate MAF sensor, fuel contamination, or vacuum hoses that are split, kinked, or improperly connected.
Verify that spark plugs are of proper heat range.
Low engine coolant.
Restricted air flow to radiator or restricted coolant flow through the radiator.
Inoperative cooling fan
Excessive oil in combustion chamber, may be caused by leaking valve seals.
Low cylinder compression. See CYLINDER COMPRESSION PRESSURE under MECHANICAL INSPECTION under PRELIMINARY INSPECTION & ADJUSTMENTS in BASIC DIAGNOSTIC PROCEDURES - 4.8L, 5.3L & 6.0L "C", "G", "H", "K", "N", "S" & "T" SERIES - FLEX FUEL & GASOLINE article.
Combustion chambers for excessive carbon build-up. Clean combustion chamber by using top engine cleaner.
Inspect for incorrect basic engine parts. Such as, camshaft, cylinder heads, pistons, etc.
Inspect the Park/Neutral Position (PNP) switch operation.
Inspect transmission Torque Converter Clutch (TCC) operation. See appropriate DIAGNOSIS article in AUTOMATIC TRANSMISSIONS. A TCC applying too soon can cause the engine to spark knock.