For 1990-2009 cars

Cause | Correction
--- | ---
Inspect for proper gear oil levels prior to performing system diagnosis.
Low gear oil levels | Fill axle to the recommended level. Refer to Rear Axle Lubricant Replacement . A low level may create intermittent or incomplete clutch park engagement.
Incorrect gear oil | Replace with gear oil, GM P/N 12378514 (Canadian P/N 88901045). Refer to Rear Axle Lubricant Replacement . Incorrect gear oil may create improper clutch pack engagement or a slippage condition.
Worn clutch pack oil pump | Replace the clutch pack assembly.
Worn clutch pack friction discs | Replace the clutch pack assembly.