Two problems related to automatic transmission have been reported for the 2003 Chevrolet Avalanche. The most recently reported issues are listed below.
2003 Chevrolet Avalanche with harsh and suden unexpected down shifts. Dealer has attemped repair several times without repairing the problem. When slowing in traffic or for a turn the transmission will downshift hard and clunk and bang as if something is going to break. This in my opinion is a safety hazard on slippery roads while making a turn as it could cause a spinout. Last call from dealer the service manager read to me a TSB from Chevrolet saying this was a normal condition and not to attempt a repair. Not acceptable. I have been an auto tech for 38 years. I feel the transmission is shifting into 1st gear instead of 2nd when speed drops just below 20mph. This just may be a shift program problem, but what damage is being done to transmission over long term?.
The truck continued to shake when depressing the gas pedal. Turns out the transmission was messed up. The dealer found lots of metal in the pan. Rebuit the broken parts @ 15,000 miles not a new one just rebuilt.