One problem related to brake sensor has been reported for the 2004 GMC Envoy. The most recently reported issues are listed below.
Bought "certified used" Envoy in October of 07. Transmission completely stopped at 57k miles 3 months after purchase. Check engine light comes on and off every couple days(have video, but sandy sansing of pensacola florida says there nothing wrong) vehicle jerks at stops and starts. After 3 trips to dealership w/ "nothing wrong" they finally decided that a O2 sensor was bad at 71k. Envoy had been back in the shop at a cost of 80. 00 bucks twice since then because of the "check engine" light and the brakes are failing, the "abs brake" light doesn't come on so they say it cant be the brakes. When trying to stop the brakes stop working and a second later they will catch and begin to stop vehicle, as if the brake system is obstructed and skips a few times. The check engine light flashes for those few seconds but doesn't stay on long and they say it isn't throwing any codes. Problem doesn't happen everyday so the dealership has said its a unfounded problem. The thermostats assembly with other parts have been replaced. Have all receipts and dates that I've taken this pos into the dealership. I will never buy another GMC vehicle. I have a 10 year old pickup truck also and has only been in the shop once. There's no excuse that we are driving these (expensive) unsafe vehicles!!.