My 2004 Envoy acts like it's going to cut off when I turn on the defrost/heat?
check your charging system,/ battery
umm.. its time to get a new one and plus its 2004 pls i would been had a new one.
Your Envoy might be having an IAC motor problem as the IAC motor control the car when it is idle. You should take it to your mechanic to have it scanned and the A/C pressure watched in the system when turned on.