2003 Envoy (v8) loses power and key won't come out of ignition. Sometimes works fine though. Won't rev above 2k and when this happens I can't get the key out, feels sluggish and doesn't even move when idleing.

You can't, that only happens in old movies.

Your immobilizer works with a chip in the key transmitting a code to the transponder in the ignition column which sends the code to the engine computer, the body control computer, the transmission computer and the ignition module. They all have a chat about it and then decide if they will let the car have power to the ignition and fuel pump. This all happens fast between the ignition lighting up and turning the starter motor.

There are always people with other ideas though and make money from YouTube videos.

https://www.google.com/search?q=2003+gmc+envoy+bypass+immobilizer+bypass

It is better to fix the problem, not find a way around it. The cheapest solution is a mobile auto locksmith.

Hotwiring and stealing cars has virtually stopped since the advent of immobilizers.

You have a SHORT, check for chafing of the wires...

hello, check your terminals on the switch wire connector, there is a yellow wire that terminal might be loose.

was the key stuck in the ignition before you change the battery? Is it the right key or it a fresh cut key

make sure you have the wires in the right places. (see pic)
Make sure you did not leave something unplugged. (sensor/vacuum line)

Could be a bad alternator.. It may not be holding charge. Or your starter could have failed.

Though I've had the same problem and it was a dead battery in my vehicle. Had enough power, but to keep the car on..

Check Engine Light flashing when accelerating ,severe engine misfire . Something in the ignition system is breaking down under a load . If the starter engages an spins the engine when you turn the key to start an it doesn't start an run , it has no spark or fuel pressure . If you don't know this or how to diagnose .suggest you take your vehicle to a qualified repair shop . Pull two plug wires off the top of the same coil an crank the engine , if you do not see a spark jump from one coil tower to the other you have an ignition system problem . ignition module or crankshaft position sensor


Flashing check engine light CarMD

I have had this happen on my 98 Voyager. Just have a new key cut. This new key will sometimes cure the problem. The older keys get worn out and won't unlock the ignition. Try this before a much more expensive and time consuming change of the ignition.

Put the transmission all the way into park. (P)

You either need a new ignition switch or there is a part that won't let the key turn if the car is not completely in park. you might need to replace that, it could be broke