ERRACTIC RPM I HAD MY THERMOSTAT REPLACED ON A 04 GMC ENVOY XUV 4.2L BECAUSE MY CHECK ENGINE LIGHT CAME ON. I TOOK IT TO AUTOZONE AND THE CODE STATED THAT WAS THE PROBLEM. ON THERMOSTAT WAS REPLACE THE BATTERY WAS DISCONNECTED-RECONNECTED TO RESET COMPUTER. NOW WHEN THE A/C OR THE DEFROST IS ON, I NOTICE THE THE RPM GAUGE WOULD BE VERY LOW AS THOUGH THE CAR IS LOSING POWER-THEN IS GOES UP! SO THE RPM GOES UP AND DOWN WHEN THOSE COMPONENTS ARE ON. WHAT COULD BE THE PROBLEM. THANKS!

It's a short in the instrument cluster.
I had the same problem with my 2004 XUV.
It started in early December of 2007.
Intermittent manifestations, than more persistent.
Took it to the GMC Dealer --
Of course, the Truck wouldn't do it's tricks for the Tech.
They replaced the ignition switch and sent me home.
It got progressively worse.
I waited until I was sure it would do it for the tech.
Took it back in to GMC Dealer--
It's a short in the instrument cluster.
They needed to replace the entire cluster.
I bought an extended warranty when I bought the truck.
The warranty is still in effect --
So I only had to pay the $100 deductible.
No Idea what it might cost otherwise.

check the fuse/relay. if it is neither of these check the switch with a multimeter. If you can hear the motor trying to move the glass when you try the switch it may be the regulator needs attention. It may be the motor needs to be replaced.

check fuse and relay along with the switch first then the regulator and motor afterwards. Replacing the small electronics is obviously easier than having to open up the tailgate to check the mechanics of the winedow.

More than likely the thermostat is stuck open. You should replace the thermostat and the temp sensor. See picture below for location.

Get codes read by auto parts store, and most do it for little or nothing.
the code(s) will give me a better idea of what may be happening.
Spark plugs could be worn out(anything over 80k miles)
Fuel regulator, injector(s), fuel pump also could be at fault.
one more a vacuum leak.
Don't be afraid to ask questions, and there aren't any dumb questions.
Codes don't give you the part to be replaced, but give you an area to test in.

The same thing happened to my '04 XUV. Discovered broken white nylon/plastic that retains the end of the lift cable. Ordered a new lift gate motor/mechanism assembly ($250) online and replaced it myself. Be sure to buy a new limit switch ($13) also. I did not and regretted it because you must remove the old limit switch from the old motor assembly so you can re-assemble it to the new motor assembly. While removing the old switch from the old assembly, I broke it. Had to then order a new switch. The cost to ship the switch separately was more than the switch. The whole thing did not really take that long and was surprisingly easy. It works great now.

Try putting your transmission in neutral and slowly shifting the 4wd selector to each position but you have to wait a few seconds between selsctions as it has to go through your vehicles computer, shifting to fast will cause your symptoms. I hope this helped you out. Roger

p0128 engine coolant thermostat below operating temp. Possible low on antifreeze. not plugged in, possible chaffed wires.

P0106 pertains to the throttle body. most cases involve cleaning the throttle body

p0614 i could not find code definition

Not without a LOT of trouble. Your pretty much hosed.

I also have an '04 Envoy XL. The problem we had was very similar. The truck lost power and the wife and kids were stuck on the interstate. It was the catalytic converter totally clogged. We were told by our trusted mechanic that we were lucky not to have blown a head gasket from the back pressure. Two days after having the converter replaced the check engine light came on. I went to Autozone to have the codes read. I don't remember the code number but it was basically that the thermostat was stuck open and the engine was running too cool causing it to run rich and clog the converter. I did not get a light until after and the light went out on it's own. I hope this helps you. GOOD LUCK!

That code represents a lower than expected reading for temperature (coolant). First thing to do is check your coolant level, as if it is low, then the sensor can not read correctly. Second, you may have a thermostat that is stuck open, allowing the coolant to cool off too much. The sensor should be mounted on top of the engine, near the thermostat housing.