2004 Envoy: Plug boots not seating properly

check for proper seating of coils / plug gap and connectors around the throttle body for possible accidental disconnect or damage

Make sure plug wires go to proper coil pack or can cause problem,easy to get one on wrong pack

check the ignition coil for that cylinder

Coil packs was the problem in the first place. When a spark plug misfires, the high voltage goes no where and the coil starts to deteriorate. After prolonged misfire the coil will not make the hi voltage necessary to fire the plug. Replace the coil packs.

Couple of things come to mind. I take it you have the I6 engine?? In some models, the big plastic resonator on top of the plugs has a hose that plugs into it at the bottom. Check for this, I've missed it many times and get the same symptoms. The other is making sure the sparkplug boot is seated correctly on every plug. Not being able to see the connection way down in there makes it difficult to know if it's on the plug correctly. It's a common problem. Take each boot off and reinstall, listen and feel for the click to make sure you have the correct connection.

There can be other reasons for misfires beside spark plugs/coil, such as a bad injector, low compression in that cylinder, burnt valve seats.

The P0106 code indicates a problem in the MAP sensor area. Did you reconnect the MAP sensor connector after the plug change or seat it properly?
The P0128 code has to do with the coolant temperature not coming up to operating temp fast enough, this usually means the thermostat is stuck open.

I6 engine I take it? A very common problem is for the coil boots not to seat on top of the plugs correctly. Double check these, I see this error all the time and have done it myself. Feel or listen for the little click when re-installing the boots. Also, make sure your pcv hose on the plastic resonator is re-attached along with the tiny hose that goes to the fuel pressure guage, another thing that is commonly missed. Again, been there made that error.

The number 3 plug is the third one back from the front of the engine. They are numbered in order 1,2,3,4,5,6 front to back, this is the cylinder numbers not the firing order. Firing order is 1-5-3-6-2-4 .

I have a 2004 GMC envoy with a V8 and its not to hard the drivers side plugs are not to bad to get to but the passanger side is a little tricky you have to go through the wheele well to get at them also the spark plug boot are really on there tight and were hard to remove them