What would cause a 04 GMC Envoy to run worst after replacing spark plugs

Do you have any codes or an engine light on, in your dash board ? Have you checked for spark at your plugs ? There are many things that could cause a vehicle to not start. Bad camshaft/crankshaft sensors. Poor ground wiring. Defective spark plugs or wires/coils.

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cylinder 1 getting too hot causing ceremic crack could have wrong heat range plug in which spark plug too hot.or need replace spark plug wires.one plug wire could be bad best replace all spark plug wires.

sounds like cat converter just about completely clogged and the back pressure from it is choking the vehicle causing the cylinder failure --the exhaust cant escape and will kill [stall] the engine--- with it running feel the exhaust coming out if there is barely any flow from tailpipe cat conv has failed [clogged ] and will need replacement -----when check engine light flashes it is to advise that u have a malfunction that needs immediate attention or further damage may occur and or vehicle shut down may occure

If the oil is on the outside of the spark plugs the most common source is the seals in the valve covers.
Unusual to have all 6 leaking on a truck that new.

engine soon light starts flashing off and on , this indicates a severe misfire which could damage the catalytic converter .There a few different thing's that will cause this .Plug wires , plugged fuel injectors low ,fuel pressure , If the check engine light is on or flashing ,DTC"S -diagnostic trouble codes will be stored .Have it check , most auto parts stores will do this for free ,an be glad to sell you parts .

Under the plastic resinator on the top of the engine. Remove this along with the hoses connected to it. When removed, you'll see the tops of 6 coils. Remove one at a time (2 screws) and lift straight up and out. I reccommend using AC Delco 41-103's for spark plugs, our engines love these and will idle better. When replacing the coils on top of the new plugs, make sure and either listen to or feel the little click. It's very easy not to get them seated correctly, which will cause an easy misfire and check engine light.

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!

I have a car in the shop right now same thing - it is being caused by a misfire. Need a real technition to diagnose the problem (auto zone - advance auto can't diagnose the problem - only read the code)
mine was caused by a failing fuel injector. good luck