2006 envoy denali engine is running rough like all cylinders are not firing, check engine light is flashing on and off.

The P0420 is probably caused the misfires. Flashing check engine light is a serious condition that can destroy the catalytic converter if not fixed. Do not keep driving it like this. Get it to a shop to have it fixed.

Shane, Base 235 75R16, SLE, SLT, XL SLE, XL SLT P245 65R17,
Denali, XL Denali P245 60R18. Check the attached links,instruction and guides, Good luck
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Well that would just be guessing.

You need to get a decent scan tool on the vehicle to see what is actually going on with the engine and then do a proper diagnosis of the cause or causes..

Or you can do what a lot of people do which is to guess what is wrong and start changing parts etc etc until they might one day stumble on the actual cause but most likely not.

What is the fault code you receive when the engine light comes on?
If you do not know try to retreive the engine trouble codes and post them here.

check for cylinder compression's,take it from there

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

A flashing check engine light means FIX IT NOW! It probably has a dead cylinder (most likely an igniton issue).Most likely,the PCM has figured out which cylinder it is aand turned off the fuel injector to it.You can get a cheap,basic code scanner(mine was 50 bucks at Harbor Freight Tools,made by Cen-Tech).It can read the code and has a book to tell you what the code means.If you find the cylinder with the misfire and correct the problem,you have to clear the code to restore fuel to that cylinder(this cheap tool does that,too).

if engine light is flashing it is not firing on all cylinders. a scanner will tell you what cylinders are misfiring. could be plugs. coil or other problem

Sounds like you have cylinder misfire. Any of the large auto parts stores can run an OBDII check and tell you which cylinder is misfiring (not firing). The engine is running with one cylinder out causing the rough idle. The faster the car goes the less perceptible the misfire is. That is why at 30mph the car seems to run smooth.

yes repace plug wire and recheck still has mis replace coil pack.I hope this helps you?