Need Help 2002 GMC ENVOY

you have to scan the code and get the definition , then figure out what the problem could be the code tells you where the problem is not what to replace , you can spend hundreds at a garage that you may or may not trust or buy a scanner for $80** and go for it , cars are run by computers I'm sure you deal with those every day

Elmo, don't drive it with a flashing check engine light. Need to read your codes, likely a coil has gone out on you. Flashing light usually means mis-fire and needs to be dealt with before you continue to drive it. Like suggested below, you need the codes scanned which will show you what cylinder is mis-firing.