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you don have to do much to that gmc as long the coil is good, just replace all 8 spark plugs, air filter and basic oil and filter change; use good brands of oil, such as castro, mobile, shells ect. if your the original owner and have high miles and never have replace thermostat, I would go ahead and have it done, I would not wait until it fail and over heat on you. if you have done lot of pulling with your vehicle and have 75-100k mile, have your transmission flush with new filter, it will give the tanny life last a bit longer.
After market spark plugs could be the problem.
Reinstall the old plugs and see if that fixes it.
This vehicle has an ion sensing ignition system and is real sensitive to the proper spark plugs. I would get the proper plugs from the dealer if the old ones work.
The codes P1311 and P1312 are ion sensing codes.
Try turning key to on only, not start, 2 or 3 times first then try to start. If it starts good first try, then I'd change my fuel filter & then have fuel pressure tested if fuel filter didn't help. You may be looking at a new pump, but lets do filter & test fuel pressure first.