I would clean the Throttle Body first. Throw a new fuel filter on for $12. Then check the fuel pressure. Could also be the Mass Air Flow Sensor. But you should be able to read any codes with an OBD2 reader, get one if you don’t have one. The cats can cause this. Check your exhaust and see how hot it is. If it’s super (burn the hair off your skin) hot then it’s the cat’s.Hello I'm new on here, I hope yall can help me out. I have a 2015 GMC 3500HD 6.0l gas. I recently bought it and then noticed that when in idle it began to run rough and I can give it some gas and at times it will go but very slowly and sluggish then it will pop and start running better or it will shut off at times. I was thinking it was the cats and when I took it to shop,I had them do a complete scan and they found PIP5644 with four o2 sensors bulletin. It just acts weird, I cleaned all ground wires,used a can of seafoam on intake and one in fuel tank, changed all spark plugs and O2 sensor bank 2 sensor 2 since that was the only fault I had at first. I can't seem to find any air leaks but I did noticed that when purging valve comes on truck runs a lil rough and also noticed lights dimming down at the same time. I checked all battery cables and found them to be loose, so I tighten them and also checked purging valve and seems to be working fine, I'm completely lost on this one any taught or advice I would really appreciate it.