Thread: P0300 and P0305 Codes

Ok. I have an 06 Grand Prix GT Supercharged with 55,000 miles on her. I just bought the car and I'm geting these two codes randomly. The p0305 set's quite often but I get the p0300 sometimes. The car stumbles hard when under heavy load ans has a somewhat rough idle. The porcelain on the #5 plug was cracked so I changed the plug but the issue came back. I've swapped coil packs around but the issue doesn't move.
I've read a lot of info on here and most seem to point to a plugged cat but I'd like some more input before I just buy a new cat for it. Any advice welcome.....

Might be time to do all the plugs and wires. And be sure to install either the stock irridium plugs, or copper only. NO PLATINUM PLUGS!! Also, you are running at least 91 octane in it, right?

Could also check the connection to the ICM (the part the coils bolt to) and check your grounds. Also check to be sure that your plug wires are in the correct order. It happens often and depending on which wires are switched around the car can run just fine except for a miss.