View Full Version : OT- OBD P300 Code

12-21-2016, 07:02 PM
My 04 Chevy K2500 HD with 6.0 Vortec has been setting P300 codes off and on for a pretty good while now (random multiple misfires). It normally happens when the fuel tank is full and the speed is up around 70 MPH with cruise on. Yesterday it was enough to not only set the 'Check Engine' light, but it also was blinking (the "do something NOW!" warning).

Only problem is that I can assess nothing wrong with it. It runs like a scalded dog, has zero hesitation, starts easily, has the same fuel mileage as on day one, pulls the guts out of my fifth wheel, and sounds great. I can find little to nothing to fix. I hate to just start changing parts, because I'd never know if I did any good. And the Internet has been no help. They usually assume something is wrong, and then help with the repairs.

I'm getting stumped on this one. Anyone else seen this kind of "problem"?

12-21-2016, 07:30 PM
Any random misfire codes that come into the shop I work at get plugs and wires right off the bat.

12-21-2016, 08:58 PM
Plugs and wires done. New oxygen sensor. Vacuum leak check done. New gas cap.

EGR verify, not done. Cam/crank position sensor, not done (not sure which one it has).

If it only didn't run so well....

12-21-2016, 10:52 PM
P0300 is supposed to be "multiple misfires".

A p030X (where X is a cylinder number) will point to one or more cylinders that is misfiring.

I had misfire problems on my truck because the wires were not run properly in the loom. According to the mechanic, two plugs were firing (weakly) at once.


12-21-2016, 11:51 PM
..... but it also was blinking (the "do something NOW!" warning).

Like maybe it is misfiring enough to load up your catalytic converter and it's starting to melt??? :eek:

12-22-2016, 07:46 AM
Try looking over at http://www.silveradosierra.com/

Lot of good info, and helpful people over there.

Learned a lot about my 2002 Silverado 2500HD with the 8.1L allision set-up.


12-22-2016, 08:24 AM
Hm funny that this topic comes up. Our sts sat for a week, needles to say i would not start, no fuel going in the injection. After 3 attempts to start the battery went dead, ok no problem i figured the car sat for a while so i put it on the charger. Again i tried to start no luck after a few times battery dead again. I took the battery out and got a new one. Now time expired and there was no battery in the car overnight and i knew all the miss start codes where erased. Ok now i figured i was home free. Well i still had no gas to the injectors and a security lock showed on the dash. That meant the security shut of the fuel pump in case of theft. Again i unhooked the battery for a few hours and bingo after that i had a start. Now the car is at the dealer to have all the codes reset to normal.
Keep in mind if you let your vehicle set for over 3 month dump some seafoam in the tank. The gasoline we buy today with all the additives and mostly ethanol in the fuel will cause these problems the fuel has a life of 3 month after that you will have problems. It may not be the plugs or wires. I would test all items i can before replacing them

12-22-2016, 02:07 PM
Well, after an entire morning of reading about the dreaded P300 code, it is apparent that it will be difficult to diagnose the cause(s) without diagnostic tools of which I do not own, and never will. P300 is basically a generic misfire code that could be tied to anything from a poor engine ground to a roasted catalytic converter; along with countless sensors and gizmos. Without fuel trims, MAF and MAP readings and a host of other data in real time, it's all just a guess.

Life seemed so much easier with a carburetor, a distributor and a timing light... I know, I cussed the bloody things back then too.

Maybe I'll just run it until it explodes. Then at least I'll know what to fix- maybe.

Thanks for the help folks.