Shifting problem Code P0300

I KNOW

MEMBER
2004 GMC C2500
6.0L
V8
4WD
AUTOMATIC
365,000 MILES

HD 2500. The engine light is flashing when I drive it down the road, but when I have it parked and the engine running then the light is on continuously and it is giving a P0300 code. I have replaced spark plugs and wires no difference. It also has problems shifting since the engine light started flashing, the torque converter does not lock up anymore since the light is flashing. I already changed the torque converter, but it did not help.

3 Replies

STEVE W.

MECHANIC
13,572 POSTS

P0300 = Random multiple misfires. Can be caused by a lot of things, usually not plugs/wires though, those will set a code for the particular plug or wire that is bad. It is caused by something that controls all the cylinders. Bad coil, MAF sensor out of range, O2 sensors reporting incorrect data, even something like an exhaust leak at the manifold can cause it.

Run a fuel pressure test to see if you have low pressure. Should have 55-62 psi.
https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

Check for a vacuum leak.
https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

If you have access to a scan tool that will read live data check the fuel trim numbers both short and long term, and the O2 sensor response.