P0300, chasing the problem.
 

Hey, I have an 04 trailblazer ls 4.2l
just recently purchased it
It was idling very rough. It was showing a P0300, as well as an hvac code for climate control.
I replaced the spark plugs with the ac Delco factory spec replacements.
I installed new coil packs.
This vehicle only has 119,354 miles on it, didn't think the packs were bad, but figured it's better to change them now then to have a future problem.
After all that, it's still throwing a p0300.
I checked fuel pressure and wasn't completely happy with it so I replaced the fuel pump.
still throwing a p0300. At this point I'm scratching my head because my scanner is showing a misfire on cylinder 4, but I can't find anything wrong with it. And the coil pack is good because I swapped them around and the misfire never moved. I can't see any breaks, Knicks, cuts, or faults in the wiring anywhere. Is it possible that I'm fighting a dogey pcm?

First, check and make sure that the ignition coil is properly seated on the spark plug.

Second, check and make sure the electrical connectors to the ignition coils are properly seated AND that all the wires going into the connectors are secure and not being held in by a few stands of wire and that the pins are clean and not bent.

Third, check and make sure the electrical connectors to the fuel injectors are properly seated AND that all the wires going into the connectors are secure and not being held in by a few stands of wire and that the pins are clean and not bent.

Forth, bad knock sensors have been known to be the cause of P0300 or P030X codes.

Where did you purchase the new ACDelco 41-103 spark plugs?

What brand ignition coils did you use and where did you purchase them?

Advance auto is where I purchased the plugs. The coil packs are Bosch and I purchased them from advance auto as well. I was watching the misfire data with the vehicle running, and it was showing misfire in cylinder 4. So I swapped coil packs between cylinders 2 and 4. After that the misfire data was showing a misfire on cylinder 3 only. So I started triple checking all connections and I can't find any damage to any of the wiring or connections. I shut the vehicle off and restarted it after triple checking everything, and the misfire has moved to cylinder 1. So it's changing, that's why I'm thinking possible pcm. I just want to be certain before spending that much money just for it not to be the issue. When I bought the vehicle it had a P0606 that was a stored code, the previous owner had cleared it, and it hasn't come back, but that's what has me questioning the pcm.

That is why you are getting a P0300 which is a random misfire. My son's truck was giving a P0300 and it turned out to be a catalytic converter failure. It was clogged causing back pressure.

Pcm ordered
 

Went ahead and ordered a pcm today. I didn't think about the converters being clogged. It does have just under 120,000 miles on it. Guess that will give something else to look at if the pcm doesn't solve the misfiring issue.

Well, finding out about the P0606 throws a curve ball into the diagnosis of the P0300 or P030X. Until you get the P0606 taken care of, all other diagnostic trouble codes may or may not be valid.

Regarding the spark plugs, OK on getting them at Advance - stay away from spark plugs from Amazon and ebay - extremely high percentage of plugs sold there are counterfeit.

I did not know Bosch had ignition coils for the TrailBlazers.

After you get the new PCM let us know if any of the misfire DTCs come back.

Will do
 

Will do. I'm hoping it clears everything up. I didn't know Bosch made coils for the trailblazer either but they do, and I don't order plugs online anymore, got burnt once before by them, won't ever happen again. I usually get my plugs coil packs and most of my parts from Napa, but they were a little more pricey with the plugs this timeso I went with advance. But the pcm should be in in a few days, I'll post the results. Thanks for all the help guys. It's nice being able to have more than one brain thinking on problems.

Good luck and you are welcome!

how did the plugs look? the plug wells oily? i think i would have went with a can of techron or seafoam in the tank and run that through with an oil change before i went to ordering a pcm or cat. just me though. can of seafoam can take care alot of codes in an old chevy sometimes.

Pcm didn't fix the problem but got rid of the P0606 code. Have run a can of seafoam through it, changed nothing. Still have a rough idle, and now I smell a fuel smell while it's idling. Giving it throttle it smooths out and runs great. Thinking possibly a fuel pressure regulator issue maybe? I'm not sure at this point. I've done new plugs, new coils, new pcm, seafoam, crank sensor, can sensor, vvt solenoid, and the fuel pump was just put in by me before I ever bought the vehicle. So I am scratching my head yet again.