Misfire - p0340 and p0300 trouble codes

[PawnofChaos]

04 trailblazer, 4.2. Running rough and SES light on. Had codes pulled and showed p0340 (CMP sensor circuit malfunction)and p0300 (Multiple engine misfire detected). Replaced the CMP sensor and left negative battery cable unhooked for 20 minutes or so to clear the codes. Upon start up no more SES light and idle was slightly better. Within seconds it SES was back and it is again running poorly. Cruise control is no longer functional as well. Any suggestions would be greatly appreciated.

[maxwedge]

You need to get the code again.

[Schurkey]

Originally Posted by PawnofChaos

Had codes pulled and showed p0340 (CMP sensor circuit malfunction)and p0300 (Multiple engine misfire detected).

Were the codes pulled using a REAL scan tool, or some consumer-grade code reader? Did you follow the proper diagnostic chart indicated by those codes, or did you slap in a replacement sensor and hope for the best?

If you weren't using a proper scan tool, you may not be getting the REAL codes. Consumer-grade crap tools substitute generic codes for the manufacturer-specific code that is actually stored.

I'm thinking that your "P0300" is very suspicious. You MIGHT have a random misfire, but it's also possible that you have a misfire on a specific cylinder that isn't being reported properly.

[PawnofChaos]

Code was pulled at Autozone. Had them pull it after changing the sensor and it's misfire cylinder 5 (I don't remember the code). I changed the plug (way overdue anyway) and again seemed better, but not for long. SES came back and again its telling me misfire cylinder 5. I plan on changing the other 5 plugs as well as the coil pack on cylinder 5. Plugs are needed so I don't mind replacing them. Am I going in the right direction by changing the coil pack? My thought was yes and if that didn't solve my problem a trip to a mechanic would be next.

[Schurkey]

Originally Posted by PawnofChaos

Code was pulled at Autozone. Had them pull it after changing the sensor and it's misfire cylinder 5 (I don't remember the code). I changed the plug (way overdue anyway) and again seemed better, but not for long. SES came back and again its telling me misfire cylinder 5. I plan on changing the other 5 plugs as well as the coil pack on cylinder 5. Plugs are needed so I don't mind replacing them. Am I going in the right direction by changing the coil pack? My thought was yes and if that didn't solve my problem a trip to a mechanic would be next.

Sounds like your first code reading was indeed substituting generic codes for the real code.

Why don't you MOVE the coil pack from #5 to #1, and see if the misfire transfers to #1? If it does, the coil pack is bad. If it doesn't, you'd better do some compression testing and leakdown testing, as well as checking the wiring to the #5 coil pack.

[PawnofChaos]

Alright, I changed the other 5-plugs (#2 and #4 were wet with fuel, #5 looks to be firing as normal). Swapped the coil packs around, as suggested and I've still got the same SES light and misfire cylinder 5 code. Have an appointment at the Chevy dealer on Wednesday. Will advise.

[PawnofChaos]

Car has been repaired. Was a bad coil pack.

[PawnofChaos]

Car has been repaired. Was a bad coil pack.

Thanks to all for their help!