Misfire problem

I get that, the sensor was just from changing rhe plugs, the misfire existed before the sensor was unplugged

There's a valve on the fuel rail that a gauge attaches to.

Plug that sensor back in. It'll invalidate any testing you do until then. What else did you do? A plug job doesn't necessitate unplugging it

Wow curious about this one

When I went through this my mechanic had some kind of magic wand tool that read the coils for spark. Can't find that tool online to save my life. Wound up being a bad plug in my situation.

Jeremy the 3 said:

I've got an 03 Denali xl that is giving me misfire problems. Changed the plugs and wires before I had my buddy hook it up to his readers. Cheap reader gives P0300(random misfire) and P1380(misfires repeated) codes. His other reader is a little bit more in depth. Shows all the cylinders misfires. This cant be all the coils CAN IT? It's got to be further in then the coils right? Any input helps thanks

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I have had a 2004 Sierra 5.3 and Currently own a 2003 GMC Yukon XL both motors presented the say as yours and on both truck the same answer after tracking it all down. On the Sierra I put plugs in, Tried replacing the injector on the 2 cylinders that were misfiring and also a coil on both of them and no change, did a leak down test for compression no leaks no fuel filer problems no fuel Pressure problems and Finally we tore into the motor and found one lobe on the CAM worn bad so it was missing bad at low RPM but running fine going down the road. I had already found a motor in a wreck to replace it but I had to tear into it to prove it to myself this was also the case on a friends truck a year after 2005 and in mine now 2003 anyone elese have the problems with the CAM Lobes wearing down to a point of causing the misfire at low idle...... Also I replaced the Cat. (however I would plug that sensor in for coolant then have your buddy run the same test again)