My 2002 GMC 5.3 will not fire the injector in number 7 cylinder. I can plug the same injector into the wires for another cylinder and it works fine. I can feel and hear it working. No injector I put in the 7 plug will not fire. I am getting 8.5 volts on the plugs , checking #7 and #3. Any help PLEASE.

going from battery ground to pos. on the plug I get 12 volt. Pos. battery to neg on plug I get 8.
any help please

Sounds like an adventure in wire tracing is in your near future.

How many miles on it?

Has it had any major damage?

Has it had any recent repairs?

Is the engine stock?

218,000 miles
no major damage
no recent repairs other than working on the issue.
first step was replace all spark plugs, spark plug wires, and the coil pack on #7 cylinder. Then injector on #7.

Nothing really suspicious there. I'd trace wires . . . or at least check for continuity between the PCM and the plug for the injector.

What?

Why are you going Pos bat to neg on plug? That is not a ground that is an electronic circuit....

Obviously, there is something wrong with the control side of the injector since it fires with another plug end....

You will have to disconnect the neg terminal bat cable....then disconnect C1 of the PCM and do an ohm check between terminal A of injector harness(red/blk) and terminal terminal 43 of C1 of PCM......should be zero ohms.....now take one lead off the end and touch ground...should be OL.....now reconnect battery neg cable(C1 still disconnected).....insert key in ignition, and just turn to on.....with injector not connected, have one probe on terminal 43 and the other on a good ground......we are now reading volts, so change your multimeter.....should read zero volts......now attach injector to harness....should read 12 volts(bat).....if you get those readings and the contacts(male/female) in the injector harness/injector and C1 terminal 41 and corresponding terminal of PCM, then you have a PCM problem.....