This just started. Truck has been in the south since April, just got back to New Mexico Sunday towing a 4000 lb U-Haul. Was fine with that. Yesterday morning went to return the trailer, and while going down our canyon the CEL came on, rpms were high and tranny wouldn't shift. Stopped the truck, opened the hood, only thing I found was the oil was low. Figured that might be it.

Started back up, truck drove fine although the CEL stayed on. Dropped the trailer, CEL went out, got the oil changed and headed back up the mountain.

Left early this morning to catch a flight in old El Paso, a repeat of the previous day. CEL came on at nearly the same spot in the canyon. This time I drove a little farther and did some troubleshooting. I put my foot in it and it refused to upshift. So I tried manually shifting but when I went to M, the M wasn't underlined and there were no numbers: just M, not M4, M3 etc. Never seen that before.

So we pulled over, shut the truck off, removed the key and opened the door to shut everything off. Waited 5 minutes, cranked back up and she drove normally the rest of the day. CEL went out after the third restart.

So light comes on steady, tranny won't shift even in manual, key WILL come out of the ignition, system resets after being off for a few minutes. The manual says the steady CEL is an emissions thing but says nothing about the tranny locking in one gear. Is this a bare wire touching metal thing? I'm on the road till Saturday so my wife is having to deal with it.

Going to have her stop by Autozone and see if any codes are stored. On a side note that may be related, the Hummer manual calls the CEL an OBDII light. We have a HUM transmitter plugged into the OBDII port but it hasn't seen this problem. It's a Verizon device that keeps track of where the truck is and watches for problems with the vehicle. We've had it about 3 months now so don't really think that's the problem, but who knows.