So I was told, which is third gear. FYI, these are the facts and this was the fix. About a quarter mile into the first trip of the morning heading to the range, all of my instrument lights came on as if I just started the vehicle. This was the first time I noticed that but the Hummer continues to run fine. Next trip, leaving the range, stopping at the first stop sign and while trying to cross the intersection the Hummer would barely move and with traffic bearing down on me I basically had to floor it. Got through the intersection, pulled off the road, put the Hummer in park and turn the ignition off. The engine turned off but I noticed the key didn’t quite turn all the way off and I couldn’t remove it. Started the Hummer, backed it up a little bit put it in park turned it off same thing. Called for a tow. Before the tow truck got there I thought I would put all the windows up, so I turn the key on, put the windows up and turned the key off, it went all the way to off and I was able to remove the key.
customer states vehicle had no power, check engine and transmission light came on, towed in.
technician comments:
multiple transmission codes
has only 6 Volts to transmission
has 6 Volts to fuse
sometimes 12 Volts
also noticed ignition key won’t come out at times
ignition switch has internal intermittent fault
replaced ignition switch, cleared codes, road test OK
$415 later I’m on the road again.
at some point on the 12 mile ride home I noticed the speedometer was indicating 110 mph. I stop for gas, turned it off with same indication.
started up, it indicated about 90 mph, as I increase speed it went past 120, so when I got home I disconnected the battery for 10 minutes, hooked it back up and it reset back to zero and has been working fine ever since.
hopefully this can help someone else, happy Hummering
05 Hummer SUT 58,054 miles

 

I have the same issue and after numerous repairs, i concluded the issue is most likely a short in the harness. The problem is where and how to fix it. I cannot find a good electrician and I still have that problem and on top of that GM does not make that harness anymore, so you cannot just change it.

 

To supplement what I wrote, the problem is that in the short, your ECU reads different speed on each wheel and as a result of that difference, the ECU assumes your transmission is shot or one of the wheels is locked. Out of precaution, your ECU either go to safe mode (2nd gear and not third I believe) or if you are driving at highspeed the ECU will force the transmission to neutral - you will be rolling forward but without any gear. To correct it, I bought a bluetooth OCB II, and when its happen, I reset the ECU codes. Sadly, while this worked great a year ago, the frequency if the incident is increasing making it more difficult to use.

(If you do not have the OCB II option, shutdown your motor and wait for 5-10mn, the system will reset by itself and all you will have left is the engine light, your hummer should be out of safe mode)

GM diagnosed a transmission issue, it was not. Then an ECU issue, it was not. Then a speed sensor issue, it was not...

Anybody else has a better explanation and/or a solution?