Nedd to see the manaul transmission gear shift

If your 2007 H3 Hummer suddenly stopped shifting into reverse gear, there could be several potential causes. Some common causes of a sudden loss of reverse gear include a failed reverse gear band, a damaged valve body, or a failed reverse gear solenoid. Additionally, low transmission fluid levels, a clogged transmission filter, or a damaged transmission pan gasket could also prevent the vehicle from shifting into reverse. It is recommended to have a professional mechanic diagnose the issue to determine the exact cause and provide the necessary repairs. They may need to perform diagnostic tests or inspect the transmission to pinpoint the issue.

No not the whole transmission. Could be a band or servo. It still has to come out and be tore down to fix.

Both error codes you have are transmission related. P0736 is an error for reverse has wrong gear ratio. P0700 is a general transmission problem code
http://www.obd-codes.com/p0700
http://www.obd-codes.com/p0736
for more information.

Transmission is done. Nothing electronic will cause that, only mechanical

Sounds like the shift cable is off the transmission under the hood. Raise the hood and check to see if the cable is moving the linkage going into the transmission when the shift lever is moved in the car. Be sure to have park brake on.

Sorry bud you need a transmission repair or replacement.

I suspect sun shell gear in the transmission either way the transmission will need repaired or replaced.

The first thing to check is the linkage. Sometimes the linkage will slip, Block the wheels on the car (both front and rear) to prevent rolling. Locate the shift linkage on your transmission, and loosen the nut. You should be able to move the shift lever through all the gears, including "P" and "R". Place the transmission shift lever in "Park" (on the transmission) then move the shift lever (inside the car) in the "P" position and tighten the nut (not a bad idea to use a new locking nut here) You should now be able to shift through all gears. If this does not work, you have a more serious problem that may include a worn or broken belt or broken parking pawl inside the transmission.

hi mate , sounds like the SPIGOT bearing its a little bearing that allows the gearbox to be at a different seed than the engine flywheel and if this is in bad condition it will take the gearbox front shaft to a higher speed than comfortable for gearshift , in the meantime when you go for 1st or reverse gear just engage a gear like 4th without taking your foot off the clutch and then go for 1st or reverse, this will stop the gearbox spining long enough for a smooth shif, oNLY USE THIS METHOD WHILE STaTIONARY and see a professional when you get time.

try shifting in to first and then reverse. reverse might be worn out. you can have that gear replaced without replacing the whole transmission.