4WD switch not working on a Hummer H2

If the 4WD service light comes on when you start your Hummer H2, it could indicate a problem with the four-wheel drive system. This warning light could indicate a problem with the transfer case, the drive shaft, the front or rear differential, or other components of the 4WD system.
It is recommended to take your vehicle to a certified mechanic or a Hummer dealership for proper diagnosis and repair. The service light coming on could be a sign of a serious problem, and it is important to address it promptly to avoid further damage to your vehicle and ensure safe operation.

Where these lights added, I suspect they where, if so you will need to trace the wiring it could be a bad connection to the vehicle factory wiring.

Hummer 4wd service light reset Questions & Answers - Fixya

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Hummer h2 2003 55000 mill ... service 4wd the liht is on all time Remote the battery wire from the car for 20 ... My 2006 H3 will not come out of 4 wheel drive?

If this is like most GM products, you push it in to adjust, then turn to lock in place.

there is many contacts in the ignition switch sounds like the switch is bad

The Hummer H2 and H3 are both all time 4WD. There is no way to put it in 2WD. Now, you can select 4WD High or 4WD Low. You can also, if your vehicle is equipped with it, lock the front or rear wheels through the differential selector. This is one of the primary reasons Hummers get terrible gas mileage is because they are always in 4WD.
The only other solution is to swap the transmission to an Allison 6Spd, which is often done with diesel conversions, and they usually give you a 2WD option, and can get up to 25 MPG out of an H2.......but that conversion is about $30k.

On the dash directly in front of the right side of drivers seat is a trio of selector switches. The top switch is a constant amber light indicating all wheel drive if equiped, the switch below it is 4h, or four wheel high gears, this is only operational in trans selction mode of D, the switch on the bottom of the trio is 4L, which is only operational in 1st gear. There is also a switch for axle interlock, which gives the truck positive traction in 4 wheel only.

Use 2 wheels, the other 4wd low ,for climbing at slow speeds: 4wd high, for driving on low traction roads,like mud or snow,at medium speeds: 4wd auto full time all wheel drive, for highway speeds on slippery surfaces, like rain or light snow.

It is All Time 4x4. The light should only be on when you Shift into 4 Lo or 4 Hi Though.

Remote the battery wire from the car for 20 minutes make and plug it back this will reset the computer of the car if the light is on you need to service center and get it checked....................