GM-GMC: 1999 gmc yukon, gmc yukon, shift linkage

Question
sometimes when i start the yukon it wont go into gear
is there such a thing as a break sensor that will cause this problem?

Answer
There is a small shift interlock on the gear selector that is responsible for stopping the truck from being shifted when the brake is not depressed.  If your truck won't go into gear when the brake is depressed it is possible that the BTSI (brake transmission shift interlock) solenoid is faulty.  It is a simple solenoid that when supplied power retracts a small pintle.  
The BTSI solinoid is located at:

Brake/Transmission Shift Interlock (BTSI) Solenoid
LH of the IP, to the RH of the steering column, under the support bracket

It is located on the steering column under the support bracket. It has two wires on it.



Here is the description of the BTSI:



The brake transmission shift interlock (BTSI) system prevents the automatic transmission from being shifted out of PARK unless the brake pedal is depressed. When the brake pedal is depressed, the BTSI solenoid releases a lock paw. This enables the movement of the shift linkage and the operation of the shift lever.

When service operations are required, overriding the BTSI is possible. In order to override the BTSI, turn the ignition switch to OFF (not LOCK), and remove the TURN-B/U fuse (# 16) to remove the battery voltage to the solenoid.

Hope this helps