1999 Chevrolet Tahoe It won't start

P0108 - Manifold Absolute Pressure/Barometric Pressure Circuit High Input

It's possible for the sensor to set that code, but it's much more common to find a wiring problem. Specifically, a break in the ground wire will set it, and a break in the signal wire could set it. Start by inspecting the wires to see if one is cut, and check any connectors to see if the terminals are stretched or corroded.

The map sensor is a three-wire sensor. All three-wire sensors have a 5.0 volt feed, a ground, and a signal wire. In this case the ground wire doesn't actually bolt to the engine or body like normal. It goes to ground through the Engine Computer so the computer can monitor it. The easiest way to test it is to measure for voltage on it. With the ignition switch on, you're either going to find the proper 0.2 volts or, if there's a break in that wire, you'll find close to 5.0 volts.

You missed the important comment in my first reply. Fault codes never say to replace parts or that they're bad. The sensor will only set that code about half of the time. The other half it's due to wiring problems. That's where voltage measurements will tell whether to replace the sensor or look for something else.