I have a 2003 gmc Sierra 2500hd 4X4 that is stuck in second gear, and it's automatic. Gas engine.I just put a new k and n transmission filter in it and it started not wanting to shift.

Your transmission computer is telling the transmission that the load for that gear is incorrect and will automatically put the transmission in the gear that will not overload your engine. Most of the time when you are going up hill the transmission will do the same. It is just trying to save you an engine rebuild.

on the side of transmission where manual shaft is ,there the neutral safety switch that the problem water go inside
change it watch out to install

I would say the torque converter is shot.

Sounds like a weak fuel pump. Have you checked fuel pressure? It should be at least 50 psi key on engine off.

REAR WHEEL LIFTED OFF THE GROUND OR ON A CARRIER

out of commision right now? sounds like it needs a new transmission, or out of fluid, but if the fluid is low or empty theres probably some dammage thats been done. i bet if you run that computer to check that code it will only confirm your guesses...

try a new one or try fuel filter

To reset the oil life......turn the key on....do not start....pump the gas pedal 3 times within 5 seconds.....then turn the key off and back on and it should be reset....sometimes u have to do it a couple times.......and what heater fan r u talking about?.....

Check fluid level in clutch master cylinder, if fluid is full then bleed clutch system.

If you still have no pedal resistance you will need a new master or slave cylinder for the clutch.

check owners manual,or call a gm service dep.