2007 Chevy Silverado 1500 Classic won't shift out of 3rd?

[danny]

Just installed head unit, amp, and speakers. It will go into reverse and park but in drive it is stuck in 3rd gear. Check engine light is on and the codes it flashes are P0740 P0758 and P2761.
Just installed head unit, amp, and speakers. It will go into reverse and park but in drive it is stuck in 3rd gear. Check engine light is on and the codes it flashes are P0740 P0758 and P2761. The engine is a 4.3 L V6

 

[Blaine]

I do not think that the equipment you installed has created this problem, however stranger things have happened. You need to go to a GM or Chevy dealer so they can hook up a scan tool and find out what exactly the codes are. That way they can treat the problem effectively so you do not have anymore problems. You may need a new transmission but I feel it is highly unlikely. There is probably another problem preventing the transmission from shifting properly.

PLEASE NOTE: You need to have the vehicle looked at as soon as possible. Avoid all highways to the dealer at all costs, you do not want to blow the engine since it will not shift out of third.