Originally Posted by lawsoncl

Neither shift solenoid is engaged in 4th, so it's unlikely to be a solenoid if you're staying in 3rd. The torque converter lockup solenoid could be bad, preventing the lockup (what some people incorrectly describe as overdrive). While driving evenly at highway speed, lightly press the brake pedal with your left foot. The rpm should jump up some as the torque converter locks up. A bad brake pedal switch can also cause no TC lockup, as the trans computer will think you have your foot on the brake. I'd test the TPS and NSS outputs just to make sure they're working correctly a well, since bad input from those can cause no 4th gear as well.