Chevrolet Express 3500 speed gear will past the 3rd but not the 4th

Hello, diagnostic trouble codes do not direct you to replace parts. It gives you a place to start performing testing. Depending on the code, the circuit may not be in question, but rather the performance of the component which could also be affected by internal transmission mechanical concerns. Hands on testing using proper test equipment and workshop manual information is needed. Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops