Please HELP....6.5 non turbo diesel 4L80E in a chevy p30 school bus will not shift into overdrive.

[92Z24]

There is a temp sensor in the wiring harness in the trans . Maybe check that out .

 

[Glagulator]

Has the van worked correctly before these problems or did you buy it like it is?
It would help if you could get the code and part number off the eprom chip in the TCM.

EXAMPLE -