Have Transmission Questions? Ask a Mechanic for Answers ASAP
Is it an automatic or manual transmission?
Also, I don't get online much on Sundays. Family and church tend to take up the majority of my day. If you'd prefer I can opt out and see if someone else will be more available today, otherwise I'll get going on this more in depth on with you on Monday.
We need to figure out whether this is a mechanical problem inside the transmission or a problem with the control module.
Disconnect the transmission electrical connection. This will place the transmission in failsafe mode and reverse and 3rd gear should both be available via the manual valve. Make sure the transmission does not slip in either reverse or 3rd gear.
If operation is normal as described above with the transmission electrically disconnected there is likely a control problem, and the transmission control unit should be checked for fault codes.If the transmission still does not engage gears or slips under failsafe conditions with the transmission and transmission control unit disconnected, check for proper fluid level and condition.If fluid level and condition are OK, there is likely a mechanical or hydraulic problem in the transmission, and it will have to be repaired or rebuilt.