E3b 740iL Trans Failsafe Prog warning

[eb2112]

I'm new here May 16, 2021
My 2000 740iL (143.000 mile) will not give out OBD codes except Cannot Read. My mechanic says it is probably an electrical issue...a short or bad control module ECM, TCM etc. I know electrical is expensive to diagnose. I have another car so no hurry to fix. I could try to trouble shoot point to point but need How to guide.
Sugesstions to pin point cause of not reading OBD codes?
Most common reason for
Trans Failsafe Prog message?

My car will drive fine then soon drops to 4th gear after "Ding" Trans code message pops up. Speedometer will work then stop, then work again. The tach has started to fail off and on as well. Help! Thank you.