Hyundai Repair: 2001 Hyundai Accent auto trans., obd ii scanner, overdrive switch

Question
The transmission will shift from fourth to third gear. I used an OBD II scanner, but there were no trouble codes.
Fluid and filter were recently changed, fluid level good. The car has 87,000 mi. on it.
Any ideas? Thanks.

Answer
It's possible that the overdrive switch is faulty, causing the transmission computer to shift out of fourth gear.  

But it's also possible that your scanner doesn't communicate well with the transmission computer.  What you describe is also consistent with going into failsafe.  If your car remains in third gear until you turn it off and restart it, I expect this is the case.  If so, you should probably have the dealer do a basic diagnosis for you.