Hyundai Repair: Gear shifting problem, transmission control module tcm, transmission control module

Question
I have a 2001 Hyundai Sonata 4cyl. bought used.  When driving at both city and highway speeds it will suddenly down shift causing the car to lurch.  At highway speeds it never goes back into overdrive or 4th gear. This is an intermittent problem I cannot recreate, it just happens then goes away.

Answer
Chances are very good that this is occurring because your transmission control module (TCM) sees a problem.  Depending on the problem the TCM sees, it will command the transmission to third gear and request a check engine lamp.  The transmission will remain in third gear until the vehicle is turned off.  Once restarted, the transmission will operate normally until the TCM sees the problem again.

You can find out the nature of the problem by reading the trouble codes stored in the ECM/TCM.  AutoZone and AAMCO have advertised they'll check the trouble codes for free.