Hyundai Repair: Transmission shifting abnormally, transmission control module tcm, transmission control module

Question
I have a 2001 V6 Sonata purchased in Canada.  83K's.  No malfunction light comes on but while driving the car shifts from overdrive to third and locks in that gear.  It will drive okay when started again until it happens all over. I had some information about it could be a speed sensor.  Would you comment on that?  If so, do you know approx. the cost of replacing?  Thanks for any advice.

Answer
Your transmission control module (TCM) is placing the transmission in failsafe.  I'm surprised that you have no check engine lamp, but the operating system may be different in Canada than in the U.S.  The input and output speed sensors are the top reasons for what you describe, but there are many other possible causes.  

Start with having the diagnostic trouble codes checked.  In the U.S., there are a couple chains that will check them for free, but I'm not sure if facilities in Canada will do this.  If you'll need to pay to have the codes read, you should probably pay for a complete diagnosis.  Choose a good driveability shop to do the diagnosis.  I recommend not taking the car to a transmission repair facility.  I've heard too many stories about transmission overhauls that weren't necessary.

I cannot even begin to comment on pricing in Canada.  As auto repairs go, however, replacing the input or output speed sensor is a minor repair that's on the inexpensive end of the auto repair spectrum.