Hyundai Repair: Hyundai Santa Fe 2001, hyundai santa fe, obd ii code reader

Question
While driving my 2001 V6 Santa Fe today, the car sounded and felt different than normal.  As I got onto the freeway, the car would not go into 4th gear (ie sounded horrible when I went 55 mph and wouldn't go any faster) and the engine light was on.  So, I got off the freeway and after stopping at a stop sign and pressing on the gas, the car inched forward for about 5 seconds, then eventually got up to a normal speed until I could find a service center.  Each time I had to start the car from a stop, it would only speed up very slowly.  The man at the service center tried to run a diagnostic test and said the "computer isn't talking" so I'd have to get that fixed at the dealership first.  I also noticed (by switching over to the manual shifting option), I could tell that the engine seemed to be stuck in 3rd gear because even when the car was stopped, and I switched to manual shifting, the light on the dashboard said the car was in 3rd.
After driving away from the service center, the "check engine" light was off and everything felt normal (so I just drove home).  Any idea what's going on?  

Answer
What happened was that your transmission control module (TCM) saw something irregular and put your transmission in failsafe (third gear only).  When this happened, it also requested the engine control module (ECM) to turn on the check engine lamp.  

Reading the trouble codes in the ECM and TCM will be the key to finding out what sort of problem the TCM saw.  In most cases, it's something simple like an input or output speed sensor.  

I'm not sure why the service station was unable to read your codes.  They should at least be able to read the ECM codes with an OBD-II code reader or scanner.  The most useful information, however, will be obtained from the TCM.  You should take the vehicle somewhere that's capable of reading the TCM trouble codes.  The dealer will be able to do this.  If  you'd like to take the vehicle to a shop other than the dealer, you should probably call first to see if they have the capability of reading the TCM codes.

The reason the vehicle operated normally after you stopped at the service station is that the vehicle was turned off and restarted.  This restores the transmission to normal operation until the TCM detects the issue again.