Hyundai Repair: Hyundai Elantra 2000 check engine & running in 1 gear only, transmission control module tcm, hyundai elantra

Question
Hi,
I had my 60k miles servicing done by a private mechanic. On completion, the car is not accelerating. After some driving at ~20m/hr, it suddenly jumps a bit into higher speed...the max it goes is 40-50m/hr. The check engine light comes at the start of the car itself. On checking with Autozone, the code was soemthing like T0173 (don't remember exactly)...they interpreted it as Transmission Control Module (TCM)...which is a dealer part & costs ~$400.
After reading another question on this forum, realised that may not be the issue.
If you can advice...Thx much for any info.
Rgds.
Vinod

Answer
The actual trouble code is the most important piece of information here.  I'll need you to get the actual code for me.  (I know it's not P0173 since this code isn't used on your car).  

I can almost guarantee the interpretation as a problem with the TCM is inaccurate.  I've never seen a trouble code on a Hyundai that resulted in being able to determine the TCM was defective without significant diagnosis.  And the cases where TCMs are needed are very rare.

Another option is returning to the mechanic that did the servicing.  If reputable and capable, he'll probably do some basic checks at no charge since this issue occurred immediately after the car was serviced.