Hyundai Repair: 2001 Hyundai Elantra P0720, hyundai elantra, speed sensors

Question
HT,
I have the problem with my Hyundai Elantra. I saw there were a number of complain on P0720. I dont want to repeat the symptoms here as they have been pretty well described. It is the same essentialy: Going at 30-50 mph the car suddenly jumps and lock the transmission into the third gear. It takes off very slowly (most likely from third gear).

Checked the codes: P0720 and P1529. Replaced myself the output speed sensor, then after couple of days the problem came back. Then put another output sensor and after couple of days the problem came back.

I have the following suspicions:

1. Output sensor (PGB) are bad, as I purchased them from Autozone (aftermarket -nonOEM)
2. Wiring (harness)
3. TCM

I changed the tranny oil recently at independent shop, not sure if they put non Hyundai oil. Could it contribute to the problem if they put non SPIII product? I only drove 6000-8000 miles ever since.

HT here answered that he saw many cases here and only one was not output sensor related. What was that 1 case related to?  

Answer
Hello Ed,
When you have speed sensor code, we reccommend you to replace both input and output speed sensors at the same time. Also , aftermarket speed sensors are not reliable and I saw  a lot of vehicles with faulty aftermarket speed sensors. SPIII transmission fluid is so important for your trany. I reccommend, replace both speed sensors with OEM parts and change the transmission fluid.