Car does not Move when transmission is in Drive or Low 1/2, But moves when in Reverse?

Hi Kenny, thanks for the response and the information provided.

On your question about metal finding, yes that's how I drained and replaced the transmission oil and found no metal shavings in the pan.

Can you provide more details on how I should get the pressure information you are looking for? Do I need to get my hand on an advanced diagnostic scan tool? If yes, is there any budget scan tool for DIYers you can recommend, so I don't spend a fortune on it?

Also, if you can elaborate more on this:
"I suspect we are going to have to dig into the unit in order to figure this out."

This is what I did: car jacked up and on a cold start, I measured the pressure (using an Engine and Transmission Oil Pressure Test Kit) through the UD tap located next to the drain plug.
- In Park and Rear - No Pressure
- In Drive or Low 1/2 - Pressure spikes to 150+ PSI with a jolt, then drops and maintains to 45 PSI on idle (750 RPMs). TCS and ABS light get on, now only one wheel moving. At RPMs 1,250, it goes to 195 PSI, still one wheel moving. And then further increase in RPMs say 1,500+ the pressure needle moves swiftly between 210 - 240 PSI, still one wheel moving.
Note: Surprisingly, seconds later suddenly both wheels come to full rotation, pressure drops to 0 PSI, and even when I take the foot off the gas pedal the RPMs maintains at 1,500. That's weird never seen that before but maybe since the car is jacked up, which triggered TCS and ABS, and thus it's acting up.

Hope this data helps anyway.

It's hard to replicate the issue of no motion in forwarding gear when the car is jacked up since it has to be a hot start. But jolting is always a replicable cold or hot start. However, if you for sure want the pressure at the tap point when the car doesn't come in motion, then I will try hard to replicate it. Let me know.

What do you suggest as the next step?

Also, with a lot of google research, thinking if a bad Transmission Fluid Temperature sensor is causing all this? Seems like that's an obvious failure point with Hyundai and Kia's.

Hi Kenny, thanks again for your prompt response. As I stated in the original post, there is no check engine light and no DTC stored in the system.

Hi Kenny, I really appreciate your help so far here. I was able to get hold of the advanced diagnostic tool, and I replicated the issue to monitor some TCM params. Here are some more observations or data points for you:

- Issue of "Car not moving forward in Drive gear" is easily replicable at Transmission Oil temperature reading at 85 Celsius. Monitored the temperature reading from cold start to hot, it was going up gradually which means the temperature sensor is good.
- In Drive gear when the Brakes are on and Input Speed reads 0, the car will move forward when Brake Switch turns off. However, when the Input Speed is not 0, the car won't move forward. Attached also found a picture of diagnostic steps from Hyundai on another forum, stating this condition as Internal Transmission Failure and suggesting replacement.
- When in Drive gear, foot off the brake, and the car doesn't move, the physical pressure reading at UD tap point is 10-15 PSI. Then if I rev up the engine to 1,500 RPMs, the pressure suddenly spikes to 75-90 PSI, the car will jolt forward and come back to normal forward motion.
- Attaching both, when the car moves forward and when it doesn't, snapshots of the diagnostic record.

I understand at this point I definitely have to look inside the transmission, but just wondering if you can pinpoint me specifics on what I should be looking for damage and repairing. Maybe some video or guide links will help. Thanks again.

Hi Kenny, I am thinking of replacing it with used transmission instead. I think it will be quicker compared to trying to rebuild. And the cost difference is marginal, so it is worth just replacing it. My Hyundai is US-built but I am only finding Korea-built transmissions online or local. US builts are out of stock everywhere. I was wondering if it doesn't matter and both built are interchangeable? Here is my VIN# 5NPDH4AE5EH501830, the transmission used is A6GF1.

Also, will it require to do Transmission Relearn through Scan tool after replacing it with a used one?