I have a 2008 Hyundai Accent 4 speed automatic gearbox. First it sometimes shift aggressively and select to 3 gear after legal 70kmp won't shot to 4 gear. My mechanics said we shld change the transmission fluid. We did using Hyundai sp3 stated in the manual. Secondly after the atf change when gear is switch to drive move it takes time before it moves and suddenly takes off after a while.
After a week now after iignition and gear shots to drive the car don't move meaning it stalls and then I hear gun shots from the engine. It moves for 5seconds and stop unroll I turn of and turn again. What could be the problem with the car as the mechanic is saying it's the gearbox sensors. Any help. Thanks

 

Check fluid level in transmission
And it sounds like your mechanic is guessing
But, I would have guessed the same.
Unfortunately, guessing can become very expensive if it doesn't fix the issue

Ask about the cost of fixing this one??
Good luck