Hyundai Elantr 2002, automatic. When the car running, I sense changing gear (at ~30 mph) and there is jerking. Then the engine light comes on. Then the speed pickup is bad. suggest what could be wrong. Thanks

Kick down switch error.
I would check/replace the switch.

engine starts with no problem; but, when I put in gear and ...

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CarGurusNov 15, 2012 - The engine just fall dead when you step on the gas; it seems to jerk before ... My car does the same thing but i put it in neutral n it starts up. ... I have a 2004 kia optima it jerks when I go into reverse it was driving ... start and run fine butwhen I put it in drive or any other gear it dies! ... 2002 Hyundai Elantra GT.

Engine stalls when tranny put in Drive or Reverse - Kia Forum

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check the trans vac lines

was the oil clean of residue ?
you might have a faulty valve that makes the shifts rough

sounds more likely to be a warn sensor could be cam or crank sensor, its not being able to read engine speed to adjust time.

run the fault codes to find the reported fault
engine lights report on engines and transmissions
could be actuator or vacuum to actuator, dirty transmission oil or pump pressure problems

Your transmission is slipping which can cause a huge amount of damage both to the car and your wallet. Take it in for service as soon as you can.

My experience with Hyundai vehicles strongly suggests you have a bad TPS (Throttle Position Sensor). Replace it... see if that CURES it. (OR, you can have a qualified technician bench test it to see if it is defective). Your solution.

CHECK ENGINE light??? Whenever the 'CHECK ENGINE' (Service Engine Soon) light comes on, the ECU (Engine Control Unit) sets a Diagnostic Trouble Code (DTC). These codes identify the particular engine management system which caused the light to come on.
You'll need to use/find an OBDII (On-Board Diagnostic) Scanner to retrieve any code which have been stored in the ECU computer. Afterward, we might be able to help you DIAGNOSE the problem and perhaps eventually fix the problem.

im lookin forward to jerkin we i approach old age too-about time for a mech chec up huh-the engine lite probly indicates for new O2 sensors or the transmission sensors-$$$-hav car scanned for free at ur friendly discount place-befor u run the marathon