2000 sonata check engine lite came on, transmission went to 2 nd gear. computer said idler control valv bad.put in new, reset computer, drove 100 yds. check eng lite 2nd gear again, now computer said speed control sensor bad. auto zone computer had no info on speed sensor where it is. looking for more info

Code 100 and 101 are for problems with the Mass air flow meter.
Have you checked it ?
You can pin test the computer but you may need a shop manual to get the wiring diagrams.

hi , i am sorry to tell you that you are right you wasn't need a new transmission , the hole problem is in the controler and thank you for using fixya

that is just a basic 10,000 km service
check engine light on
undo battery terminals count ten reconnect
clears the codes
if check engine light returns
the code will be stored in the ecu

description sounds like an 02 sensor

Okay, wish you would of listed the code # but thats okay, the way you explained it sounds like the gear selector switch is bad, this switch is located on the side of ******, the computer and ****** think its in d3 this is cause most of the time 90% of the time to a bad selector switch also called a neutral safety switch,

replace switch reset codes and check ****** fluid , not sure how far you drove like this but if fluid is brunt i would have it flushed, not drain and fill but flushed, a BG flush.

I think with a flush and selector switch you will be all set, good day.

the transmission sensors(in or out)went bad .i had the same problem.change the two in and out sensors on the tranny then take the battery terminal off for ten seconds and it will reset your computer and take off engine check lite. ps the sensors are under your air box.

its not that hard to fix, i did it my self on 3000gt.

YOU NEED TO CHECK POWER AND GROUND CIRCUITS TO TCM

they set the timing WRONG and that is an interference motor so the valves hit the pistons

SHift solenoid pressure switch

P1723 A/t control relay - Stuck on.
This Code is showing that the main control relay of the transmission is stuck on. When that happens transmission goes in to the fail safe mode which is being stuck in 3rd gear to prevent damage to the transmission