After owning my elantra for over 200,000 miles, it finally had a problem that can't seem to fix. I recently did a large amount of repairs as I was selling my "very reliable" elantra to a friend. Ie... Timing belt and all other belts, water pump, valve cover gasket, spark plugs, wires, fuel pump, air filter...

And ahhhhh the transmission.
So out of the blue my check engine light comes on and the tranny throws itself into 3rd gear (protect mode). This randomly persists for 1,000 miles, but it resets after I turn the car off and back on... Since I'm about to sell it to my friend, I have autozone do the reading and it was indeed the input speed sensor, so I replace it and it still happens

So I bring it to the Hyundai dealer and do a tranny flush $160, a diagnostic scan $93, and sure enough it needs an input speed sensor ($167 installed by them).
This does the trick for 500 miles and then SLAM... except now the tranny randomly and more often wont shift out of 1st gear and then slams down into 3rd gear againAutozone machine says its the output speed sensor now. So I bought one and put this one in myself and no change!! Grrrrrr. What gives?
Autozone says they dont know what to say, since after they read the code and cleared it (after i replaced the output sensor) now it reads that the check engine light is on but no codes!

The tranny is still acting like this and i dont want to go on a wild goose chase paying the dealer chasing ghosts!!

My main questions are:
1. Does the dealer have a special way of clearing these codes that I'm not doing? (I disconnect the battery and wait 30min while changing the sensor for it to reset and have autozone clear it with their machine). Or is there a waiing period for the car's ecm or tcm to go through and acknowledge the new sensor?

2. Is this common for a ripple effect among sensors going bad?

3. At the start of the tranny issues I also notice a small leak in the top plastic part of the radiator and have been attempting fixes wich only last a few weeks at a time. I just got a new radiator and am going to install it in the next week, but is there a connection between the sensors and the radiator leak? (Even though I check it very often and keep it full)

Any input or thoughts are appreciated...

 

After 6 hours of installing my first radiator, it appears as though that was the problem!!! Once I hooked it up and filled the tranny fluid and coolant back up, the check engine light went away and the car is back to its reliable smooth shifting self!