Hyundai Repair: check engine light, 2002 hyundai sonata, hyundai sonata

Question
My check engine light came on as I was driving down the street in my 2002 Hyundai Sonata. Just before the check engine came on my car would jump forward. That's the best way to describe it. I took my car to Advanced Auto and they told me it was my input speed sensor. They never gave me a code. I had the output speed sensor replace a couple of years ago,it did the same thing. I want to know how to replace the input speed sensor and is this something normal for a sensor to go? I could have sworn I've replaced the input speed sensor before. And if I don't replace it what will it do to the car?

Answer
This is a reasonably common failure, but two years is a little sooner than normal.  Perhaps check the wiring going to the sensor to make sure it's not chewed in half by a rodent or similar animal.

To replace the input speed sensor, remove the air box (three 10mm bolts) and unbolt the transmission shift cable (two 12mm bolts) from the transmission.  Move the shift cable sideways out of the way, remove the 10mm bolt holding the input speed sensor in place, and drive the conidition.

If the speed sensor (or whatever problem is causing any failsafe, then continued driving may damage the transmission.  Starting from a stop or driving slowly in gearshift in 3rd gear (failsafe) puts a significant stress on the engine/transmisison from this morning.