I have a 1998 Hyundai Starex van which I bought brand new in 1999. My problem came about in the middle of last year (2013) when the "Check Engine" light turned on in the vehicle's dashboard. Af

SOURCE: Temperature gauge replaced 3 times and temp sensor, but check engine light still on

TRY THE THERMOSTAT ??

SOURCE: Hyundai Elantra 1999 timing issue

It's a '99 Elantra, have you ever replaced the timing belt? It is a belt not a chaing, so it is very much should have the belt changed. After 13 years if you haven't done the belt yet now appears to be the time to change. I have an '05 and we changed it over a year now.

Good luck!!
EJ Kudelka