Hyundai Repair: Lantra 1996 Check Engine Light is on all the time, engine control module ecm, diagnostic capabilities

Question
QUESTION: When I bought this used car sometimes the check engine light was on and sometimes was off, there is no relation if I just start driving or I am driving for along time sometimes on sometimes off. And nowadays it is always on. The heat pointer does not going higher than the middle. Another point the tank for extra water for radiator is making confuse for me. the full mark is almost at the bottom of the tank, so why did they make it that large. when I add water it is going down a little bit below the full mark and the pipe is hardly touch the water. Can you help me please
ANSWER: In regard to your check engine lamp, you should have the trouble codes in the engine and transmission control module read.  Once I know what the trouble codes are, I'll know the nature of the problem and can advise you on diagnosis and repair.

In regard to the coolant reservoir, the reason it's large is to allow for coolant expansion as the engine temperature increases.  As the coolant in the engine expands do to temperature increase, coolant pushes out of the radiator and into the reservoir.  When the engine cools, the coolant contracts, and some is sucked back into the radiator.  There should be a hose attached to the reservoir cap that extends to the bottom of the reservoir to enable the coolant to be sucked back into the engine.  If, after filling the reservoir to the full mark, the coolant falls significantly below that mark in a short period of time, you should check your cooling system for leaks.

---------- FOLLOW-UP ----------

QUESTION: Thanks for quick response, but could I ask you what is the engine and transmission control module read? and how can I find it?

Answer
The engine control module (ECM) and transmission control module (TCM) have self-diagnostic capabilities.  When the check engine lamp comes on, there should be a self-diagnostic trouble code (DTC) stored in at least one of the ECM or TCM.  Someone will need to read these DTCs in order for you to know the nature of the problem.  

Since you refer to your vehicle as a "Lantra," I presume you're not in the U.S.  This means that I don't know whether generic code readers will read the DTCs or who may be able to do so.  At the very least, Hyundai service facilities should be able to do so.