Honda Repair: Temperature, coolant temperature sensor, engine coolant temperature sensor

Question
I Have a 1995 Honda accord regular 4cyl, and i have this problem that, when I'm driving in the city the heater temperature is normal, but when i get to the freeway, the temp dives past the white line and the heater gets cold, and i just want to know if that is related to a defective thermostat, and can that cause the "check engine light" to turn on?

Answer
The most likely cause is a defective thermostat. If you have a cooling issue with your vehicle, it may cause a problem with the engine coolant temperature sensor, but you have to have the PCM flashed so you can find out the code in the PCM. Also, check the coolant in the radiator when the engine is cold. I need more info to be of further help.