Hey guy i was driving to work this morning and the service engine soon light popped on, so i went to auto zone to check it out and a PO128( the pcm has determined that the input from the coolant temperature sensor indicates a temperature value that is below the thermostat regulating temperature) code came up. I dont know what it is or what i need to do to fix it. The only thing i noticed is now sense the temperature outside is getting a little warmer my car temp gauge stays at 200 degrees. :(

This would indicate that your PCM is not seeing the car at 200F.

Your car should only have the one coolant sensor that feeds both the pcm and gauge.

Is the code cleared?

Today which is the next day going to work i had the heat on and the car stayed 180 degrees or less the whole time i had the heat on. Also the check engine light went off by itself, before i made it to work which is only a 20 min drive.

Not sure why this is in Performance and Brainstorming. It sounds like a mechanical problem to me.

Sounds very much like a stuck thermostat. I'll move this to the Mechanical section for you. We discuss Performance ideas here.

I had the same problem on my 2000 SSEi. You need to check the coolant level. I found out that I would get the same SES code when the coolant was low. I subsequently replaced the thermostat and the LIM gasket and I have not had the code since. I would probably agree with BillBoost that the problem is with a sticking thermostat.