First, make sure you aren't putting straight coolant in it. Most coolant has to be diluted or it can damage the system, however there are some pre-mixed coolants available.
Second, how low was the antifreeze and have you checked it again? Normally your cooling system should not lose fluid. If it keeps losing antifreeze you need to find the leak, running out could cause severe engine damage. If it's leaking into the motor it could be the cause of your sputtering, but more importantly antifreeze is corrosive and can damage the pistons.
If the system is too low, the coolant won't circulate through the engine. You may get a false read from your sending unit because it will not be submerged, causing the check engine light, and no coolant in the heater core means no heat inside the car.
If the system was not very low, I would start by checking the thermostat. If it gets stuck open, the coolant constanly running through the engine will prevent it from ever reaching normal operating temp. Your ECM may take this as a failure in the temp sending unit. That would throw the light on. It could also cause the gas-to-air mixture to be off creating your sputtering issue.
Generally speaking, the thermostat is in a housing at the engine end of one (usually the top) of the radiator hoses. Locate it and take it out with the engine cold. There is a spring that reacts to heat, opening and closing the stat. It should be shut when you look at it cold. If not it needs replaced. If you want to check how it operates, my "backyard operation check" is to put in in a pot of water and throw it on the stove. Most are set to open at 180F. If it doesn't open before the water boils, it needs replaced.
Hope this helps.
Sunday, November 23rd, 2008 AT 8:22 PM