Weak cabin heating and the engine light comes on.

now that the coolant isn't leaking out....have the thermostat replaced!

You may have a thermostat stuck open. It is possible that a piece of debris is holding it open. Or it could even have a weak spring. The check engine light could lead you to the issue. Most likely it is a code telling you that the cooling system took too long to reach temperature.

POSS THERMOSTAT STUCK OPEN WHICH WILL NOT LET COOLANT GET HOT ENOUGH TO HAVE THE HEATER WORK CORRECTLY