By following the instruction at the link below you can find out what the codes are as well as clear them to see if they return. That black plastic box houses your Control Units for the Engine and Transmission (ECM, TCM).

The guess about oil in your coolant bottle making the level sensor inside stick sound good to me, next time it's on(low coolant) try opening the cap and pushing down on the float to see if you can get the light to go out. If its hot careful of the hot pressure in the bottle. Also check the connector on the bottom of the bottle to ensure it's connected.