My S80's coolant level low warning keep displaying even though the coolant level in the reservoir stays the same, while the engine tempreature is normal

 

you may need to replace the sensor. Not sure about your year - on some models the coolant level sensor is in the top (like my 850), others the level sensor is built into the reservoir which would need to be replaced. First thing I'd do is check and clean up the connectors to the sensor.

 

I am having the same thing happen on my S80. She keeps telling me the coolant is low and all I can tell her is, "Honey, your new sensor is on the way. You will be fine."

I found one for $30.00 OEM online. Part #30645812. For the 2007 S80 V8 model, the coolant level sensor is located in the bum of the coolant reservoir.

 

Just replaced the sensor. I didn't have to add any coolant, although, a rag set under the work area should keep it clean from the few drips that happen. I kept the large coolant hose on the bottom connected.

The process was relatively easy! just....

Unclip the two smaller hoses at the top
Unscrew the reservoir
Pull the reservoir off the metal mounting tab.
Pull the sensor out of the hole - use needle nose clamps or pliers because the bruja is really in there.
- I pulled on the wires and the internal components of the sensor came out.
Use a flat head to push the sensor/harness out of the reservoir clip on the bottom.
Detach the sensor from the harness.
Attach the new sensor.
Push the connected sensor/harness back in the reservoir clip on the bottom.
You can try to get the wire in the clamp leading to the sensor hole, but i found it difficult.
Push the sensor in the hole. Push until you feel/hear it click.
Put it on the mounting tab
Reconnect your hoses.
Secure the reservoir with the bold.
Check your coolant level.
Start test and the light should go off, as mine did!

EDIT: If the process needs love, just let me know. I couldn't find anything anywhere on it, so i figured this will help someone, somewhere on the internet.

Much Love.