The coolant sensor light in my 6 cyl. 98 Mercury Mystique stays on all the time even though the coolant level is full.
Can I fix it or do I need a new sensor??
Unplug the coolant level switch connector to make sure the connections are clean. If that doesn’t solve the problem, then most likely the coolant level switch requires replacement.
Tester
Where are you checking the coolant level? If you’re just eyeballing the plastic overflow reservoir, the coolant level can still be low in the radiator itself (which is where the sensor is). A faulty radiator cap can cause this, as could blockage in the lines from the reservoir to the radiator.
If you check your coolant level on a regular basis - both radiator (while car is cool!) and reservoir, then you can safely ignore that light. Or replace the sensor.
The sensor appears to be in the plastic overflow reservoir??
Yes. The coolant level switch is in the coolant reservoir.
Tester