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The sensor in the reservoir is probably bad. It is located in the bottom of the reservoir and fairly easy to change.
Near the bottom of the coolant reservoir ( overflow tank)
in the trunk on the o/s
Add coolant to the reservoir. If the low coolant light still stays on, replace the low coolant sensor.
On a 2002 Mercury Sable : The engine coolant reservoir is at the front of the engine compartment , on the passenger side of the engine compartment ( the engine coolant reservoir has a thread on cap on it , that is your pressure cap / radiator cap )
For anyone interested, you take out the engine coolant reservoir and you can access it from there.
On a 2002 Ford Ranger : The reservoir would be in the engine compartment There are reservoirs for brake fluid , windshield washer fluid , power steering fluid , engine coolant , etcetera
The thread on cap on the engine coolant reservoir of a 2002 Mercury Sable is the pressure cap / radiator cap
On a 2002 Lincoln LS : The engine coolant reservoir is in the engine compartment , on the drivers side of the engine compartment , next to the firewall . According to a drawing I was looking at there is some kind of trim piece over the top of it , but the trim piece has a hole in it to gain access to the engine coolant reservoir cap
This would be a primary symptom of a blown head-gasket.
On a 2002 Mercury Cougar : No , the thread on cap on your engine coolant reservoir is the pressure cap / radiator cap
On a 2002 Ford Explorer the thread on cap on the engine coolant reservoir is the pressure cap / radiator cap