Crucially, you should NOT blank off the heater core inlet and outlet together while the engine is running. Doing so will create a blockage in the cooling system, causing the engine to overheat and potentially leading to serious and expensive damage. The coolant needs to flow through the heater core (even if you're not using the heater) to maintain proper engine temperature.
If your heater isn't working, the problem is likely one of the following:
* A faulty heater core: This requires replacing the core, which is a significant job involving dashboard disassembly.
* Blocked heater core: This might be treatable with a flush, but it's often a precursor to failure.
* Problems with the heater control valve: This valve regulates coolant flow to the heater core. It could be stuck closed or malfunctioning.
* Low coolant level: Check and refill as needed (use the correct coolant type).
* Air in the cooling system: This can be bled out using the proper procedure for your vehicle.
* Faulty blend door actuator: This is responsible for mixing hot and cold air.
* A clogged or restricted hose: Check hoses for kinks or blockages.
Before attempting any repairs, consult a repair manual specific to your 1996 Mercury Sable 3.0L. This will provide detailed diagrams and instructions for accessing and working on the heater system. If you're not comfortable with automotive repairs, it's best to take your vehicle to a qualified mechanic.
