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The common cause for the heater to produce poor heat in most case is plugging of the heater core.
DUE TO THE INTERNAL DESIGN OF THIS HEATER CORE, REPLACEMENT IS THE ONLY ADVISEABLE WAY TO REPAIR A POOR HEAT CONDITION. Flushing these cores in most cases, IS NOT EFFECTIVE. This concern is caused by a bank to bank coolant flow imbalance. To reduce the imbalance concern, Ford has released a coolant bypass kit to install in the heater hoses. This kit basically contains an H fitting to allow extra coolant to circulate between the heater hoses. The heater core concern is caused by the corrosive nature of the coolant. This corrosion eats away at the metal in the cooling system. We have seen cases where the impellers on water pumps have been eaten away. A majority of this sediment is not removed through normal flushing procedures and with normal vehicle operation will continue to circulate and eventually plug another heater core. It is recommended that the water pump be replaced, a strong chemical flush be used, a heater hose bypass be installed and frost plugs be removed to completely drain the engine block. Run the engine after filling to see if any discoloration is still evident. If discoloration is present, flushing again may be necessary. When complete, replace the heater core and recheck operation. Ford has issued TSB 01-11-6 addressing the bypass kit and flushing procedure. A related document, TSB 01-11-6, is available.
The labor on this job can run as high as 6.0 hours and the core usually runs about $79.00. This is a very common concern on the Sable but I would have it checked to be sure before replacing the core.