2000 Ford Taurus heater core

I've heard a lot of people get good results by reverse flushing the core. Disconnect the hoses and hook a garden hose to the core. Flush untill water comes out clear. Removing the hoses from the engine instead of the core makes the flushing easier. This works in most cases. Lot easier than changing the core.

Another common reason for no heat is air trapped in the core. Clamping off the bypass hose will force water into the core forcing the air out. After clamping rev engine to about 2000 rpms.
Remove the clamp ( vice grip works good) after you get heat Do not drive with clamp on! Use care not to puncture an old brittle hose.