Start with the basics, inspect the condition of the:
P0303 means that an engine cylinder misfire has been detected in the ( number three engine cylinder ) The engine cylinder locations on a 1996 Mercury Sable are numbered : firewall 1-----2-----3 4-----5-----6 front of vehicle > driver
You have the code read with a scan tool to determine which cylinder.
On a Mercury Sable : firewall 1-----2-----3 4-----5-----6 front of vehicle > driver
stuck closed thermeostat or blown cylinder head gasket.
On a Mercury Sable V6 engine : firewall 1-----2-----3 4-----5-----6 front of vehicle > driver
On a 2000 Mercury Sable the engine cylinder locations are numbered : firewall 1-----2-----3 4-----5-----6 front of vehicle > driver
On a Mercury Sable V6 engine : firewall 1-----2-----3 4-----5-----6 front of vehicle > driver
If you have a 4 cylinder engine, you will have two. On a 6 cylinder, you will have four.
On a 2000 Mercury sable : firewall 1-----2-----3 4-----5-----6 front of vehicle > driver
( 6 ) one per engine cylinder
Click the link for a diagram.
If your check engine light is blinking that means an engine cylinder misfire has been detected - have the vehicle scanned for any trouble code(s) to help diagnose and then repair the problem