How do i fix my 1990 mustang code 66

SOURCE: CODE 66 LOW INPUT VOLTAGE

oxygen sensor

SOURCE: Why would my 1990 Mustang GT be runing extremely

The coolant temp. sensor you replaced - was the new one plastic or metal? There is a reported very high failure rate in the plastic sensors, and it is recommended that you go back to the metal style. Check the signal wire for the sensor - it should be less than .7 volts. If it is out of range, replace the sensor with a metal one.

It could also be caused by excessively high fuel pressure - caused by a sticking fuel pressure regulator, or a restricted fuel return line.
If you are able to check fuel pressure, it should have 35 - 45psi with key on, pump running - and 30 - 45 psi idling

Let me know how you make out!