A circuit high fault can be caused by the following things according to the FSM:
- ECT sensor voltage above 4.96 volts
- Faulty ECT sensor
- ECT signal shorted to battery voltage
- ECT signal circuit open
- Sensor ground circuit open
- Faulty PCM
If the thermostat caused the code, I would expect a P0125 or P0128. Having said that, you should put the correct temperature thermostat in the vehicle for it to run properly and go into closed-loop operation as quickly as possible.
I would carefully check the connector at the Engine Coolant Temperature sensor - you may have a bent pin, a pushed out terminal, debris/crud in the connector causing poor contact, etc.