my car was tested for check engine light and code was need a thermostat.

Do you not trust the facility that performed the diagnosis? ... Is there another reason for you to think the diagnosis was incorrect?

Hello, the code may have been for an engine coolant temperature reading, but not "need a thermostat". There is no diagnostic trouble code that directly condemns a thermostat. All that said, the diagnostic steps may have revealed the need to replace the thermostat. What is your further question?