Will a missing thermostat cause error code po-135?

No, a missing thermostat is not typically the cause of error code P0-135. This code generally indicates a problem with the oxygen sensor circuit, specifically the heater circuit for the sensor. Issues related to the thermostat may affect engine temperature and performance but would not directly trigger a P0-135 code. It's important to diagnose the oxygen sensor and its associated wiring to resolve this specific error.