Our Dart has a P0128 code that goes off intermittently. If we reset it may not go off again for a week or 2. I know it is typically a thermostat or sensor. It appears to have good heat and the temp gauge comes up just shy of the half way mark. Some one stated we should try the sensor in that case. Can this be changed with out draining the entire system? Will changing it introduce too much air into the system?