Got a 2001 OBDII car which is throwing a PO128 code defined as Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature) and I've also seen it called a Thermostat Malfunction. I replaced the obvious Coolant Temp Sensor but no luck. My fans are running constantly, hot or cold. Have not tried replacing the Thermostat yet., but my temperature gauge in the dash seems to not work at all. It should read from 100 degrees and up... surely my engine is getting to 100 degrees even with a stuck thermostat??? But maybe not with the fans running and considering the temperature outside. IOW, how would a car detect that the thermostat is not working? Any ideas? Thanks!