Problem: This vehicle came to the shop because the temp gauge intermittently dropped to 165° F after engine had completely warmed-up. The MIL (malfunction indicator light) was on.
Details: The tech connected a scan tool and pulled a P0128 DTC (Engine coolant temperature sensor does not meet the minimum thermostat regulating temperature in the pre-set amount of time). The technician first replaced the ECT (Engine coolant temperature) sensor and the thermostat with OEM parts but the problem remained. On advice from the Tech-Assist consultant, the technician checked the actual engine temperature with a non-contact thermometer (laser infrared type) and compared that temperature with the scan tool’s ECT PID. It was the same and below the required threshold temperature.
Confirmed Repair: Because the technician had already replaced the thermostat, the next possible cause was that an incorrect radiator had been installed. NOTE: Some replacement radiators may be missing a restricted orifice that is built into the radiator tank, which is located in the radiator hose fitting that feeds coolant to the surge tank. The engine may not reach operating temperature or it may set DTC P0128 due to excessive coolant flow to the radiator surge tank.
NOTE: If the actual temperature is correct and the ECT sensor PID is incorrect then check the electrical circuit and the connector pins for the ECT sensor. If the circuit and connector are good, you may have a bad “New” sensor.
The tech confirmed that there was no restrictor in the radiator fitting and that the radiator had been recently replaced. He replaced the radiator with a different brand that had the restrictor and the problem was solved.