seen ECM software updates (Figure 9) which are supposed to "increase robustness of environmental temperature model." A reflash however, did not repair this vehicle. e. Lowering the heat to your pan / water container should then reverse the process and allow the wax in the sensing section of the thermostat to contract. The stat's spring should then slowly close it. Failure to open or close in this manner means the thermostat is faulty. Finally Repaired . . . with a THIRD Thermostat! Some technicians have reported that even new OEM thermostats can fail the ECM's thermostat monitor and set P0128 DTCs. Most of them had repaired the vehicle with an aftermarket thermostat. Mike replaced the second new OEM stat with a leading aftermarket brand and cleared the DTC. It did not reset. Some of Mike's fellow technicians suggested he look at GM's exact DTC parameters (Figure 8) to see how the vehicle's ECT and ambient temperatures were moving prior to the DTC setting. A couple of techs experienced in programming modules had Diagnosing P0128 Thermostat Performance DTCs GM As stated before, a P0128 is a DTC that can set if the cooling system temperature doesn't rise at the correct rate. The very first thing to check is of course coolant level. The ECT sensor is the ECM's input to report the cooling system's temperature. If it's not completely immersed in coolant (as in the case of a cooling system low on coolant), it can't accurately report the engine's temperature. If you're wondering exactly how this DTC sets, you can find quite a bit in OEM documents like the GM DTC parameter chart (Figure 8) and the Ford OBD-II documents which are often used to obtain Mode $06 information. GM Figure 8: GM DTC Criteria / Enabling Conditions Chart. Figure 9: GM calibration update for P0128? A Google search for "GM CAL ID" gets you to the no charge https://tis2web.service.gm.com/tis2web website where the vehicle's VIN can be entered. Selecting ECM, we see there has been a calibration update for the diagnostic section of the ECM's software to "increase robustness of environmental temperature model." That's not exactly a smoking gun to lead you to try a calibration update quite yet, but if you've done all you can do . . . try the updated flash! April 2020 4 MACS Service Reportshttps://tis2web.service.gm.com/tis2web