Thermostat Monitor
The Thermostat Monitor checks that the thermostat is operating properly by modeling Engine Coolant Temperature (ECT) based on engine fueling, engine speed, vehicle speed, and the ambient temperature. There are increment and decrement portions to the model; the increment is based on engine speed and fuel quantity, while the decrement is derived from calculated radiator efficiency based on coolant delta temp to ambient and vehicle speed. The model is delayed by 60 seconds after engine start to negate potential errors due to block heater use. It is also suspended while in catalyst warm-up mode due to errors in fuel quantity heat being contributed to the coolant.
Once that estimation reaches the thermostat start-to-open temperature, if the actual measured ECT has not reached a minimum warm-up temperature and the driver has not spent too much time in part fuel cut off (over 30%), too low load (over 70%), too high vehicle speed (over 70%), or too low vehicle speed (over 70%) - then the thermostat is determined to be stuck open.
When ECT drops below 70 degrees C, the thermostat model and monitor are re-initialized.
| DTCs | P0128 -Coolant Temp Below Thermostat Regulating Temperature | ||
| Monitor Execution | Continuous | ||
| Monitor Sequence | None | ||
| Sensors OK | Engine Coolant Temperature (ECT), Intake Air Temperature (IAT), Vehicle Speed (VS) | ||
| Typical Monitoring Duration | Nominal time it takes for engine to warm up to thermostat "Start-To-Open" temperature - see approximate times below.
NOTE:
Unified Drive Cycle is 23.9 minutes long
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| Ambient Temperature | Drive Cycle | Completion Time | |
| -7 deg C | Unified Drive Cycle x2 | 40 min | |
| 21 deg C | Unified Drive Cycle | 19 min | |
| 38 deg C | Unified Drive Cycle | 14 min | |
| Entry Condition | Minimum | Maximum |
| Modeled engine coolant temperature | 90 deg C | |
| Engine coolant temperature at start | -7 deg C | 54 deg C |
| Intake air temperature at start | -7 deg C | |
| Ratio of time that the vehicle speed is above, 85 km/hr, to the total monitoring time | 70% | |
| Ratio of time that the engine fueling is below 20 mg/str to the total monitoring time | 35% | |
| Ratio of time that the engine torque is below 60 n/m to the total monitoring time | 70% | |
| Ratio of time that the vehicle speed is below 45 km/hr to the total monitoring time | 70% |
| Measured Engine Coolant Temperature < 70.2 deg C when modeled coolant temp > 90 deg C |