Advanced Diagnostics

Advanced Diagnostics - DTC P0128: Cooling System Malfunction






General Description
The thermostat is closed when the engine coolant temperature is low, and it stops the circulation of engine coolant to speed engine warm up. When the engine coolant temperature increases, the thermostat opens and circulates engine coolant to control its temperature. When the engine coolant temperature decreases, the opening area of the thermostat is reduced to regulate the engine coolant temperature. If the thermostat sticks open, engine warm up is delayed, and exhaust emissions are adversely affected. The powertrain control module (PCM) measures the rise in the coolant temperature after the engine starts at the engine block and at the radiator, and it estimates the characteristics of the engine coolant temperature by calculations based on those two temperatures and the driving conditions. When ECT 2 immediately increases from the starting value, it is defined as the thermostat stuck open. When ECT 2 does not increase to the specified value, it is defined as a thermostat malfunction.

Monitor Execution, Sequence, Duration, DTC Type, OBD Status






Enable Conditions






Malfunction Threshold
Malfunction determination 1:
If the difference between the current measured coolant temperature at the radiator (ECT 2) and the initial coolant temperature at the radiator (ECT 2) is at least 13 °F (7 °C) when the estimated coolant temperature at the engine (ECT 1) reaches 160 °F (71 °C), a malfunction is detected (thermostat stuck open); or if the coolant temperature at the radiator (ECT 2) only reaches 67 °F (19 °C), a malfunction is detected (thermostat malfunction).

Malfunction determination 2:
When the calculated engine coolant temperature at the engine (ECT 1) reaches 158 °F (70 °C) before the measured engine coolant temperature at the engine (ECT 1) reaches 158 °F (70 °C), a malfunction is detected.

Driving Pattern
1. Start the engine specified under Enable Conditions (see Engine off time , "Initial engine coolant temperature (ECT 2)", "Initial intake air temperature", and "The difference between initial intake air temperature and current intake air temperature").
2. Drive the vehicle at a speed between 15 - 75 mph (24 - 120 km/h) for at least 30 minutes.

- Drive the vehicle in this manner only if the traffic regulations and ambient conditions allow.

Diagnosis Details

Conditions for setting the DTC
When a malfunction is detected during the first drive cycle with the engine coolant temperature and intake air temperature at engine start-up within the specified temperature range, a Pending DTC is stored in the PCM memory. If the malfunction returns in the next (second) drive cycle with the engine coolant temperature and intake air temperature at engine start-up within the specified temperature range, the MIL comes on and a Confirmed DTC and the freeze data are stored.

Conditions for clearing the DTC
The MIL is cleared if the malfunction does not return in three consecutive trips in which the diagnostic runs. The MIL, the Pending DTC, the Confirmed DTC, and the freeze data can be cleared with the scan tool Clear command or by disconnecting the battery.