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 1 does not reach a specified value at a certain condition, it is defined as a thermostat malfunction.
Monitor Execution, Sequence, Duration, DTC Type, OBD Status
| Execution | Once per driving cycle |
| Sequence | None |
| Duration | Depending on the driving conditions |
| DTC Type | Two drive cycles, MIL on |
| OBD Status | PASSED/FAILED/NOT COMPLETED (STILL TESTING) |
Enable Conditions
| Condition | Minimum | Maximum |
|---|---|---|
| Engine off time | 6 hours | - |
| Initial intake air temperature [IAT SENSOR (1)]* 1 |
20 °F (-7 °C) | 123 °F (50 °C) |
| Estimated initial outside air temperature* 2 | 20 °F (-7 °C) | - |
| Initial engine coolant temperature (ECT 2) [ECT SENSOR 2]* 1 |
20 °F (-7 °C) | 123 °F (50 °C) |
| Initial engine coolant temperature (ECT1) [IAT SENSOR (1)]* 2 |
- | 123 °F (50 °C) |
| The difference between initial intake air temperature [IAT SENSOR (1)] and current intake air temperature [IAT SENSOR (1)] |
- | 3 °F (2 °C)* |
| 7 °F (4 °C)**, * 1 9 °F (5 °C)**, * 2 |
*: Intake air temperature decrease judgment when there is no driving record of 25 mph (40 km/h) in this drive cycle.
**: Intake air temperature decrease judgment when there is a driving record of 25 mph (40 km/h) in this drive cycle.
*1:: '14-15
*2:: '17
[ ]: HDS Parameter
Malfunction Threshold
Malfunction determination 1:
When the change in the engine coolant temperature at the radiator (ECT 2) (except for when the engine is stopped) has exceeded the criterion under any one of the following conditions:
- When the estimated engine coolant temperature [EST ECT1] reached to 162 °F (72 °C).
- When the estimated engine coolant temperature [EST ECT1] changed 36 °F (20 °C).
| Elapsed time after starting the engine | Temperature change at the radiator (ECT 2) | |
|---|---|---|
| Judgment temperature | 0 second* 1, * 2 | More than 13 °F (7 °C) |
| 50 seconds* 1 | More than 13 °F (7 °C) | |
| 2 minutes 10 seconds* 1 | More than 13 °F (7 °C) | |
| 4 minutes 30 seconds* 1 | More than 13 °F (7 °C) | |
| 8 minutes 20 seconds* 1 | More than 15 °F (8 °C) | |
| 11 minutes 40 seconds* 1, * 2 | More than 15°F (8 °C) |
Malfunction determination 2:
The estimated engine coolant temperature [EST ECT1] reaches to 158 °F (70 °C) before the actual engine coolant temperature at the engine (ECT 1) [ECT SENSOR 1] reaches to 158 °F (70 °C).
Possible Cause
NOTE: The causes shown may not be a complete list of all potential problems, and it is possible that there may be other causes.
- Thermostat failure
- Engine coolant temperature (ECT) sensor 1 failure
- Engine coolant temperature (ECT 2) sensor 2 failure
- Intake air temperature (IAT) sensor 1 failure
- Intake air temperature (IAT 2) sensor 2 failure
- Manifold absolute pressure (MAP) sensor failure
- Crankshaft position (CKP) sensor failure
- Barometric pressure (BARO) sensor failure
- Ignition system failure
- Fuel supply system failure
- Radiator fan failure
- VTEC system failure
- Hood opened
- EGR system failure
Confirmation Procedure: '14-15
Operating Condition
- 1.
Enter the maintenance mode, then start the engine under Enable Conditions (see "Engine off time", "Initial intake air temperature [IAT SENSOR (1)]", "Initial engine coolant temperature (ECT 2) [ECT SENSOR 2]", and "The difference between initial intake air temperature[IAT SENSOR (1)] and current intake air temperature [IAT SENSOR (1)]").
- 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.
With the HDS
None.
Confirmation Procedure: '17
Operating Condition
- 1.
Enter the maintenance mode, then start the engine under Enable Conditions (see "Engine off time", "Estimated initial outside air temperature", "Initial engine coolant temperature (ECT 1) [ECT SENSOR 1]", and "The difference between initial intake air temperature [IAT SENSOR (1)] and current intake air temperature [IAT SENSOR (1)]").
- 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.
With the HDS
None.
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 12 volt battery.