DTC P0128

DTC P0128:  Cooling System Malfunction

General Description 

Courtesy of HONDA, U.S.A., INC.

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 

Execution	Once per driving cycle
Sequence	None
Duration	Depending on the driving conditions
DTC Type	Two drive cycles, MIL on

Enable Conditions 

Condition	Minimum	Maximum
Engine off time	6 hours	-
Estimated initial outside air temperature	20 deg.F (-7 deg.C)	-
Initial engine coolant temperature (ECT 1) [ECT Sensor 1]	-	122 deg.F (50 deg.C)
The difference between initial intake air temperature [IAT Sensor (1)] and current intake air temperature [IAT Sensor (1)]	-	4 deg.F (2 deg.C)*
		9 deg.F (5 deg.C)**

*: 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.

[ ]: HDS Parameter

Malfunction Threshold 

Malfunction determination 1:

The difference between the current engine coolant temperature at the radiator (ECT 2) [ECT Sensor 2] and the initial engine coolant temperature at the radiator (ECT 2) [ECT Sensor 2] is at least 4 deg.F (2 deg.C) when either of these condition is met:

• When the estimated engine coolant temperature [EST ECT1] reached to 162 deg.F (72 deg.C).
• When the estimated engine coolant temperature [EST ECT1] changed 36 deg.F (20 deg.C).

Malfunction determination 2:

The estimated engine coolant temperature [EST ECT1] reaches to 158 deg.F (70 deg.C) before the actual engine coolant temperature at the engine (ECT 1) [ECT Sensor 1] reaches to 158 deg.F (70 deg.C).

Possible Cause 

Confirmation Procedure 

Operating Condition 

1. 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, the freeze data, and the on-board snapshot 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, the freeze data, and the on-board snapshot can be cleared with the scan tool Clear command or by disconnecting the 12 volt battery.