Fuel System > Engine Control System > P0128 Coolant Thermostat (Coolant Temperature below Thermostat Regulating Temperature)

General Description

When the engine is first started, the fuel control system operates in an open loop operation, ignoring the HO2S(Heated Oxygen Sensor) signal and calculating the air/fuel ratio based on inputs from the engine coolant temperature, the throttle position and the mass air flow sensors. The PCM will begin by using the oxygen sensor signal for controlling the fuel delivery(closed loop) when the following conditions are met:

-	The engine has run a minimum amount of time at engine start up.

-	The HO2S has a varying voltage output showing that it is hot enough to operate properly.

-	The ECT has increased a minimum amount based on the ECT at start up.

The PCM monitors the amount of time for the cooling system to achieve closed loop operating temperature. The engine coolant temperature sufficient to allow closed loop operation is not achieved within a predetermined time frame, PCM sets DTC.

DTC Description

Checking time and coolant temperature under detecting condition, if the coolant temperature dose not reach the target temperature as the test timer reaches the threshold time, PCM sets P0128. MIL(Malfunction Indication Lamp) turns on when the malfunction lasts till consecutive 2 driving cycle.

DTC Detecting Condition

Item

Detecting Condition

Possible Cause

DTC Strategy

•	Monitor the engine coolant temperature and time

•	Thermostat stuck

Enable condition

•	Engine running

•	Thermostat target temperature - Startup coolant temperature ≥ 20°C(36°F)

•	ntake air temperature > - 10°C( 14°F)

•	Engine Running

threshold value

•	The coolant temperature dose not reach the target temperature as the test timer reaches the threshold time.

diagnosis time

•	Once per driving cycle

MIL ON condition

•	2 driving cycles

Monitor DTC Status

Check DTC Status

(1)	Connect scantool to Data Link Connector(DLC).

(2)

IG "ON".

(3)	Select "DTC" button, and then Press "DTC Status" to check DTC's information from the DTCs menu.

(4)	Read "DTC Status" parameter.

(5)

Is parameter displayed "Present fault"?

	▶ Substitude with a known-good Thermostat and check for proper operation. If the problem is corrected, Go to "Verification of Vehicle Repair" procedure.
	▶ Fault is intermittent, go to "Verification of Vehicle Repair" procedure.

Component Inspection

Remove the thermostat and check the following items:

(1)	Stuck or damaged

(2)

Verify the temperature at which the valve is open

A.	Valve opening temperature : 82°C(177°F)

B.	Full opening temperature : 95°C(205°F)

Is the thermostat noraml?

	▶ Clear DTC and Test-drive under enable conditions above-mentioned. ▶ After the test, If this DTC is set, recheck it thoroughly. If not, it is intermittent fault, go to "Verification of vehicle Repair" procedure.
	▶ Substitute with a known - good Thermostat and check for proper operation. ▶ If the problem is corrected, Go to "Verification of Vehicle Repair"procedure.

Verification Of Vehicle Repair

After a repair, it is essential to verify that the fault has been corrected.

1.	Connect scantool and select "DTC" button.

2.	Press "DTC Status" button and confirm that "DTC Readiness Flag" indicates "Completed". If not, drive the vehicle within conditions noted in the freeze frame data or enable conditions

3.	Read "DTC Status" parameter

Is parameter displayed "History(Not Present) fault"?

	▶ System performing to specification at this time. Clear the DTC
	▶ Go to the applicable troubleshooting procedure.