P0128

P0128

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 ECM 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 ECM 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, ECM 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:

Monitor DTC Status
1. Connect scantool to Data Link Connector (DLC).
2. IG "ON".
3. Select "Diagnostic Trouble Codes (DTCs)" mode, and then Press F4 (DTAL) to check DTC's information from the DTCs menu.
4. Read "DTC Status" parameter.






5. Is parameter displayed "Present fault"?

YES - Substitute with a known - good Thermostat and check for proper operation.
If the problem is corrected, Go to "Verification of Vehicle Repair" procedure.

NO - Fault is intermittent, go to "Verification of Vehicle Repair" procedure.

Component Inspection
1. Remove the thermostat and check the following items:
1. Stuck or damaged
2. Verify the temperature at which the valve is open
- Valve opening temperature : 82 °C (177 °F)
- Full opening temperature : 95 °C (205 °F)

2. Is the thermostat normal?

YES - 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.

NO - 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. Monitor and record the Freeze Frame Data for the Diagnostic Trouble Code (DTC) which has been diagnosed.
2. Using a Scantool, Clear the DTCs
3. Operate the vehicle within conditions noted in the freeze frame data or enable conditions
4. Monitor that all readiness test have been verified as "Complete"
5. Are any DTCs present?

YES - Go to the applicable troubleshooting procedure.

NO - System is performing to specification at this time.