DTC P0128:00 [MZI-3.7 (PCM CONNECTOR (3 TYPES))]

DTC P0128:00	Coolant thermostat (coolant temp below thermostat regulating temperature)
DETECTION CONDITION	• P0128:00 indicates that the thermostat monitor has not achieved the required engine operating temperature within a specified amount of time after starting the engine.
POSSIBLE CAUSE	• Insufficient warm up time • Low engine coolant level • Leaking or stuck open thermostat • Inoperative CHT sensor • PCM malfunction

DTC P0128:00

Coolant thermostat (coolant temp below thermostat regulating temperature)

DETECTION CONDITION

• P0128:00 indicates that the thermostat monitor has not achieved the required engine operating temperature within a specified amount of time after starting the engine.

POSSIBLE CAUSE

• Insufficient warm up time

• Low engine coolant level

• Leaking or stuck open thermostat

• Inoperative CHT sensor

• PCM malfunction

STEP	INSPECTION	ACTION
1	VERIFY FREEZE FRAME DATA HAS BEEN RECORDED • Has FREEZE FRAME DATA been recorded?	Yes	Go to the next step.
No	Record FREEZE FRAME DATA on the repair order, then go to the next step.
2	VERIFY RELATED SERVICE INFORMATION AVAILABILITY • Verify related Service Information availability. • Is any related Service Information available?	Yes	Perform repair or diagnosis according to the available Service Information. • If the vehicle is not repaired, go to the next step.
No	Go to the next step.
3	INSPECT ENGINE COOLANT LEVEL Warning • Never remove the cooling system cap or loosen the radiator drain plug while the engine is running, or when the engine and radiator are hot. Scalding engine coolant and steam may shoot out and cause serious injury. It may also damage the engine and cooling system. • Turn off the engine and wait until it is cool. Even then, be very careful when removing the cap. Wrap a thick cloth around it and slowly turn it counterclockwise to the first stop. Step back while the pressure escapes. • When you're sure all the pressure is gone, press down on the cap using the cloth, turn it, and then remove it. • Is the engine coolant fill level correct?	Yes	Go to the next step.
No	Inspect for a loss of engine coolant. Repair if necessary. (See ENGINE COOLANT LEVEL INSPECTION [MZI-3.7].) Go to Step 6.
4	INSPECT CHT SENSOR OPERATION • Run the engine until the engine temperature stabilizes. • Verify that the radiator hoses are hot and the cooling system is pressurized. • Access the PCM and monitor the CHT PID. • Is the temperature more than 77 °C {171 °F}?	Yes	Go to the next step.
No	Inspect the CHT sensor. Replace if necessary, then go to Step 6. (See CYLINDER HEAD TEMPERATURE (CHT) SENSOR INSPECTION [MZI-3.7].)
5	INSPECT THERMOSTAT • Cool down the engine. • Inspect the thermostat. (See THERMOSTAT INSPECTION [MZI-3.7].) • Is the thermostat normal?	Yes	The test is complete. Go to the next step.
No	Replace the thermostat, then go to the next step. (See THERMOSTAT REMOVAL/INSTALLATION [MZI-3.7].)
6	VERIFY TROUBLESHOOTING OF DTC P0128:00 HAS BEEN COMPLETED • Verify that all disconnected connectors are reconnected. • Turn the ignition switch to the ON position (Engine off). • Clear the DTC from the PCM memory using the M-MDS. • Cool down the engine and radiator. • Start the engine and run it until the engine temperature stabilizes. • Retrieve DTCs using the M-MDS. • Is the PENDING CODE for this DTC present?	Yes	Repeat the inspection from Step 1. • If the malfunction recurs, replace the PCM. (See PCM REMOVAL/INSTALLATION [MZI-3.7].) Go to the next step.
No	Go to the next step.
7	VERIFY AFTER REPAIR PROCEDURE • Perform the “AFTER REPAIR PROCEDURE”. (See AFTER REPAIR PROCEDURE [MZI-3.7 (PCM CONNECTOR (3 TYPES))].) • Are any DTCs present?	Yes	Go to the applicable DTC inspection. (See DTC TABLE [MZI-3.7 (PCM CONNECTOR (3 TYPES))].)
No	Troubleshooting completed.

STEP

INSPECTION

ACTION

VERIFY FREEZE FRAME DATA HAS BEEN RECORDED

• Has FREEZE FRAME DATA been recorded?

Yes

Go to the next step.

Record FREEZE FRAME DATA on the repair order, then go to the next step.

VERIFY RELATED SERVICE INFORMATION AVAILABILITY

• Verify related Service Information availability.

• Is any related Service Information available?

Yes

Perform repair or diagnosis according to the available Service Information.

• If the vehicle is not repaired, go to the next step.

Go to the next step.

INSPECT ENGINE COOLANT LEVEL

Warning

• Never remove the cooling system cap or loosen the radiator drain plug while the engine is running, or when the engine and radiator are hot. Scalding engine coolant and steam may shoot out and cause serious injury. It may also damage the engine and cooling system.

• Turn off the engine and wait until it is cool. Even then, be very careful when removing the cap. Wrap a thick cloth around it and slowly turn it counterclockwise to the first stop. Step back while the pressure escapes.

• When you're sure all the pressure is gone, press down on the cap using the cloth, turn it, and then remove it.

• Is the engine coolant fill level correct?

Yes

Go to the next step.

Inspect for a loss of engine coolant. Repair if necessary.

(See ENGINE COOLANT LEVEL INSPECTION [MZI-3.7].)

Go to Step 6.

INSPECT CHT SENSOR OPERATION

• Run the engine until the engine temperature stabilizes.

• Verify that the radiator hoses are hot and the cooling system is pressurized.

• Access the PCM and monitor the CHT PID.

• Is the temperature more than 77 °C {171 °F}?