Hino P0128 Fault Code

TECHNICAL DESCRIPTION

• The thermostat prevents coolant from flowing into the radiator when the engine coolant temperature is less than 82°C {179.6°F}, in order to secure the engine warm-up function. When the temperature rises above 82°C {179.6°F}, the valve opens and allows coolant to flow into the radiator, thereby preventing the engine from overheating.
• If the thermostat fails and the valve stays open, it is difficult for the engine coolant temperature to rise, so the exhaust components that work after the engine coolant temperature has reached a certain value (warm-up) will not operate for a long time. The thermostat must therefore be diagnosed to see if it is working properly.

DESCRIPTION OF MALFUNCTION

In order to detect a malfunction of the thermostat, the above failure is determined when the actual engine coolant temperature is a certain amount lower than the predicted temperature.

DTC SET CONDITION

Check Conditions:

• Engine speed ≥ 700 r/min.
• Engine coolant temperature ≤ 52°C {125.6°F}.
• Engine operating time ≥ 1 minute.
• When the predicted engine coolant temperature reaches 82°C {179.6°F}. (The predicted temperature is calculated using the engine speed, fuel injection rate, EGR opening, intake temperature, and vehicle speed).

Judgment Criteria:

When the engine coolant temperature ≤ 71°C {159.8°F}.

RESET CONDITION

• Problem does not recur in the same driving cycle.
• Normal operation is restored when in subsequent driving cycles, readings are in the normal range within the above detection conditions.
• Three driving cycles must be deemed normal for MIL to be OFF.

INDICATION, WARNING OR SYSTEM CONTROL REGULATION WHEN THE DTC IS SET

• MIL: ON
• SVS light: OFF

PRE-INSPECTION WORK

Check that the battery voltage is in the normal range.

AFTER-INSPECTION WORK

• Clear all past DTCs.
• Check that no DTC is detected after test drive.

POSSIBLE CAUSES

• Inclusion of foreign matters in the thermostat.
• Damaged spring in the thermostat.
• Engine coolant temperature sensor failure.

INSPECTION PROCEDURE

TROUBLESHOOTING STEPS

Step 1) Inspect the engine coolant temperature sensor connector.

Check the connection of the engine coolant temperature sensor connector (Looseness and poor contact).

Were any failures found?

Step 2) Inspect the engine coolant temperature sensor.

1. Check the installation of the engine coolant temperature sensor.
2. Make sure there is no dirt or damage to the engine coolant temperature sensor.

Were any failures found?

Step 3) Inspect the engine coolant temperature sensor unit.

1. Set the starter switch to the “LOCK” position.
2. Disconnect the engine coolant temperature sensor connector.
3. Use the electrical tester to measure the resistance between the terminals of the engine coolant temperature sensor.

Do the measurements meet the standard value?

Step 4) Inspect the thermostat.

Check if the thermostat is damaged due to the incursion of foreign substances or if the spring is damaged.

Were any failures found?

Step 5) Inspect the thermostat valve-opening temperature.

Check valve opening temperature. Submerge in hot water (95°C {203°F}) and check if the valve lift is 13 mm {0.512 in.} or greater.

Were any failures found?

Step 6) Inspect the thermostat valve-open time.

Check valve opening time. After taking out from hot water, submerge in water and check that the valve closes within 5 minutes.

Were any failures found?

Step 7) Check the DTC detected (Engine ECU) [Diagnostic Tool].

1. Perform engine warm-up. (Engine coolant temperature: 60ºC {140ºF} or more).
2. Stop the engine and set the starter switch to the “LOCK” position.
3. Connect the vehicle to the diagnostic tool.
4. Set the starter switch to the “ON” position.
5. Select [Engine] and check if P0128 has been detected.

Has DTC P0128 been detected?

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