Dodge Dakota (ND). Manual — part 645

P0125-INSUFFICIENT COOLANT TEMP FOR CLOSED-LOOP FUEL CONTROL (CONTINUED)

2.

THERMOSTAT OPERATION

NOTE: This test works best if performed on a cold engine (cold soak)
Ignition on, engine not running.

With a scan tool, read the Eng Coolant Tmp Deg value. If the engine was allowed to sit overnight (cold soak), the
temperature value should be a sensible value that is somewhere close to the ambient temperature.

NOTE: If engine coolant temperature is above 82°C (180°F), allow the engine to cool until 65°C (150°F) is
reached.
Start the Engine.

During engine warm-up monitor the Eng Coolant Tmp Deg value. The temp deg value change should be a smooth
transition from start up to normal operating temp 82°C (180°F). Also monitor the actual coolant temperature with a
thermometer.

NOTE: As the engine warms up to operating temperature, the actual coolant temperature (thermometer read-
ing) and the scan tool Eng Coolant Tmp Deg values should stay relatively close to each other.
Using the appropriate service information, determine the proper opening temperature of the thermostat.

Did the thermostat open at the proper temperature?

Yes

>> Go To 3

No

>> Replace the thermostat.

Perform POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)

3.

ECT SENSOR OPERATION

Ignition on, engine not running.

With a scan tool, read the Eng Coolant Tmp Deg value. If the engine was allowed to sit overnight (cold soak), the
temperature value should be a sensible value that is somewhere close to the ambient temperature.

NOTE: If engine coolant temperature is above 82°C (180°F), allow the engine to cool until 65°C (150°F) is
reached.
Start the Engine.

During engine warm-up monitor the Eng Coolant Tmp Deg value. The temp deg value change should be a smooth
transition from start up to normal operating temp 82°C (180°F). Also monitor the actual coolant temperature with a
thermometer.

NOTE: As the engine warms up to operating temperature, the actual coolant temperature (thermometer read-
ing) and the scan tools Eng Coolant Tmp Deg value should stay relatively close to each other.

Is the thermometer reading relatively close to the scan tool ECT reading?

Yes

>> Test Complete.

No

>> Replace the Engine Coolant Temperature Sensor.

Perform POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)

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ENGINE ELECTRICAL DIAGNOSTICS

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P0128-THERMOSTAT RATIONALITY

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ENGINE ELECTRICAL DIAGNOSTICS

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P0128-THERMOSTAT RATIONALITY (CONTINUED)

For the Engine circuit diagrams (Refer to 9 - ENGINE - SCHEMATICS AND DIAGRAMS).

For a complete wiring diagram Refer to Section 8W.

Theory of Operation

The PCM predicts what the engine coolant temperature should be, based on the engine coolant temperature at
start-up, ambient temperature and how the vehicle is subsequently driven. The predicted engine coolant tempera-
ture is compared to the Engine Coolant Temperature Sensor reading. The error between the two is calculated and
integrated with respect to time. When the Thermostat diagnostic runs, the integrated error is compared to a cali-
brated threshold and pass/fail is determined. Separate pass and fail thresholds are used in order to improve accu-
racy of the diagnostic.

•

When Monitored:
Engine running.

•

Set Condition:
The PCM predicts a coolant temperature value that it will compare to the actual coolant temperature. A sig-
nificant difference results in an error. Two Trip Fault. Three good trips to turn off the MIL.

Possible Causes

LOW COOLANT LEVEL

THERMOSTAT OPERATION

SIGNAL CIRCUIT SHORTED TO BATTERY VOLTAGE

TEMPERATURE SENSOR

SIGNAL CIRCUIT OPEN

(K900) SENSOR GROUND CIRCUIT OPEN

SIGNAL CIRCUIT SHORTED TO GROUND

SIGNAL CIRCUIT SHORTED TO (K900), (G931) SENSOR GROUND

OTHER POSSIBLE CAUSES

PCM

Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. (Refer to 9 - ENGINE -
DIAGNOSIS AND TESTING).

Diagnostic Test

1.

ACTIVE DTC

NOTE: If any ECT, AAT, CMP or CKP sensor DTCs have set along with P0128, diagnose them before con-
tinuing.

NOTE: Make sure that the Pinion Factor has been programmed correctly into the PCM.
Ignition on, engine not running.

With a scan tool, read DTCs.

Is the DTC active at this time?

Yes

>> Go To 2

No

>> Refer to the INTERMITTENT CONDITION Diagnostic Procedure.

Perform POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)

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ENGINE ELECTRICAL DIAGNOSTICS

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P0128-THERMOSTAT RATIONALITY (CONTINUED)

2.

LOW COOLANT LEVEL

NOTE: If a Engine Coolant Temperature (ECT) DTC is set along with this code, diagnose the ECT DTC first.

NOTE: Inspect the ECT terminals and related PCM terminals. Make sure the terminals are free from corro-
sion and damage.

NOTE: The best way to diagnose this DTC is to allow the vehicle to sit overnight outside in order to have
a totally cold soaked engine.

NOTE: Extremely cold outside ambient temperatures may have caused this DTC to set.

WARNING: Never open the cooling system when the engine is hot. The system is under pressure. Failure to
follow these instructions can result in personal injury including extreme burns, scalding, or death. Allow the
engine to cool before opening the cooling system.
Check the coolant system to make sure that the coolant is in good condition and at the proper level.

Is the coolant level and condition OK?

Yes

>> Go To 3

No

>> Inspect the vehicle for a coolant leak and add the necessary amount of coolant.

Perform POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)

3.

THERMOSTAT OPERATION

NOTE: This test works best if performed on a cold engine (cold soak).
Ignition on, engine not running.

With a scan tool, read the ECT Deg value. If the engine was allowed to sit overnight (cold soak), the temperature
value should be a sensible value that is somewhere close to the ambient temperature.

NOTE: If engine coolant temperature is above 82°C (180°F), allow the engine to cool until 65°C (150°F) is
reached.
Start the Engine.

During engine warm-up, monitor the ECT Deg value. The temp deg value change should be a smooth transition
from start up to normal operating temp 82°C (180°F). Also monitor the actual coolant temperature with a thermom-
eter.

NOTE: As the engine warms up to operating temperature, the actual coolant temperature (thermometer read-
ing) and the scan tool, ECT Temperature value should stay relatively close to each other.
Using the appropriate service information, determine the proper opening temperature of the thermostat.

Did the thermostat open at the proper temperature?

Yes

>> Go To 4

No

>> Replace the thermostat.

Perform POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)

4.

AMBIENT AIR TEMP SENSOR OPERATION

Ignition on, engine not running.

With a scan tool, read and record the AAT Sensor Temperature value.

Using the DRB Temperature Probe #CH7050, or an equivalent temperature measuring tool, measure the ambient air
temperature near the AAT Sensor.

Is the AAT Sensor value with -15°C (5°F) of the temperature probe reading?

Yes

>> Go To 5

No

>> Go To 7

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