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For 1990-2009 cars only
Makes 🢒 Isuzu 🢒 1999 🢒 S/T Isuzu - 4WD 🢒 DTC 🢒 DTC P0125 Engine Coolant Temperature (ECT) Insufficient for Closed Loop Fuel Control

Object Number: 50791 Size: SF
Engine Controls Components
Cell 20: MAP, IAT, TPS and ECT Sensors
OBD II Symbol Description Notice
Powertrain Control Module Connector End Views
Handling ESD Sensitive Parts Notice
Handling ESD Sensitive Parts Notice

Circuit Description

When the vehicle is first started, the engine operates in an Open Loop operation, ignoring the oxygen sensor signal and calculating the air/fuel ratio based on inputs from the engine coolant temperature (ECT), the throttle position, and the manifold absolute pressure sensors only. The powertrain control module (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 based on ECT at engine start up.
    • The oxygen sensor (O2S 1) 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 engine start up.

Conditions for Running the DTC

Region 1 - Ambient Temperature Above 10°C (50°F)

    • The IAT is greater than 10°C (50°F).
    • The ECT is greater than 10°C (50°F).
    • The engine run time is greater than 120 seconds.
    • The maximum idle time is less than 90 seconds.

OR

Region 2 - Ambient Temperature Above -7°C (19°F)

    • The IAT is greater than -7°C (19°F).
    • The ECT is greater than -7°C (19°F).
    • The engine run time is greater than 300 seconds.
    • The maximum idle time is less than 225 seconds.

OR

Region 3 - Ambient Temperature Below -7°C (19°F)

    • The IAT is greater than -7°C (19°F).
    • The ECT is less than -7°C (19°F).
    • The engine run time is greater than 1350 seconds
    • The maximum idle time is less than 1013 seconds

The conditions below are present for all three regions.

    • No DTCs P0105, P0107, P0108, P0112, P0113, P0117, P0118, P0122, P0123, P0171, P0200, P0300, P0301, P0302, P0303, P0304, P0335, P0341, P0440, P0442, P0446, P0506, P0507, P0601, P0602, or P1441.
    • The minimum calculated airflow is less than 10 gps to be considered at idle for all three temperature regions.

Conditions for Setting the DTC

The conditions below are present for all three regions.

    • The ECT is less than 40°C (113°F).
    • The vehicle is in an Open Loop operation.

Action Taken When the DTC Sets
    • The malfunction indicator lamp (MIL) will illuminate after two consecutive ignition cycles in which the diagnostic runs with the malfunction present.
    • The PCM will record the operating conditions at the time that the diagnostic fails. This information will store in the Freeze Frame and Failure Records buffers.
    • A history DTC stores.

Conditions for Clearing the MIL/DTC
    • The malfunction indicator lamp (MIL) will turn OFF after 3 consecutive ignition cycles in which the diagnostic runs without a fault.
    • A history DTC will clear after 40 consecutive warm up cycles without a fault.
    • Use a scan tool to clear the DTCs.

Diagnostic Aids

A skewed ECT sensor or a stuck open thermostat will cause DTC P0125 to set.

An intermittent may be caused by a poor connection, rubbed through wire insulation or a wire broken inside the insulation.

    • Check for a poor connection or a damaged PCM harness. Inspect the ECT sensor signal circuit and the ground circuit terminals for the following conditions:
       - Improper mating
       - Broken locks
       - Improperly formed connectors
       - Damaged terminals
       - A poor terminal to wire connection
       - Damaged harness
    • Perform an intermittent test. If the connections and the harness check OK, monitor a digital voltmeter connected between ECT sensor signal circuit and ground circuit terminals while moving the related connectors and the wiring harness. If a fault is induced, the resistance reading will change. This may help in order to isolate the location of the malfunction.
    • Use the Temperature vs. Resistance table below in order to evaluate the possibility of a skewed sensor. Refer to Temperature Versus Resistance .

Test Description

The number(s) below refer(s) to the step number(s) on the Diagnostic Table.

  1. The Powertrain OBD System Check prompts you to complete some of the basic checks and to store the freeze frame and failure records data on the scan tool if applicable. This creates an electronic copy of the data captured when the malfunction occurred. The scan tool stores this data for later reference.

    2.

  2. Allow the engine to fully cool. The ECT sensor and the IAT sensor will read close to the ambient temperature in order to check for a possible skewed ECT sensor.

    3.

  3. Measure the engine coolant temperature with a thermometer in order to determine what the actual value the ECT sensor should be. Take into consideration if the engine has been operating and if the engine coolant has been warmed.

    4.

  4. This step simulates a DTC P0118. The PCM and wiring are OK if the PCM senses a change.

    5.

  5. Reprogram the replacement PCM and perform the crankshaft position system variation learn procedure. Refer to the latest Isuzu Technical Communication System (ITCS) information for PCM programming and the Crankshaft Position System Variation Learn for the crankshaft position system variation learn procedure.

DTC P0125-Engine Coolant Temperature (ECT) Time to Closed Loop Fuel Control

Step

Action

Value(s)

Yes

No

1

Did you perform the Powertrain On-Board Diagnostic (OBD) System Check?

--

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

2

  1. Allow the Engine to fully cool to the ambient temperature.
  2. Turn ON the ignition switch leaving the engine OFF.
  3. Install a scan tool.
  4. Compare the ECT sensor reading to the IAT sensor readings.

Are the temperature readings close?

--

Go to Step 5

Go to Step 3

3

  1. Turn OFF the ignition switch.
  2. Disconnect the ECT sensor electrical connector.
  3. Measure the resistance across the ECT sensor terminals with the DMM.
  4. Check the ECT sensor value to the actual coolant temperature with the Temperature vs. Resistance table, refer to Fuel Injector Solenoid Coil Test - Engine Coolant Temperature Outside 10-35 Degrees C (50-95 Degrees F) .

Does the ECT sensor accurately reflect the actual engine coolant temperature?

--

Go to Step 4

Go to Step 15

4

Is the ECT sensor value close to the IAT sensor reading on the scan tool?

--

Go to Step 5

Go to Step 2

5

  1. Disconnect the ECT sensor electrical connector.
  2. Turn ON the ignition switch leaving the engine OFF.

Is the ECT sensor value less than the specified value?

-30°C (-22°F)

Go to Step 6

Go to Step 14

6

Connect a jumper wire between the following circuits at the ECT sensor electrical connector.

    • ECT sensor signal circuit terminal
    • ECT sensor ground terminal

Is the ECT sensor value greater than the specified value?

130°C (266°F)

Go to Step 7

Go to Step 8

7
  1. Check the ECT sensor electrical connector for a poor electrical connection
  2. Repair the circuit as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a repair necessary?

--

Go to Step 16

Go to Step 9

8
  1. Disconnect the jumper wire from the ECT sensor ground circuit electrical connector.
  2. Probe the jumper wire to a known good ground.

Is the ECT sensor value greater than the specified value?

130°C (266°F)

Go to Step 11

Go to Step 12

9
  1. Turn OFF the ignition switch.
  2. Reconnect the ECT sensor electrical connector.
  3. Start the engine.
  4. Operate the engine for five minutes.
  5. Observe ECT on the scan tool. The temperature should rise more than the specified temperature

Does the temperature rise more than the specified temperature?

5°C (9°F)

Go to Step 10

Go to Step 15

10
  1. Turn OFF the ignition switch.
  2. Disconnect the ECT electrical connector.
  3. Disconnect the PCM electrical connectors.
  4. Turn ON the ignition switch.
  5. Connect the DMM to ground.
  6. Probe the ECT sensor ground circuit with the DMM.

Does the DMM read a voltage greater than the specified value?

0.5 volt

Go to Diagnostic Aids

Go to Step 13

11
  1. Check the ECT sensor ground circuit for an open or a poor connection at the PCM.
  2. Repair the circuit or connection as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a repair necessary?

--

Go to Step 16

Go to Step 14

12
  1. Check the ECT sensor signal circuit for an open or a poor connection at the PCM.
  2. Repair the circuit or connection as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a repair necessary?

--

Go to Step 16

Go to Step 14

13

Repair the ECT sensor ground circuit for a short to voltage. Refer to Wiring Repairs in Wiring Systesms.

Is the action complete?

--

Go to Step 16

--

14

Replace the PCM. Refer to the Powertrain Control Module Replacement .

Is the action complete?

--

Go to Step 16

--

15
  1. Turn OFF the ignition switch.
  2. Replace the ECT sensor. Refer to the Engine Coolant Temperature Sensor Replacement .

Is the action complete?

--

Go to Step 16

--

16
  1. Clear the DTCs with the scan tool.
  2. Start the engine.
  3. Idle the engine at the normal operating temperature.
  4. Operate the vehicle within the conditions for setting this DTC as specified in the supporting text.

Does the scan tool indicate that this diagnostic ran and passed?

--

Go to Step 17

Go to Step 2

17

Check to see if any additional DTCs are set.

Does the scan tool display any DTCs that you have not diagnosed?

--

Go to applicable DTC table

System OK