GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Geo 🢒 1996 🢒 Tracker - 4WD 🢒 DTC 🢒 DTC P0125 Engine Coolant Temperature (ECT) Insufficient for Closed Loop Fuel Control

Schematic Reference: Engine Controls Schematics

Circuit Description

The engine operates in open loop when first started. In open loop the engine control module (ECM) ignores the oxygen sensor signal and calculates the air/fuel ratio based on inputs from the engine coolant temperature sensor, the throttle position sensor, and the mass air flow sensor. The ECM will begin closed loop fuel control when the oxygen sensor signal is active enough and the ECT sensor reaches a predetermined coolant temperature. The ECM expects to begin closed loop fuel control within a specified amount of time.

Conditions for Running the DTC

The HO2S 1 output is varying, indicating that the sensor is hot enough to operate properly.

Conditions for Setting the DTC
    • An engine coolant temperature sufficient to allow closed loop operation 20°C (68°F) is not achieved within a predetermined time frame.
    • The diagnostic will run once per drive cycle.

Action Taken When the DTC Sets
    • The ECM illuminates the malfunction indicator lamp (MIL) after two consecutive ignition cycles in which the diagnostic runs with the fault active.
    • The ECM records the operating conditions at the time the diagnostic fails. This information is stored in the Freeze Frame buffer.

Conditions for Clearing the MIL/DTC
    • The MIL turns OFF after three consecutively passing trips without a fault present.
    • A History DTC clears after 40 consecutive warm-up cycles without a fault.
    •  Use the scan tool Clear DTC Information function or disconnect the ECM battery feed in order to clear the DTC.

Diagnostic Aids

Check for any of the following conditions:

    • A DTC P0125 that is set can indicate a skewed ECT sensor. Comparing the coolant temperature displayed on a scan tool with the actual engine coolant temperature measured with a thermometer may isolate the fault. If the displayed engine coolant temperature is not close to the actual engine coolant temperature, replace the ECT sensor.
    • A poor connection at the ECM.
    • Check for the proper operation of the thermostat.

An intermittent malfunction may be caused by a fault in the ECT sensor electrical circuit. Inspect the wiring harness and components for any of the following conditions:

    • Backed out terminals.
    • Improper mating of terminals.
    • Broken electrical connector locks.
    • Improperly formed or damaged terminals.
    • Faulty terminal to wire connections.
    • Physical damage to the wiring harness.
    • A broken wire inside the insulation.
    • Corrosion of electrical connections, splices, or terminals.

If the DTC P0125 cannot be duplicated, the information included in the Freeze Frame data can be useful in determining vehicle operating conditions when the DTC was first set.

Test Description

The numbers below refer to the step numbers in the Diagnostic Table.

  1. The Powertrain OBD System Check prompts the technician to complete some basic checks and store the freeze frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.

    2.

  2. This step compares the ECT temperature to the IAT temperature.

    3.

  3. This step measures actual coolant temperature and compares that to the ECT display on the scan tool.

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 cool to ambient temperature.
  2. Turn ON the ignition, leaving the engine OFF.
  3. Compare the ECT temperature display to the IAT temperature display on a scan tool.

Are the ECT and the IAT temperature readings close?

--

Go to Step 3

Go to Diagnostic Aids

3
  1. Start the engine.
  2. With the engine idling, monitor the ECT parameter on the scan tool.

Does the ECT display increase to above the specified value within 10 minutes?

20°C (68°F)

Go to Diagnostic Aids

Go to Step 4

4

Are any ECT sensor DTCs set?

--

Go to applicable DTC table

Go to Step 5

5

Check the cooling system coolant level. Fill the cooling system to manufacturer specifications if low.

Was coolant added to the cooling system?

--

Go to Step 10

Go to Step 6

6

Compare the ECT parameter on the scan tool to the actual coolant temperature measured with a thermometer.

Is the scan tool ECT display close to the measured temperature?

--

Go to Step 8

Go to Step 7

7
  1. Check for any of the following conditions:
    2. • High resistance in the ECT sensor wiring.
    • A poor connection at the ECT sensor.
    • A poor connection at the ECM.
  2. Repair as necessary.

Was a repair necessary?

--

Go to Step 10

Go to Step 9

8
  1. Inspect the cooling system for problems. Refer to Engine Thermostat Diagnosis in Cooling and Radiator Section.
  2. Repair the cooling system as necessary.

Is the action complete?

--

Go to Step 10

--

9

Replace the ECT sensor and recheck the ECT display on the scan tool. Refer to Engine Coolant Temperature Sensor Replacement .

Is the action complete?

--

Go to Step 10

--

10
  1. Perform the scan tool Clear DTC Information function and road test the vehicle within the Freeze Frame conditions that set the DTC.
  2. Review the scan tool data and check for DTCs.
  3. The repair is complete if no DTCs are stored.

Are any DTCs displayed on the scan tool?

--

Go to the Applicable DTC Table

System OK