For 1990-2009 cars
Engine Data Sensors
OBD II Symbol Description Notice
Handling ESD Sensitive Parts Notice

### Circuit Description

The engine operates in open loop when first started. In open loop, the powertrain control module (PCM) 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 PCM will begin closed loop fuel control when the oxygen sensor signal is active enough and the ECT sensor reaches a predetermined coolant temperature. The PCM 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

• | 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 PCM illuminates the malfunction indicator lamp (MIL) after two consecutive ignition cycles in which the diagnostic runs with the fault active.

• | The PCM 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 PCM 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. Refer to Engine Coolant Temperature Sensor Replacement .

• | A poor connection at the PCM.

• | Check for the proper operation of the thermostat. Refer to Thermostat Diagnosis in Engine Cooling.

If the DTC P0125 is intermittent, driving the vehicle under the following conditions can verify whether the fault is present. Perform the scan tool Clear DTC Information function. Road test the vehicle while monitoring the DTC P0125 diagnostic on the scan tool under the Not Ran Since Code Cleared selection in the DTC Information menu. If a DTC P0125 appears in the Not Ran Since Code Cleared list, the P0125 diagnostic has not yet run. When the DTC P0125 does not appear in the Not Ran Since Code Cleared list, the P0125 diagnostic has run. If the MIL is NOT ILLUMINATED and there is no PENDING DTC Status in DTC Information, the P0125 diagnostic has passed. DTCs MUST BE CLEARED in order to view the CURRENT STATUS of the Not Ran Since Code Cleared list. DO NOT FORGET that the Not Ran Since Code Cleared list only indicates that the test has run, not whether the test passed or failed. The DTC Information screen must be checked for CURRENT or PENDING status, in order to determine the outcome of the diagnostic test involved.

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 you cannot duplicate the DTC P0125, 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 to 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. This step compares the ECT temperature to the IAT temperature. This inspection is to be performed when the vehicle has reached ambient room temperature. At that time both the surrounding air temperature and the engine coolant temperature are nearly equal. If the ECT sensor and circuit are OK, both sensors should indicate the same temperature.

3. Fault not present indicates the condition that caused a DTC P0125 to set is intermittent and not currently present. If no other DTCs are stored, refer to Diagnostic Aids for additional information on diagnosing an intermittent P0125.

4. After performing the scan tool Clear DTC Information function, the DTC P0125 diagnostic can be monitored on the scan tool under the Not Ran Since Code Cleared selection in the DTC Information menu. If a DTC P0125 appears in the Not Ran Since Code Cleared list, the P0125 diagnostic has not yet run. When the DTC P0125 does not appear in the Not Ran Since Code Cleared list, the P0125 diagnostic has run. If the MIL is NOT ILLUMINATED and there is no PENDING DTC Status in DTC Information, the P0125 diagnostic has passed. DTCs MUST BE CLEARED in order to view the CURRENT STATUS of the Not Ran Since Code Cleared list. DO NOT FORGET that the Not Ran Since Code Cleared list only indicates that the test has run, not whether the test passed or failed. The DTC Information screen must be checked for CURRENT or PENDING status, in order to determine the outcome of the diagnostic test involved.

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 temperatures.
2. Turn ON the ignition, leaving the engine OFF.
3. Compare the ECT sensor temperature reading to the IAT sensor temperature reading on a scan tool.

Are the ECT and the IAT readings within 2 or 3 degrees of each other?

--

Go to Step 3

Go to Step 6

3 |

1. Start the engine and immediately observe the ECT on the scan tool.
2. Continue to observe the ECT while idling the engine for 10 minutes.

Did the ECT indicated on the scan tool increase by more than the specified value within the 10 minutes?

20°C (68°F)

Go to Step 5

Go to Step 4

4 | Are there any ECT sensor DTCs set? | -- | Go to Diagnostic Trouble Code (DTC) List | Go to Step 8
5 | The fault is not present. Are there any DTCs stored that have not been diagnosed? | -- | Go to Diagnostic Trouble Code (DTC) List | Go to Diagnostic Aids
6 |

1. Measure the resistance of the ECT sensor with a DMM.
2. Compare the measured resistance of the ECT sensor with the resistance values in the Temperature vs Resistance table. Refer to Temperature Versus Resistance .

Was the resistance of the ECT sensor within specifications?

--

Go to Step 7

Go to Step 9

7 |

1. Check for any of the following conditions:

• | High resistance in the ECT sensor wiring

• | A poor connection at the ECT sensor

• | A poor connection at the PCM

2. Repair as necessary. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.

Was a repair necessary?

--

Go to Step 11

Go to Step 10

8 |

1. Inspect the engine cooling system for all of the following:

• | Correct coolant level. Refer to Cooling System Draining and Filling and Cooling System Leak Testing in Engine Cooling.

• | Proper operation of the cooling system. Refer to Engine Fails To Reach Normal Operating Temperature in Engine Cooling.

• | Proper operation of the thermostat. Refer to Thermostat Diagnosis in Engine Cooling.

2. Repair as necessary.

Is the action complete?

--

Go to Step 11

--

9 | Replace the ECT. Refer to Engine Coolant Temperature Sensor Replacement . Is the action complete? | -- | Go to Step 11 | --
10 | Important: The replacement PCM must be programmed. Replace the PCM. Refer to Powertrain Control Module Replacement/Programming . Is the action complete? | -- | Go to Step 11 | --
11 |

1. Perform the scan tool Clear DTC Information function and road test the vehicle within the Freeze Frame conditions that set the DTC or until the P0125 diagnostic has run.
2. Review the scan tool data and check for DTCs. The repair is complete if no DTCs are stored.

Are any DTCs displayed on the scan tool?

--

Go to Diagnostic Trouble Code (DTC) List

System OK