For 1990-2009 cars
Engine Controls Components
HO2S 1 and HO2S 2
OBD II Symbol Description Notice
Handling ESD Sensitive Parts Notice

### Circuit Description

A three-way catalytic converter is used in order to control emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides of Nitrogen (NOx). The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting it to nitrogen. The powertrain control module (PCM) has the ability to monitor this process using the HO2S 1 and the HO2S 2. The HO2S 1 produces an output signal which indicates the amount of oxygen present in the exhaust gas entering the three- way catalytic converter. The HO2S 2 produces an output signal which indicates the oxygen storage capacity of the catalyst, indicating the ability of the catalyst to efficiently convert the exhaust gases. If the catalyst is operating efficiently, the HO2S 1 signal will be far more active than the signal produced by the HO2S 2 . If the HO2S 2 signal voltage remains at or near the 450 mV bias for an extended period of time, DTC P0140 will be set.

### Conditions for Running the DTC

• | No active TP, MAP, MAF, IAT, ECT, or AIR Sensor, Fuel Injector Circuit, or EVAP DTCs are present.

• | The AIR pump is commanded OFF.

• | The ignition voltage is between 9.0-18.0 volts.

• | The engine coolant temperature is more than 50°C (136°F).

• | The engine run time is more than 200 seconds.

### Conditions for Setting the DTC

HO2S 2 signal voltage remains between 412 mV-499 mV for more than 100 seconds.

### Action Taken When the DTC Sets

• | The PCM will illuminate the malfunction indicator lamp (MIL) during the second consecutive trip in which the diagnostic test has been run and failed.

• | The PCM will store conditions which were present when the DTC set as Freeze Frame/Failure Records data.

### Conditions for Clearing the MIL/DTC

• | The PCM will turn OFF the malfunction indicator lamp (MIL) during the third consecutive trip in which the diagnostic has run and passed.

• | The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.

• | The DTC can be cleared by using a scan tool.

### Diagnostic Aids

Inspect for the following conditions:

• | Corroded exhaust flange bolts -- Using a DMM, ensure that continuity exists between the engine block and the heated oxygen sensor shell. If resistance is excessively high, replace the corroded exhaust flange, attaching hardware as necessary.

• | A malfunctioning HO2S heater or heater circuit -- With the ignition ON and the engine not running, the HO2S voltage displayed on a scan tool should gradually drop to less than 250 mV or rise to more than 600 mV. If the voltage does not change, disconnect the HO2S and connect a test lamp between the HO2S ignition feed and the heater ground circuits. If the test lamp does not illuminate, repair the open ignition feed or the sensor ground circuit as necessary. If the test lamp illuminates and the HO2S signal and low circuits are OK, replace the affected HO2S. Refer to Heated Oxygen Sensor Replacement .

Important : Remove any debris from the connector surfaces before servicing a component. Inspect the connector gaskets when diagnosing or replacing a component. Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion.

• | Faulty terminal connections

| Inspect the harness connectors for backed-out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal-to-wire connections. Use a corresponding mating terminal order to test for proper tension. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

• | A damaged harness

| Inspect the wiring harness for damage. If the harness inspection does not reveal a problem, observe the display on the scan tool while moving the connectors and the wiring harnesses that are related to the sensor. A change in the scan tool display may indicate the location of the fault. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

• | Inspect the PCM and the engine grounds for clean and secure connections.

If the DTC is determined to be intermittent, reviewing the Failure Records can be useful in determining when the DTC was last set.

### Test Description

The numbers below refer to the step numbers on the diagnostic table:

1. If the DTC P0140 test passes while the Fail Records conditions are being duplicated, an intermittent condition is indicated.

   Reviewing the Failure Records vehicle mileage since the diagnostic test last failed may help to determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.

2. This vehicle is equipped with a PCM which utilizes an Electrically Erasable Programmable Read Only Memory (EEPROM). When the PCM is being replaced, the new PCM must be programmed.

Step | Action | Values | 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. Ensure that the engine is at operating temperature.
2. Operate the engine above 1200 RPM for two minutes while monitoring HO2S 2 voltage on the scan tool HO2S data list.

Does the scan tool indicate that the HO2S 2 voltage varies outside the specified values?

412-499 mV

Go to Step 3

Go to Step 4

3 |

1. Turn ON the ignition.
2. Review and record scan tool Failure Records data and note the parameters.
3. Operate the vehicle within the Failure Records conditions as noted.
4. Using a scan tool, monitor Specific DTC info for DTC P0140 until the DTC P0140 test runs.

Does the scan tool indicate that the DTC failed this ignition?

--

Go to Step 4

Go to Diagnostic Aids

4 |

1. Turn ON the ignition.
2. Disconnect HO2S 2 and jumper the HO2S signal and low circuits on the PCM side to a ground.
3. Using a scan tool, monitor the HO2S 2 voltage.

Is HO2S 2 voltage less than the specified value?

150 mV

Go to Step 8

Go to Step 5

5 |

1. Remove the jumper wire.
2. Using a DMM, measure voltage between the HO2S 2 signal circuit, PCM side, and the HO2S 2 heater ground circuit.

Is HO2S 2 signal voltage above the specified value?

950 mV

Go to Step 6

Go to Step 7

6 |

1. Turn OFF the ignition.
2. Disconnect the PCM.
3. Check the continuity of the HO2S 2 low circuit. Refer to Testing for Continuity in Wiring Systems.
4. If the HO2S 2 low circuit measures more than 5 ohms, repair the open or faulty connection as necessary. Refer to Testing for Intermittent Conditions and Poor Connections , and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 13

Go to Step 9

7 |

1. Turn OFF the ignition.
2. Disconnect the PCM.
3. Test the continuity of the HO2S 2 signal circuit. Refer to Testing for Continuity in Wiring Systems.
4. If the HO2S 2 signal circuit measures more than 5 ohms, repair the open or faulty connection as necessary. Refer to Testing for Intermittent Conditions and Poor Connections , and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 13

Go to Step 10

8 | Test for a faulty HO2S 2 signal or low circuit terminal connection at the HO2S 2 harness connector. Refer to Testing for Intermittent Conditions and Poor Connections , and Connector Repairs in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 13 | Go to Step 11
9 | Inspect for a faulty HO2S 2 low circuit terminal connection at the PCM. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 13 | Go to Step 12
10 | Inspect for a faulty HO2S 2 signal circuit terminal connection at the PCM. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems. Did you find and correct the condition? | -- | Go to Step 13 | Go to Step 12
11 | Replace HO2S 2. Refer to Heated Oxygen Sensor Replacement . Did you complete the replacement? | -- | Go to Step 13 | --
12 | Important : The replacement PCM must be programmed. Replace the PCM. Refer to Powertrain Control Module Replacement/Programming . Did you complete the replacement? | -- | Go to Step 13 | --
13 |

1. Turn ON the ignition.
2. Review and record the scan tool Failure Records data.
3. Clear the DTCs.
4. Operate the vehicle within the Failure Records conditions as noted.
5. Use a scan tool to monitor Specific DTC information for DTC P0140 until the DTC P0140 test runs.

Does the scan tool indicate that the DTC failed this ignition?

--

Go to Step 2

System OK