For 1990-2009 cars
Engine Controls Components
Cell 20: HO2S
OBD II Symbol Description Notice
Handling ESD Sensitive Parts Notice

### Circuit Description

The heated oxygen sensors (HO2S) increase the amount of time the sensor spends in closed loop fuel control operation or catalyst monitoring operation. The oxygen sensor heater greatly reduces the amount of time required for the fuel control HO2S (sensor 1) to become active. The oxygen sensor heater is required by the post-catalyst HO2S (sensor 2) in order to maintain the high operating temperatures. The heater helps provide accurate oxygen content readings of the engine exhaust by the HO2S.

### Conditions for Running the DTC

The engine is running.

### Conditions for Setting the DTC

• | HO2S heater current exceeds 2 amps.

• | HO2S heater current is 0.2 amps or less.

### 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.

• | The PCM will turn the HO2S 1 heater OFF.

### 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 in order to clear the DTC.

### Diagnostic Aids

The HO2S Heater diagnostic checks for many of the faults that can cause a DTC P0135 to set. If the HO2S Heater test passes the cause of the DTC P0135 is probably intermittent. Clear the DTC information. Road test the vehicle while monitoring the HO2S Heater test in the MIL/System Status selection under System Information on the scan tool. When the HO2S Heater test indicates complete with a YES status, check for a DTC P0135 under Last Test Failed. If there is no DTC P0135 indicated, the DTC is intermittent and the HO2S Heater diagnostic is indicating OK at this time.

Check for a faulty electrical connection to the PCM.

An intermittent malfunction may be caused by a fault in the HO2S 1 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 wiring connections.

• | Physical damage to the wiring harness.

• | A broken wire inside the insulation.

• | Corrosion of electrical connections, splices, or terminals.

Repair any electrical circuit faults that were found. Refer to Wiring Repairs in Wiring Systems.

The information included in the Freeze Frame data can be useful in determining the vehicle operating conditions when the DTC first set.

### Test Description

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

1. The 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. This step checks whether the malfunction that caused the DTC P0135 is still present. Operating the vehicle in check mode enhances the PCM's diagnostic capabilities. The HO2S Heater diagnostic can be monitored on the scan tool under the MIL/System Status selection of System Information. When the HO2S Heater test displays a YES status (indicating that the oxygen sensor heater diagnostic is completed) check for a DTC P0135 in the Last Test Failed screen of the scan tool. If there is no DTC P0135 displayed the HO2S Heater diagnostic has run and passed, indicating that no malfunction was present this time. DTCs MUST BE CLEARED in order to view the CURRENT STATUS of the system diagnostics being performed. Do not forget that the MIL/System Status tests only indicate that the test has run, not whether the test passed or failed. The Last Test Failed screen must be checked for related DTCs in order to determine the outcome of the diagnostic test involved. For additional information on check mode operation refer to Powertrain Control Module Diagnosis .

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. Perform the scan tool Clear DTC Information function.
2. Place vehicle into Check Mode, using the scan tool.
3. Operate the vehicle within the Freeze Frame conditions as specified or until the HO2S Heater test is complete. Refer to the Test Description.

Did a DTC P0135 set?

--

Go to Step 3

Fault Not Present-Go to Diagnostic Aids

3 |

1. Turn ON the ignition, leaving the engine OFF.
2. Disconnect the HO2S 1 connector.
3. Probe the HO2S 1 sensor connector ignition feed circuit (engine harness side), using a test lamp connected to ground.

Does the test lamp illuminate?

--

Go to Step 4

Go to Step 8

4 |

1. Turn ON the ignition, leaving the engine OFF.
2. Probe the HO2S 1 heater control circuit (engine harness side) using a test lamp connected to B+.

Does the test lamp illuminate?

--

Go to Step 6

Go to Step 5

5 | Start and idle the engine. Does the test lamp illuminate? | -- | Go to Step 9 | Go to Step 7
6 |

1. Check the HO2S 1 heater control circuit for a short to ground,
2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a repair necessary?

--

Go to Step 12

Go to Step 11

7 |

1. Check the HO2S 1 heater control circuit for an open circuit.
2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a repair necessary?

--

Go to Step 12

Go to Step 11

8 |

1. Check for an open in the ignition feed circuit.
2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Is the action complete?

--

Go to Step 12

--

9 | Check connections at the HO2S 1 connector and repair as necessary. Was a repair necessary? | -- | Go to Step 12 | Go to Step 10
10 | Replace the HO2S 1. Refer to Heated Oxygen Sensor Replacement . Is the action complete? | -- | Go to Step 12 | --
11 | Replace the PCM. Refer to Powertrain Control Module Replacement . Is the action complete? | -- | Go to Step 12 | --
12 |

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. The repair is complete if no DTCs are stored.

Are any DTCs displayed on scan tool?

--

Go to the applicable DTC table

System OK