For 1990-2009 cars

### Circuit Description

The DTC P0134 HO2S Circuit Insufficient Activity Sensor 1 diagnostic monitors the HO2S switching time. The heated oxygen sensor (HO2S) produces a voltage that varies between 100 mV and 900 mV under normal operating conditions. The powertrain control module (PCM) monitors this voltage and determines if the exhaust is indicating a lean or rich fuel mixture. If the voltage input at the PCM is about 100 mV the fuel mixture is lean, and if the voltage input is about 900 mV the fuel mixture is rich. The PCM constantly monitors the HO2S signal during closed loop operation and compensates for a rich or lean condition by decreasing or increasing fuel injector pulse width as necessary. If the HO2S 1 voltage remains at or near 450 mV for an extended period of time, DTC P0134 will be set.

### Conditions for Running the DTC

• | Engine coolant temperature (ECT) was more than -7°C (+19°F) at engine start up.

• | Intake air temperature (IAT) was more than -7°C (+19°F) at engine start up.

• | Barometric pressure is more than 75 kPa.

### Conditions for Setting the DTC

• | HO2S 1 voltage remains lower than or at 450 mV for 30 seconds.

• | Maximum HO2S 1 voltage remains less than 450 mv.

### Action Taken When the DTC Sets

• | The malfunction indicator lamp (MIL) illuminates 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 will be stored in the Freeze Frame buffer.

### Conditions for Clearing the MIL/DTC

• | The MIL will turn OFF after three consecutively passing cycles without a fault present.

• | A History DTC will clear after 40 consecutive warm-up cycles without a fault.

• | DTCs can be cleared by using a scan tool or by disconnecting the PCM battery feed.

### Diagnostic Aids

Check for any of the following conditions:

• | If DTCs other than Fuel Trim (P0171, P0172), Misfire (P0300 to P0304), EGR (P0400) and HO2S 1 (P0131 to P0134) are set, diagnosis those DTCs first.

• | Vacuum leaks -- Check for disconnected or damaged vacuum hoses and for vacuum leaks at the intake manifold, throttle body, EGR system, and crankcase ventilation system.

• | Exhaust leaks -- Check for exhaust leaks near the HO2S.

• | Fuel pressure -- The system will go rich or lean if pressure is too high or low. The PCM can compensate for minor changes, however, if fuel pressure is too high or low, a DTC may be set.

• | Check the EVAP canister for fuel saturation. If full of fuel, check canister control and hoses.

• | Check for a leaking fuel pressure regulator diaphragm by checking vacuum line to regulator for the presence of fuel.

• | A contaminated HO2S -- A contaminated oxygen sensor can fail to enter or remain in closed loop. Silicon contamination of the HO2S can cause a lazy HO2S voltage to be indicated. Silicone contamination is indicated by a powdery white deposit on the portion of the HO2S exposed to the exhaust stream. If contamination is found, determine the cause and correct the condition before replacing the HO2S.

• | A poor connection at the PCM -- Inspect the harness connectors for the following:

- | Backed out terminal

- | Improper mating

- | Broken locks

- | Improperly formed or damaged terminals

- | Poor terminal to wire connection

• | A damaged wiring harness -- Inspect the wiring harness for damage. If the harness appears to be OK, observe the HO2S 1 display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the HO2S 1 display will indicate the location of the fault.

• | Follow DTC P0171 or P0172 for fuel system checks.

An intermittent malfunction may be caused by a fault in the HO2S sensor circuit. Inspect the wiring harness and components for an intermittent condition. Refer to Intermittent Conditions .

If you cannot duplicate the DTC P0134, 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 Diagnostic System Check-Engine Controls 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 checks whether the HO2S is operating within the normal range. The vehicle must be at operating temperature.

3. This step checks whether the malfunction that caused the DTC P0134 is still present. Driving the vehicle under these conditions will verify that the fault is present. The DTC P0134 diagnostic can be monitored on the scan tool under the MIL/System Status selection of System Information. When the HO2S test displays a YES status (indicating that the purge system diagnostic is completed), check for a DTC P0134 in the Last Test Failed screen of the scan tool. If there is no DTC P0134 displayed the HO2S system 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.

4. This step checks the HO2S 1 signal circuit. The DMM supplies a safe (low current) voltage to the PCM HO2S signal circuit when connected to B+. The scan tool should display near the high limit (1000-1275 mV) of the HO2S input circuit, if the PCM and the signal wire are OK.

5. This step checks the HO2S 1 ground circuit for a poor connection or an open.

6. This step checks the HO2S 1 signal circuit.

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
Schematic Reference: Engine Controls Schematics
1 | Did you perform the Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | Go to Diagnostic System Check - Engine Controls
2 |

1. Install a scan tool.
2. Start the engine.
3. Run the engine under the following conditions for 1 minute or more:

• | ECT is more than 80°C (176°F)

• | Engine speed is more than 2,000 RPM

4. Observe the HO2S 1 voltage on the scan tool.

Does the HO2S 1 voltage switch below and above the specified value?

300 mV-600 mV

Go to Step 3

Go to Step 4

3 |

1. Perform the scan tool Clear DTC Information function.
2. Operate the vehicle within the Freeze Frame conditions as specified, or until the HO2S test is complete.

Did HO2S 1 voltage stay within the specified values?

300 mV-600 mV

Go to Step 4

Go to Diagnostic Aids

4 |

1. Disconnect the HO2S 1 electrical connector.
2. Probe the HO2S 1 ground circuit (PCM harness side), with a test lamp connected to B+.

Did the test lamp illuminate?

--

Go to Step 5

Go to Step 6

5 |

1. Use the DMM and select the millivolt DC scale.
2. Connect one lead of the DMM to B+.
3. Connect the other lead of the DMM to the HO2S 1 signal circuit.

Does the scan tool indicate an HO2S 1 voltage above the specified value?

1000 mV

Go to Step 8

Go to Step 7

6 |

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

Was a repair necessary?

--

Go to Step 10

Go to Step 9

7 |

1. Check the HO2S 1 signal circuit for a open or short.
2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Was a repair necessary?

--

Go to Step 10

Go to Step 9

8 | Important: Before replacing the oxygen sensor, inspect the sensor for contamination. If contamination is found, determine and correct the cause of the contamination. Replace the HO2S 1. Refer to Heated Oxygen Sensor 1 Replacement . Is the action complete? | -- | Go to Step 10 | --
9 | Replace the PCM. Refer to Powertrain Control Module Replacement . Did you complete the replacement? | -- | Go to Step 10 | --
10 |

1. Use the scan tool in order to clear the DTCs.
2. Turn OFF the ignition for 30 seconds.
3. Start the engine.
4. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC run and pass?

--

Go to Step 11

Go to Step 2

11 | With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? | -- | Go to Diagnostic Trouble Code (DTC) List | System OK