Circuit Description
The HO2S is a sensor used to detect oxygen-content in the exhaust. The PCM supplies the sensor with a signal circuit and a ground circuit. The PCM supplies a bias voltage between the circuits. The sensor varies the voltage based on the oxygen content in the exhaust. When the exhaust stream is lean (more oxygen), the sensor produces a low voltage signal. When the exhaust stream is rich (less oxygen), the sensor produces a high voltage signal. The PCM monitors the HO2S signal during closed loop. To correct for rich or lean conditions, the PCM adjusts the injector pulse width. If HO2S bank 1 sensor 1 remains too low for a calibrated period of time, DTC P0131 will set.
Conditions for Running the DTC
| • | MAF, TP, EVAP, IAT, MAP, ECT, AIR (if equipped), or injector DTCs are not set. |
| • | EGR Flow diagnostic inactive |
| • | AIR diagnostic inactive |
| • | Catalyst diagnostic inactive |
| • | Closed loop fuel enabled |
| • | No injectors disabled |
| • | System voltage between 9 and 19 volts |
| • | Air/Fuel ratio is between 14.5 and 14.8. |
| • | TP is between 3 and 25 percent. |
| • | The above conditions must be met for 3 seconds. |
Conditions for Setting the DTC
| • | HO2S bank 1 sensor 1 signal voltage remains below 75 mV (0.075 V) in closed loop mode. |
| OR |
| • | HO2S bank 1 sensor 1 signal voltage remains below 575 mV (0.575 V) during Power Enrichment (PE) mode. |
Action Taken When the DTC Sets
| • | The PCM illuminates the malfunction indicator lamp (MIL) during the second consecutive trip in which the diagnostic test runs and fails. |
| • | The PCM stores the conditions present when the DTC set as Freeze Frame/Failure Records data. |
Conditions for Clearing the MIL/DTC
| • | The PCM will turn the MIL OFF after the third consecutive trip in which the diagnostic runs and passes. |
| • | The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction. |
| • | The DTC can be cleared by using the scan tool Clear DTC Information function. |
Diagnostic Aids
Notice: Do not solder heated oxygen sensor wires. Soldering the wires will result in the loss of the air reference to the sensor. Refer to Engine Electrical for proper wire and connection repair techniques.
Check for the following conditions:
| • | HO2S wire routing -- The sensor wiring may be routed incorrectly and contacting the exhaust system. |
| • | Fuel pressure -- Fuel pressure that is too low will cause a lean condition. The PCM will compensate for some decrease. However, if fuel pressure is low enough, DTC P0131 may set. |
| • | Fuel injectors -- If the fuel injectors deliver improper fuel volume at the specified pressure, DTC P0131 may set. Refer to Fuel Injector Balance Test with Special Tool or Fuel Injector Balance Test with Tech 2 . |
| • | Exhaust leaks -- Exhaust leaks may cause outside air to be drawn into the exhaust gas stream, and pass the HO2S sensor. This causes the HO2S signal to indicate a false lean condition. |
| • | AIR system leaks -- Check the AIR system hoses, check valves, and piping for leaks, poor connections, or signs of damage. |
| • | Vacuum leaks |
| Check for the following: |
| - | Disconnected or damaged vacuum hoses. |
| - | Vacuum leaks at the throttle body, EGR valve, EVAP purge solenoid, and intake manifold. |
| • | Fuel contamination -- Water, even in small amounts, can be delivered to the fuel injectors. The water can cause a lean exhaust. Also, excessive alcohol in the fuel can cause a lean condition. |
| • | Check with the customer to verify the conditions under which the DTC set. Check to see if the vehicle ran out of fuel. |
If none of the above conditions are present, replace the HO2S sensor. Refer to Heated Oxygen Sensor Replacement - Bank 1 Sensor 1 and Heated Oxygen Sensor Replacement - Bank 1 Sensor 2 .
Review the front and rear long term fuel trim values on the scan tool fail records data. If both banks were lean, check for a condition that would cause a lean engine operation. If only the rear bank was lean, the lean condition is isolated to that bank (such as an intermittent short to ground on the signal circuit).
If the condition is not present, check with the customer about the type of fuel used (i.e., over 10 percent ethanol fuel). Recommend using fuels of 10 percent or less ethanol.
If the condition is intermittent, refer to Intermittent Conditions .
Test Description
The numbers below refer to the step numbers on the diagnostic table.
-
Verifies whether or not the HO2S sensor voltage is switching from below 300 mV to above 600 mV, or remaining below 300 mV.
-
Verifies the bias voltage from the PCM and the condition of the circuit wiring. With the sensor disconnected, the scan tool should display a bias voltage near 450 mV.
Step | Action | Values | Yes | No |
|---|---|---|---|---|
1 | Did you perform the Powertrain On Board Diagnostic (OBD) System Check? | -- | ||
Does voltage remain below the value specified? | 75 mV | |||
3 |
Does the scan tool indicate that DTC P0131 failed last test? | -- | Go to Diagnostic Aids | |
Does the scan tool display an HO2S bank 1 sensor 1 voltage near the specified value? | 450 mV | Go to Diagnostic Aids | ||
5 |
Did you find and correct the condition? | -- | ||
6 |
Important: The replacement PCM must be programmed. Replace the PCM. Refer to Powertrain Control Module Replacement/Programming . Is the replacement complete? | -- | -- | |
7 |
Does the scan tool indicate that DTC P0131 passed? | -- | System OK |
