P0131
DTC P0131 HO2S Circuit Low Voltage Bank 1 Sensor 1
Circuit Description
IMPORTANT: If the voltage is measured with a 10 Megohm DMM, the voltage may read as low as 0.32 volt.
The HO2S does not produce a voltage at temperatures below 200 °C (392 °F). When the HO2S is below 200 °C (392 °F), the VCM will default to a 450 mV reference voltage. When the HO2S temperature is below 200 °C (392 °F), the oxygen sensor voltage displayed on the scan tool will be near the 450 mV reference signal. The VCM holds the fuel control system in open loop operation until the sensor reaches operating temperature. The HO2S heater provides for a faster sensor warm-up which allows the sensor to become active in a shorter period of time. When the HO2S temperature exceeds 200 °C (392 °F), the sensor begins producing a voltage that varies in response to exhaust oxygen levels. This voltage ranges from a high of approximately 900 mV under rich exhaust conditions, to a low of approximately 100 mV under lean exhaust conditions. These voltage transitions from rich-to-lean and lean-to-rich occur rapidly when the VCM is in control of the air/fuel ratio (closed loop operation).
This DTC is designed to detect an HO2S voltage that remains at a low (lean) voltage for more than a specified number of seconds during the test conditions. This DTC is set under the following conditions:
- There is an HO2S circuit fault that results in a false lean exhaust condition.
- The HO2S is correctly detecting a lean air/fuel ratio resulting from either a vacuum leak or a fuel control system fault.
Conditions for Running the DTC
- No active TP sensor DTCs
- No active EVAP DTCs
- No active IAT sensor DTCs
- No active MAP sensor DTCs
- No active ECT sensor DTCs
- No active MAF sensor DTCs
- No active misfire DTCs
- No intrusive test in progress
- No device controls active
- The system voltage is between 11.7 - 18 volts
Lean Test Enable
- The system is in closed loop
- The air/fuel ratio is between 14.5 - 14.8
- The throttle position is between 3.5 - 99%
- The above conditions are met for 5 seconds
Power Enrichment Lean Test Enable
- The system is in closed loop
- Power enrichment mode is active
- High speed fuel cut-off not active
- The elapsed time since test enabled is greater than 1 second
Conditions for Setting the DTC
Lean Test
The O2 sensor voltage remains less than 86 mV for more than 50 seconds.
Power Enrichment Lean Test
The O2 sensor voltage remains less than 598 mV for more than 30 seconds
Action Taken When the DTC Sets
- The VCM illuminates the MIL (Malfunction Indicator Lamp) if a failure is detected during 2 consecutive key cycles.
- The VCM will set the DTC and records the operating conditions at the time the diagnostic fails. The VCM stores the failure information in the scan tools Freeze Frame and/or the Failure Records.
Conditions for Clearing the MIL/DTC
- The VCM turns OFF the MIL after 3 consecutive drive trips when the test has Run and Passed.
- A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles (coolant temperature has risen 22 °C (40 °F) from the startup coolant temperature and the engine coolant temperature is more than 70 °C (158 °F) during the same ignition cycle).
- Use the scan tool Clear Information function.
Diagnostic Aids
IMPORTANT: Never solder the HO2S wires. For proper wire and connection repair refer to Wiring Repairs or Connector Repairs.
Using the scan tool, observe the LT fuel trim values at the different RPM and the air flow conditions. The scan tool also displays the fuel trim cells, so the LT fuel trim values can be checked in each of the cells in order to determine when the DTC may have set. If the conditions for this DTC exist, the LT fuel trim values measure around 158 or greater.
Check for the following conditions:
- Display the HO2S voltage with a scan tool. If the voltage is less than 0.086 The DTC (86 mV) and increases to about 0.450, the DTC (450 mV) after the HO2S is disconnected, then the HO2S is internally shorted and should be replaced.
- A sensor pigtail may be mispositioned and contacting the exhaust system.
- An intermittent short to ground in the signal circuit between the VCM connector and HO2S.
- A poor VCM to the engine block ground.
- Lean injectors: Perform the Injector Balance Test. Refer to Fuel Injector Balance Test with Tech 2.
- Fuel Contamination: Water, even in small amounts, near the in-tank fuel pump inlet can be delivered to the injectors. The water causes a lean exhaust and can also set this DTC.
Refer to Alcohol/Contaminants-in-Fuel Diagnosis.
- Fuel pressure: If the pressure is too low, the system will be lean. In order to confirm, monitor a fuel pressure while driving the vehicle at various speeds and loads. Refer to Fuel System Diagnosis.
- Exhaust leaks: If there is an exhaust leak, the engine may pull the outside air into the exhaust and past the sensor. Refer to Exhaust System Inspection in Engine Exhaust
- Vacuum or the Crankcase leaks can cause a lean condition or a possibly a high idle. An intermittent may be caused by any of the following conditions:
- A poor connection
- Rubbed through wire insulation
- A broken wire inside the insulation
Thoroughly check any circuitry that is suspected of causing the intermittent complaint. Refer to Intermittents and Poor Connections Diagnosis.
If a repair is necessary, then refer to Wiring Repairs or Connector Repairs.
Steps 1 - 4:
Steps 5 - 6:
Steps 7 - 10:
Step 11:
Test Description
The numbers below refer to the step numbers on the diagnostic table.
3. In order to determine if the engine is lean during power enrichment operate the vehicle under higher throttle/acceleration conditions while monitoring scan tool HO2S voltage and the power enrichment parameter. A lean condition will cause HO2S voltage to be less than 598 mV during power enrichment.
5. Monitor the HO2S voltage of the opposite bank sensor. If the voltage activity of the opposite bank sensor is similar to the voltage activity of the suspect sensor check for lean conditions that would affect both cylinder banks. An opposite bank sensor with normal HO2S voltage activity indicates the suspect HO2S needs replacement or a lean condition exists only on the suspect HO2S cylinder bank.