P0131

Diagnostic Chart (Part 1 Of 2):

Diagnostic Chart (Part 2 Of 2):

Schematics:

CIRCUIT DESCRIPTION
The Heated Oxygen Sensor (HO2S) is a sensor designed to create a voltage relative to the oxygen content in the engine exhaust stream. The control module supplies the HO2S with signal high and low circuits. Ignition voltage and ground are supplied to the HO2S heater by independent circuits. The oxygen content of the exhaust indicates when the engine is operating lean or rich. When the HO2S detects that the engine is operating rich, the signal voltage is high, and decreases the signal voltage as the engine runs leaner. This oscillation above and below the bias voltage, sometimes referred to as activity or switching, can be monitored with the HO2S signal voltage.

The HO2S contains a heater that is necessary in order to quickly warm the sensor to operating temperature and to maintain the operating temperature during extended idle conditions. The HO2S needs to be at a high temperature in order to produce a voltage. Once the HO2S has reached operating temperature, the control module monitors the HO2S bias, or reference, voltage as well as the HO2S signal voltage for Closed Loop fuel control. During normal Closed Loop fuel control operation, the control module will add fuel, or enrich the mixture, when the HO2S detects a lean exhaust content and subtract fuel, or learnout the mixture, when the HO2S detects a rich exhaust condition.

Certain vehicle models utilize an oxygen sensor after the catalytic converter in order to monitor catalyst efficiency.

This Diagnostic Trouble Code (DTC) determines if the HO2S is functioning properly by checking for an adequate number of HO2S voltage transitions above and below the bias range of 300-600 mV. This DTC sets when the Vehicle Control Module (VCM) fails to detect a minimum number of voltage transitions above and below the bias range during the test period. Possible causes of this DTC are listed below:
^ An open or a short to voltage on either the HO2S signal or HO2S low circuits
^ A malfunctioning HO2S
^ A problem in the HO2S heater or its circuit
^ A poor HO2S ground

This DTC is designed to detect an HO2S voltage that remains at a low 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 Throttle Position (TP) sensor DTCs.
^ No active Evaporative Emissions (EVAP) system DTCs.
^ No active Intake Air Temperature (IAT) sensor DTCs.
^ No active Manifold Absolute Pressure (MAP) sensor DTCs.
^ No active Engine Coolant Temperature (ECT) sensor DTCs.
^ No active Mass Air Flow (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 percent.

Power Enrichment Lean Test Enable
^ The system is in Closed Loop.
^ The Power Enrichment mode is active.
^ The sigh speed fuel cut-off is not active.
^ The elapsed time since test enabled is more 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 control module illuminates the Malfunction Indicator Lamp (MIL) if a failure is detected during 2 consecutive key cycles.
^ The control module sets the DTC and records the operating conditions at the time the diagnostic failed. The failure information is stored in the scan tools Freeze Frame and Failure Records.

CONDITIONS FOR CLEARING THE MIL/DTC
^ The control module 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. A warm-up cycle occurs when the coolant temperature has risen 22°C (40°F) from the startup coolant temperature and the engine coolant reaches a temperature that is more than 70°C (158°F) during the same ignition cycle.
^ A scan tool can clear the DTCs.

DIAGNOSTIC AIDS

IMPORTANT: Never solder the HO2S wires. For the proper wire and connection procedure refer to Wiring Repairs or to Connector Repairs in Diagrams.

Using the scan tool, observe the Long Term FT values at the different RPM and air flow conditions. The scan tool also displays the fuel trim cells, so the Long Term FT 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 Long Term FT values measure around 158 or more.
^ Inspect for the following conditions:
- 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 the HO2S.
- A poor VCM-to-engine block ground
- Any lean injectors
Perform the Injector Balance Test. Refer to Fuel Injector Balance Test with Tech 2. Component Tests and General Diagnostics
- A contaminated fuel supply
Any 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.
- The fuel pressure
If the pressure is too low, the system will be lean. In order to confirm, monitor the fuel pressure while driving the vehicle at various speeds and loads. Refer to Fuel System Diagnosis. Component Tests and General Diagnostics
- Any exhaust leaks
If there is an exhaust leak, the engine may pull outside air into the exhaust. and past the sensor.
- Any vacuum or the crankcase leaks can cause a lean condition or a possibly a high idle
- Any intermittents

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 inspect any circuitry that is suspected of causing the intermittent complaint. Refer to Testing for Intermittent and Poor Connections in Diagrams.

If a repair is necessary, refer to Wiring Repairs or Connector Repairs in Diagrams.

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 mode, 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.