GM Service Manual Online
For 1990-2009 cars only

Diagnostic Instructions

    • Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    •  Review Strategy Based Diagnosis for an overview of the diagnostic approach.
    •  Diagnostic Procedure Instructions provide an overview of each diagnostic category.

DTC Descriptors

DTC P0131: HO2S Circuit Low Voltage Bank 1 Sensor 1

DTC P0151: HO2S Circuit Low Voltage Bank 2 Sensor 1

Diagnostic Fault Information

Circuit

Short to Ground

Open/High Resistance

Short to Voltage

Signal Performance

HO2S Bank 1 Sensor 1 Signal

P0131

P0134, P0135

P0132

P0133, P2096, P2097

HO2S Bank 1 Sensor 2 Signal

P0137

P0140

P0138

P0140

HO2S Bank 2 Sensor 1 Signal

P0151

P0154, P0155

P0152

P0153, P2098, P2099

HO2S Bank 2 Sensor 2 Signal

P0157

P0160

P0158

P0160

Low Reference

--

P0134, P0135, P0154, P0155

--

--

Typical Scan Tool Data

HO2S 1 or 2

Circuit

Short to Ground

Open

Short to Voltage

Operating Conditions: Engine operating in Closed Loop

Parameter Normal Range: Fluctuates above and below 350-500 mV

Sensor Signal

0-60 mV

445-450 mV

1275 mV

Low Reference

--

445-480 mV

1275 mV

Circuit Description

The heated oxygen sensors (HO2S) are used for fuel control and catalyst monitoring. Each HO2S compares the oxygen content of the surrounding air with the oxygen content of the exhaust stream. When the engine is started, the control module operates in an Open Loop mode, ignoring the HO2S signal voltage while calculating the air-to-fuel ratio. The control module supplies the HO2S with a reference, or bias voltage of about 450 mV. While the engine runs, the HO2S heats up and begins to generate a voltage within a range of 0-1,000 mV. This voltage will fluctuate above and below the bias voltage. Once sufficient HO2S voltage fluctuation is observed by the control module, Closed Loop is entered. The control module uses the HO2S voltage to determine the air-to-fuel ratio. An HO2S voltage that increases above bias voltage toward 1,000 mV indicates a rich fuel mixture. An HO2S voltage that decreases below bias voltage toward 0 mV indicates a lean fuel mixture.

The heating elements inside each HO2S heat the sensor to bring the sensor up to operating conditions faster. This allows the system to enter Closed Loop earlier and the control module to calculate the air-to-fuel ratio sooner.

Conditions for Running the DTC

    • DTCs P0116, P0117, P0118, P0119, P0201, P0202, P0203, P0204, P0205, P0206 P0261, P0262, P0264, P0265, P0267, P0268, P0270, P0271, P0273, P0274, P0276, and P0277 are not set.
    • The ignition voltage is greater than 10.5 volts.
    • The engine is operating for more than 1 second.
    • The engine coolant temperature (ECT) during the previous ignition cycle was more than 60°C (140°F).
    • The ECT at startup is less than 40°C (104°F).
    • The HO2S heater is active for more than 5 seconds.
    • The calculated exhaust temperature is less than 800°C (1,472°F).
    • DTC P0131 and P0151 run continuously once the conditions above exist.

Conditions for Setting the DTC

    • The ECM detects that a primary HO2S voltage is less than 60 mV while the corresponding secondary HO2S voltage is greater than 500 mV.
    • The ECM detects that a primary HO2S voltage is less than 60 mV after a cold start with the engine coolant temperature (ECT) less than 40°C (104°F).
    • Either condition exists for more than 4 seconds, or a cumulative of 50 seconds.

Action Taken when the DTC Sets

DTCs P0131 and P0151 are Type B DTCs.

Conditions for Clearing the MIL/DTC

DTCs P0131 and P0151 are Type B DTCs.

Reference Information

Schematic Reference

Engine Controls Schematics

Connector End View Reference

    •  Engine Control Module Connector End Views
    •  Engine Controls Connector End Views

Electrical Information Reference

    •  Circuit Testing
    •  Connector Repairs
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

    •  Scan Tool Data List
    •  Scan Tool Data Definitions
    •  Scan Tool Output Controls

Circuit/System Verification

  1. Engine idling, observe the HO2S voltage parameter. The voltage should fluctuate above and below the range of 350-550 mV.
  2. If the vehicle passes the Circuit/System Verification test, then operate the vehicle within the conditions for running the DTC. You may also operate the vehicle within the conditions that are captured in the Freeze Frame/Failure Records Data List.

Circuit/System Testing

  1. Ignition OFF, disconnect the harness connector at the appropriate HO2S sensor.
  2. Ignition ON, verify that the HO2S voltage parameter is between 350-500 mV.
  3. If less than the specified value, test the signal circuit for a short to ground. If the circuit/connections test normal, replace the ECM.
    If greater than the specified value, test the signal circuit for a short to voltage. If the circuit/connections test normal, replace the ECM.
  4. Install a 3A fused jumper wire between the signal circuit terminal B and ground. Verify the HO2S voltage parameter is less than 60 mV.
  5. If greater than the specified value, test the signal circuit for an open/high resistance. If the circuit/connections test normal, replace the ECM.
  6. Connect a 3A fused jumper wire between the signal circuit terminal B and the low reference circuit terminal A. Verify the HO2S voltage parameter is less than 60 mV.
  7. If greater than the specified value, test the low reference circuit for an open/high resistance or for a short to voltage. If the circuit/connections test normal, replace the ECM.
  8. Verify that the following conditions do not exist:
  9. • Lean fuel injectors--Refer to Fuel Injector Balance Test with Special Tool or Fuel Injector Balance Test with Tech 2 .
    • Low fuel system pressure--Refer to Fuel System Diagnosis .
    • Fuel that is contaminated--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Exhaust leaks near the HO2S
    • Engine vacuum leaks
         ⇒If you find any of the above conditions, repair as necessary.
  10. If all circuits/connections test normal, replace the HO2S sensor.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    •  Heated Oxygen Sensor Replacement - Bank 1 Sensor 1
    •  Heated Oxygen Sensor Replacement - Bank 2 Sensor 1
    •  Control Module References for engine control module replacement, setup, and programming