P0151

Diagnostic Chart:

Wiring Diagram:

CIRCUIT DESCRIPTION

IMPORTANT: If the voltage is measured with a 10 megohm Digital Voltmeter, the voltage may read as low as 0.32 volt (320 mV).

The Vehicle Control Module (VCM) supplies a voltage of about 0.45 volt (450 mV) between the HO2S Signal and HO2S Low circuits. The Heated Oxygen Sensor varies the voltage within the range of about 1.0 volt (1000 mV) if the exhaust is rich, down to about 0.10 volt (100 mV) if the exhaust is lean.

The sensor is like an open circuit and produces no voltage when it is below 360°C (600°F). The DTC P0151, P0152, an open signal circuit, or a cold sensor causes Open Loop operation. The DTC P0151 determines if the HO2S sensor or the signal circuit is shorted to low by checking for a lean condition during a steady throttle and power enrichment (PE).

CONDITIONS FOR SETTING THE DTC

The following conditions will set the DTC:

^ No Throttle Position (TP) sensor DTCs.

^ No EVAP DTCs.

^ No Intake Air Temperature (IAT) sensor DTCs.

^ No Manifold Absolute Pressure (MAP) sensor DTCs.

^ No Engine Coolant Temperature (ECT) sensor DTCs.

^ No Mass Air Flow (MAF) sensor DTCs.

^ No intrusive test in progress.

^ No device controls active.

^ The system voltage measures at least 9.0 volts.

Test Enable Lean Test

^ Closed Loop low Manifold Absolute Pressure (MAP) not active.

^ Closed Loop.

^ No fuel injectors turned off.

^ Air to fuel ratio is at least 14.5 but less than 14.8.

^ The TP is greater than 5% but less than 99%.

^ The above met for 5 seconds.

^ Related HO2S DTCs not active since code clear.

Test Enable PE Lean Test

^ A Closed Loop.

^ The Power Enrichment mode active.

^ The high speed fuel cutoff not active.

^ Related HO2S DTCs not active since code clear.

^ Time elapse since test enable is at least 1 second.

ACTION TAKEN WHEN THE DTC SETS

A current DTC P0151 causes the system to operate in open loop. With the DTC P0151 set, the Malfunction Indicator Lamp (MIL) turns ON at the first test failure.

CONDITIONS FOR CLEARING THE MIL/DTC

^ The VCM turns the MIL OFF after 3 consecutive drive trips when the test has Run and Passed and not Failed.

^ 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 start-up coolant temperature and the engine coolant temperature exceeds 70°C (160°F) during the same ignition cycle).

^ Use the scan tool Clear Information function.

^ Disconnect the VCM battery feed for 30 seconds.

DIAGNOSTIC AIDS

Using the scan tool, observe the Long Term (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 V (86 mV) and increases to about 0.450 V (450 mV) after the HO2S is disconnected, then the HO2S is internally shorted and should be replaced.

^ A heated oxygen sensor wire: A sensor pigtail may be mispositioned and contacting the exhaust manifold.

^ An intermittent ground in wire between the connector and the sensor.

^ A poor VCM to the engine block ground.

^ Lean injectors: Perform the Injector Balance Test. Refer to Fuel Injector Balance Test for the 4.3L.
Fuel Injector Balance Test

^ 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.

^ 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.
System Diagnosis - Fuel System

^ Exhaust leaks: If there is an exhaust leak, the engine may pull the outside air into the exhaust. and past the sensor.

^ Vacuum or the Crankcase leaks can cause a lean condition or a possibly a high idle.

^ If the above check out okay, the problem is a faulty heated sensor.

\e1mIMPORTANT\e0m: Never solder or attempt to repair HO2S wires.

TEST DESCRIPTION

The numbers below refer to the step numbers in the diagnostic table.

2. Running the engine at 1200 RPM keeps the oxygen sensor hot in order to ensure the oxygen sensor remains active and can indicate the exhaust oxygen accurately.

3. Opening the HO2S Signal circuit should result in displayed voltage between 350 and 550 mV. If the display is fixed below 350 mV, the fault is a short to ground in the HO2S Signal circuit or a faulty VCM.