The engine control module (ECM) supplies a voltage near 450 mV between the heated oxygen sensor (HO2S) high signal circuit and the low reference circuit. The HO2S varies the voltage over a range from about 1,000 mV when the exhaust is rich, down through about 10 mV when the exhaust is lean. The ECM monitors and stores the HO2S voltage information. The ECM evaluates the HO2S voltage samples in order to determine the amount of time that the HO2S voltage was out of range. The ECM compares the stored HO2S voltage samples taken within each sample period and determines if the majority of the samples are out of the operating range. The ECM test for the following conditions:

The ECM detects that the rear HO2S voltage is greater than 650 mV for greater than 100 seconds, then an intrusive test is performed. The ECM will lean the fuel mixture up to -7 percent and then wait for 10 seconds at -7 percent. If the ECM detects that the HO2S voltage is still greater than 650 mV, the ECM then tests the HO2S at the next decel fuel cut-off. If the ECM detects the HO2S voltage is greater than 200 mV after 4 seconds or for a cumulative of 30 seconds in decel fuel cut-off, the DTC sets.

DTCs P2271 and P2273 are Type B DTCs.

DTCs P2271 and P2273 are Type B DTCs.

Schematic Reference

Engine Controls Schematics

Connector End View Reference

Component Connector End Views

Electrical Information Reference

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

1. Ignition ON, observe the DTC information with a scan tool. Verify that DTC P0137, P0138, P0140, P0157, P0158, or P0160 is not set.

⇒	If any of the listed DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle for further diagnosis.

2. Operate the engine above 1,200 RPM for 30 seconds, observe the HO2S voltage parameter. The reading should fluctuate above and below the range of 350-550 mV.
3. Operate the vehicle within the Conditions for Running the DTC to verify the DTC does not reset. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.

1. Engine operating, move the related HO2S harnesses for the appropriate rear sensor between the HO2S harness connector and the engine control module (ECM) while monitoring the appropriate HO2S voltage parameter. Verify that the HO2S parameter does not change abruptly while moving the related harnesses.

⇒	If the HO2S parameter does change abruptly while moving the related harnesses repair the circuit as necessary.

2. Ignition OFF, disconnect the harness connector at the appropriate HO2S.
3. Ignition ON, verify that the HO2S parameter is between 350-500 mV.

⇒	If greater than 500 mV, test the signal circuit of the HO2S sensor for a short to voltage. If the circuit/connections test normal, replace the ECM.

4. Verify that none of the following conditions exist:

•	Rich fuel injectors--Refer to Fuel Injector Solenoid Coil Test.

•	Water intrusion in the HO2S harness connector

•	High fuel system pressure--Refer to Fuel System Diagnosis.

•	Fuel that is contaminated--Refer to Alcohol/Contaminants-in-Fuel Diagnosis.

•	A collapsed air intake duct

•	A restricted air filter element

•	Exhaust system restrictions

•	Excessive fuel in the crankcase

•	An engine mechanical condition--Refer to Symptoms - Engine Mechanical.

⇒	If you find any of the above conditions, repair as necessary.

5. If all circuits/connections test normal, replace the applicable HO2S.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.