DTC P0131: Primary HO2S (Sensor 1) Circuit Low Voltage Reset the ECM/PCM (see page 11-312). Start the engine. Hold the engine at 3,000 rpm (min-1) with no load (in Park or neutral) until the radiator fan comes on. Check the primary HO2S (Sensor 1) output voltage with the scan tool during acceleration using wide open throttle. Does the voltage stay at 0.5 V or less? YES - Go to step 4. NO - Intermittent failure, system is OK at this time. Check for poor connections or loose wires at the primary HO2S (Sensor 1) and at the ECM/PCM. Check the fuel pressure (see page 11-431). Is it normal? YES - Go to step 5. NO - Repair the fuel supply system. Turn the ignition switch OFF. Disconnect the primary HO2S (Sensor 1) 4P connector. Turn the ignition switch ON (II). Check the primary HO2S (Sensor 1) output voltage with the scan tool. Does it stay at 0.5 V or less? YES - Go to step 9. NO - Replace the primary HO2S (Sensor 1).

Turn the ignition switch OFF. Disconnect ECM/PCM connector A (31P). Check for continuity between the primary HO2S (Sensor 1) 4P connector terminal No. 2 and body ground. PRIMARY HO2S (SENSOR 1) 4P CONNECTOR Terminal side of male terminals Is there continuity? YES - Repair short in the wire between the ECM/PCM (A6) and the primary HO2S (Sensor 1). NO - Substitute a known-good ECM/PCM and recheck (see page 11-313). If symptom/indication goes away, replace the original ECM/PCM.