Diagnostic Aids

The scan tool output control feature, Fuel Injector Modification, can be used at idle to check the oxygen sensor performance. Increasing the Fuel Injector Modification Commanded State to 25 percent should cause the HO2S 1 voltage to exceed 800 mV. Decreasing the Fuel Injector Modification Commanded State to -12 percent should cause the HO2S 1 voltage to drop to less than 100 mV. An oxygen sensor that fails to respond accordingly may be faulty or contaminated. When performing the Fuel Injector Modification with the scan tool, the Loop Status will remain Open Loop and the Short Term FT will be fixed at 0 percent.

Check for any of the following conditions:

• Any damaged oxygen sensor wiring-The sensor pigtail may be improperly routed and contacting the exhaust system.
• An incorrect fuel pressure-A condition that causes a lean exhaust can cause DTC P0125 to set. The system will go lean if the fuel pressure is too low. The PCM can compensate for some decrease. However, if the fuel pressure is too low, DTC P0125 may be set. Refer to Fuel System Diagnosis .
• Lean fuel injectors-Refer to Fuel Injector Balance Test with Special Tool .
• Any vacuum leaks-Check for incorrectly routed, damaged, or loose vacuum lines and hoses. Inspect for vacuum leaks at the intake manifold, the throttle body, and the crankcase ventilation system.
• Any exhaust leaks-An exhaust leak may cause outside air to be pulled into the exhaust gas stream past the HO2S, causing DTC P0125 to set. Check for exhaust leaks near the HO2S 1.
• Any fuel contamination-Water, even in small amounts, can be delivered to the fuel injectors. Water can cause a lean exhaust to be indicated. Excessive alcohol in the fuel can also cause this condition. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .

An intermittent malfunction may be caused by fault in the HO2S circuit sensor electrical circuit. Inspect the wiring harness and components for an intermittent condition. Refer to Intermittent Conditions .

Repair any electrical circuit faults that were found. Refer to Wiring Repairs .

The information included in the Freeze Frame data can be useful in determining the vehicle operating conditions when the DTC first set.