Diagnostic Aids

Scan tool output control feature, Fuel Injector Modification, can be used at idle to check oxygen sensor performance. Increasing Fuel Injector Modification Commanded State to 25 percent should cause HO2S 1 voltage to exceed 800 mV. Decreasing Fuel Injector Modification Commanded State to - 12 percent should cause 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 Fuel Injector Modification with scan tool, Loop Status will remain Open Loop and Short Term FT will be fixed at zero percent.

Check for any of the following conditions:

• Any damaged oxygen sensor wiring. Sensor pigtail may be improperly routed and contacting exhaust system.
• An incorrect fuel pressure. A condition that causes a lean exhaust can cause DTC P0125 to set. System will go lean if fuel pressure is too low. PCM can compensate for some decrease. However, if fuel pressure is too low, DTC P0125 may be set. See FUEL SYSTEMS in SYSTEM & COMPONENT TESTING - 1.8L VIBE article.
• Lean fuel injectors. See FUEL INJECTOR BALANCE TEST WITH SPECIAL TOOL in FUEL SYSTEMS in SYSTEM & COMPONENT TESTING - 1.8L VIBE article.
• Any vacuum leaks. Check for incorrectly routed, damaged, or loose vacuum lines and hoses. Check for vacuum leaks at intake manifold, throttle body, and 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 HO2S 1.
• Any fuel contamination. Water, even in small amounts, can be delivered to fuel injectors. Water can cause a lean exhaust to be indicated. Excessive alcohol in the fuel can also cause this condition.

An intermittent malfunction may be caused by fault in the HO2S circuit sensor electrical circuit. Check the wiring harness and components for an intermittent condition. Repair any electrical circuit faults that were found. The information included in freeze frame data can be useful in determining vehicle operating conditions when the DTC first set.