DTC 25 (CALIFORNIA Emissions), O2S, Air/Fuel Ratio Lean: Notes

When Oxygen sensor (O2S) reaches operating temperature, it produces a varying voltage. Voltage will vary from 0.1 volt (lean) to 1.0 volt (rich). ECM monitors voltage and determines concentration of gases in exhaust and adjusts fuel to maintain air/fuel ratio. DTC 25 will set if sensor input voltage at ECM is less than 0.45 volt for 90 seconds or more, with O2S sensor warm and engine speed greater than 1500 RPM, vehicle speed is less than 60 MPH, and engine coolant temperature is greater than 122°F (50°C). DTC 25 will also set if engine speed varies more than 15 RPM over preceding crank angle period for 30 seconds while idling with engine coolant temperature greater than 122°F (50°C).

1. Checks for faulty O2S sensor.
2. Checks for an open in Black wire between ECM and O2S sensor.
3. Checks for an open in Brown wire between ECM and G106.
4. Checks for a short to ground in Black wire between ECM and O2S sensor and a faulty ECM.