Oxygen (O2) sensor produces a varying voltage as engine reaches normal operating temperature. Voltage will vary from .1 (lean) to 1.0 (rich) volt. ECM monitors voltage and determines concentration of gases in exhaust and adjusts fuel to maintain air/fuel ratio.

If O2 sensor input voltage at ECM is less than .45 volt for 90 seconds or more with O2 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), Code 25 will set.

Code 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).

NOTE: Test numbers refer to test numbers on diagnostic chart.

Checks for faulty O2 sensor.
Checks for open in Black wire between ECM and O2 sensor.
Checks for open in Brown wire between ECM and G106.
Checks for short to ground in Black wire between ECM and O2 sensor, open in Brown wire to O2 sensor (federal emissions only), and a faulty ECM.

When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

• You smell fuel, burning insulation, or see smoke.

• Brakes feel soft, pull hard to one side, or make grinding noises.

• The engine overheats, stalls repeatedly, or misfires under load.

• You are missing required tools, torque specs, or safe lifting equipment.

• You are not confident in the next step or safety outcome.

Safety Warning

Code 25, O2 Sensor Circuit, Lean Exhaust Indicated: Notes

Use the Manual With the Right Hardware

When to See a Mechanic

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