Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Code 27, Sub-O2 Sens CKT, Open Or Shorted CKT (Ca EMIS Only): Notes
Sub-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 after converter. ECM uses this input and Oxygen (O2) sensor input, and adjusts fuel to maintain air/fuel ratio.
Code 27 will set when engine is at operating temperature and engine is off idle for 2 seconds or more with O2 sensor input at ECM .45 volt or more, and sub-O2 sensor input at ECM .45 volt or less.
- Checks for faulty sub-O2 sensor.
- Checks for open in Black wire between ECM and sub-O2 sensor.
- Checks for open in Brown wire between ECM and G106.
- Checks for short to ground in Black wire between ECM and sub-O2 sensor, open in Brown wire to sub-O2 sensor, and a faulty ECM.
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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.