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.
DTC 27 (CALIF Emissions), Sub-O2 Sensor (Sub-O2S), Open Or Shorted CKT: Notes
When sub-oxygen sensor (sub-O2S) reaches operating temperature, it produces a varying voltage. Voltage will vary from 0.1 volt (exhaust is lean) to 1.0 volt (exhaust is rich). ECM monitors voltage and determines concentration of gases in exhaust after catalytic converter. ECM uses this input along with Oxygen sensor (O2S) input to adjust fuel to maintain optimum air/fuel ratio. DTC 27 will set when engine is at operating temperature and engine is off idle for 2 seconds or more with O2S input at ECM 0.45 volt or more, and sub-O2S input at ECM 0.45 volt or less.
- Checks for a faulty sub-O2S.
- Checks for an open in Black wire between ECM and sub-O2S.
- Checks for an open in Brown wire between ECM and G106.
- Checks for a short to ground in Black wire between ECM and sub-O2S, an open in Brown wire to sub-O2S, and a faulty ECM.
Use the Manual With the Right Hardware
Pair factory procedures with proven DIY tools so the instructions are easier to execute.
Disclosure: Verified tool recommendations may earn an affiliate commission at no cost to you.
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.
Rather have a pro handle this?
A certified mobile mechanic comes to your home or office. They bring the tools, you stay comfortable. No tow truck, no waiting room, no surprises.
No obligation. See pricing before you book.