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.
1995 Geo Prizm Base, StandardFACTORY OEM SERVICE PROCEDURE
Sub-O2S (Calif. Emissions)
- A malfunction in the sub-O2S circuit will set DTC 27. For testing procedures, see TESTS W/CODES - 1.6L article. Use following procedure to test sub-O2S.
- A voltage is produced in response to amount of Oxygen in exhaust gases. A lean mixture (high oxygen content) will produce a low voltage of approximately 0.1 volt. A rich mixture (low oxygen content) will produce a high voltage of approximately 0.9 volt.
Factory Service Tools
Use the Manual With the Right Hardware
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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.
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