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.
1991 Geo Prizm LSi, StandardFACTORY OEM SERVICE PROCEDURE
Engine Controls - Tests W/Codes
- Introduction
- Self-Diagnostic System
- PRIZM GSi CODE CHARTS (VIN 5)
- Diagnostic Circuit Check
- Chart A1, No Check Engine Light
- Diagnostic Aids
- Chart A2, Check Engine Light Won't Flash On Steady
- Codes 12 & 13, No Rpm Signal/No Signal For 2 Seconds
- Code 14, IGN Signal Circuit/No Signal
- Code 21, O2 Sensor Circuit/Open Or Short Circuit
- Code 22, Coolant Temp Sensor/Open Or Short Circuit
- Code 24, Intake Air Temp Sensor CKT/Low Temp Indicated
- Code 25, O2 Circuit/Lean Exhaust Indicated
- Code 26, O2 Sensor Circuit/Rich Exhaust Indicated
- Code 27, Sub-O2 Sensor/Open Or Shorted CKT (CALIF. Only)
- Code 31, Mass Airflow Sensor/Short Circuit
- Code 41, Throttle Position Sensor/Open Or Short Circuit
- Code 42, Vehicle Speed Sensor (VSS) Circuit
- Code 43, No Crank Signal From Starter
- Code 51, No A/C Signal
- Code 52, Knock Sensor Signal/Open Or Short CKT
- Code 53, Knock Control Signal/Faulty ECM
- Code 71, EGR Gas Temp Sensor CKT (CALIFORNIA Only)
- PRIZM EXCEPT GSi CODE CHARTS (VIN 6)
- Diagnostic Circuit Check
- Chart A1, No Check Engine Light
- Diagnostic Aids
- Chart A2, Check Engine Light Won't Flash On Steady
- Codes 12 & 13, No Rpm Signal/No Signal For 2 Seconds
- Code 14, IGN Signal CKT, Open Or Short CKT
- Code 21, O2 Sensor CKT/Open Or Short CKT
- Code 22, Coolant Temp Sensor CKT/Open Or Shorted CKT
- Code 24, Intake Air Temp Sensor CKT/Low Temp Indicated
- Code 25, O2 CKT/Lean Exhaust Indicated
- Code 26, O2 Sensor CKT/Rich Exhaust Indicated
- Code 31, Map Sensor/Open Or Short CKT
- Code 41, Throttle Switch Circuit
- Code 42, Vehicle Speed Sensor Circuit
- Code 43, No Crank Signal From Starter
- Code 51, No A/C Signal
- Code 71, EGR Gas Temp Sensor CKT (CALIFORNIA Only)
- Summary
Factory Service Tools
Use the Manual With the Right Hardware
Pair factory procedures with proven DIY tools so the instructions are easier to execute.
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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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