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.
1990 Geo Prizm Base, 4D Hatchback, AutomaticFACTORY OEM SERVICE PROCEDURE
Retrieving Codes
- Perform DIAGNOSTIC CIRCUIT CHECK before diagnosing vehicle using self-diagnostic system. See DIAGNOSTIC CIRCUIT CHECK chart in this article.
- Ensure ignition switch is in the OFF position. Using a jumper wire, ground diagnostic connector terminal to ground terminal. See Fig 1 . Connector is located in the engine compartment. Turn ignition switch to the ON position. Read trouble codes.
- If no trouble codes are stored, CHECK ENGINE light will flash rapidly on then off until jumper wire is disconnected. Count the number of flashes and look for pauses in flashing to ensure no trouble codes are overlooked. If trouble codes are stored, code will flash 3 times and the lowest code will be given first.
TROUBLE CODE IDENTIFICATION CHART
| Code | Circuit Affected | Probable Cause |
|---|---|---|
| 12 | No RPM Signal | No Signal For 2 Seconds |
| 13 | No RPM Signal | No Signal For 2 Seconds |
| 14 | Ignition Signal | No Signal |
| 21 | Oxygen Sensor | Open/Short Circuit |
| 22 | Coolant Temp. Sensor | Open/Short Circuit |
| 24 | Manifold Air Temp. Sensor | Low Temp. Indicated |
| 25 | Oxygen Sensor | Lean Exhaust Indicated |
| 26 | Oxygen Sensor | Rich Exhaust Indicated |
| 27 | Sub-Oxygen Sensor | Open/Short Circuit (VIN 5-Calif.) |
| 31 | Mass Airflow Sensor | Shorted Circuit (VIN 5) |
| 31 | MAP Sensor (VIN 6) | Open/Shorted Circuit |
| 41 | Throttle Position Sensor | Incorrect Signal Voltage Circuit (VIN 5) |
| 41 | Throttle Switch | Incorrect Signal Voltage (VIN 6) |
| 42 | Vehicle Speed Sensor | Incorrect Signal Voltage |
| 43 | No Crank Signal | No Signal From Starter |
| 51 | No A/C Signal | No Signal Present |
| 52 | Knock Sensor Signal (VIN 5) | Open/Short Circuit |
| 53 | Knock Control Signal (VIN 5) | Faulty ECM |
| 71 | EGR Temperature Sensor | Out Of Calibration (Calif.) |
Factory Service Tools
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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.