1991 Geo Prizm LSi, AutomaticFACTORY OEM SERVICE PROCEDURE

Retrieving Codes

  1. Perform diagnostic circuit check before diagnosing vehicle using self-diagnostic system. See appropriate DIAGNOSTIC CIRCUIT CHECK chart under DIAGNOSTIC CODE CHARTS.
  2. Ensure ignition switch is in the OFF position. Using a jumper wire, connect diagnostic terminal to the ground terminal. See Fig 1 . Connector is located in engine compartment. Turn ignition switch to the ON position. Read fault codes.
  3. If no fault codes are stored, CHECK ENGINE light will flash on and off rapidly until jumper wire is disconnected. Count the number of flashes and look for pauses in flashing to ensure no fault codes are overlooked. If fault codes are stored, code will flash 3 times starting with the lowest code.
Fig 1: Identifying Diagnostic Connector Terminals
G121597Courtesy of GENERAL MOTORS CORP.
FAULT CODE IDENTIFICATION

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 Intake Air Temp. Sensor Low Temp. Indicated
25 Oxygen Sensor Lean Exhaust Indicated
26 Oxygen Sensor Rich Exhaust Indicated
27 Sub-Oxygen Sensor (VIN 5-Calif.) Open/Short Circuit
31 Mass Airflow Sensor (VIN 5) Short Circuit
31 MAP Sensor (VIN 6) Open/Short Circuit
41 Throttle Position Sensor Circuit (VIN 5) Open/Short Circuit
41 Throttle Switch (VIN 6) Incorrect Signal Voltage
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 (Calif.) Out Of Calibration
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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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