Trouble Codes
    Also known as Error Codes.
    The Geo Storm is equipped with an OBD-1 type Engine Control Unit/Module. OBD stands for On Board Diagnostics. The number 1 indicates that the type of computer is from the first series of computer systems to contain this technology. OBD-2 and later computer systems use a more standardized system that was mandated by the government so that all systems could be accessed by a standardized computer for emissions testing. HOWEVER, OBD-1 systems are NOT Standardized, and this is simply a grouping of similar, unrelated systems, each of which requiring a special tool to access.

    The On Board Diagnostic system can be a valuable tool for determining the source of engine control and emissions sensor problems. If the computer has registered a Trouble Code, reading the code and referencing the cause of the code can greatly reduce guessing and lead directly to the source of the problem.

    Trouble Codes can be read by two methods:
    • Scanning the codes via the Check Engine Light.
      Locate the ALDL connector. It can be found behind the plastic trim panel on the right or passenger side, front floor footwell, just below knee level, at the lower right hand corner of the dash board, next to the glove box, behind the kick panel. There will be two, nearly identical, three connector plugs. The ALDL connector is the WHITE plug. The orange plug is for the airbag system. Do not tamper with the orange, airbag connector.
      With the engine off, the car level, the emergency brake set, take the WHITE ALDL plug. Make an electrical connection between the far left and far right wires. This can be done with a wire, or, most often, a bent paperclip.
      Turn the ignition key to the accessory position, and watch the Check Engine Light blink.
      The code is shown by blinking the first digit, then a pause, then the second digit, then a longer pause before it repeats the code.
      The computer will flash each code three times, and then move on to the next trouble code recorded.
      The first code should be "12". This is the code for "Engine is off, Diagnostic system is working." This code will flash three times, and then the computer will go to the first trouble Code that has been recorded. Unless no Trouble Code has been recorded, and the computer will then flash "12" again.
      The most important thing is to sit through all the codes, and write them down, until the computer returns to flashing "12" again. This will indicate that the computer has shown all the codes and is repeating the cycle again.
    • Scanning the codes with a Tech-1 Scan Tool.
      Locate the ALDL connector as indicated above.
      Make sure the engine is off and vehicle secured as described above.
      Plug the connector into the Tech-1 scan tool, and connect the tool power cable.
      Turn the ignition key to the accessory position.
      Enter the VIN number engine code and year information into the Tech-1 as the tool asks.
      Select to display Trouble Codes.
      Toggle through the codes and write them down.

    One note about the Tech-1 Scan Tool:
    The ECM/ECU for the Geo Storm and Isuzu sister cars is similar, but not identical.
    The computer program for the Geo Storm and Isuzu sister cars is similar, but not identical.
    The Tech-1 cartridge for Geo Storm and the Isuzu sister cars is not the same, but the cartridge for one, will access and display the diagnostic functions of the other, as long as the corresponding engine identification information is entered into the scan tool.
    The Tech-2 Scan Tool has reflashing capabilities. If this function is used, the cartridge must match the vehicle ECM. Using a Geo Storm cartridge to reflash a Isuzu ECM, or a Isuzu cartridge to reflash a Geo Storm ECM, will result in destroying the ECM.

    The computer will not always record a Trouble Code when there is a problem. But if it does, diagnosis becomes a matter of matching the code with the problem.
    If the Check Engine light comes on in the instrument cluster, then the ECM has detected a problem, and in most cases, the ECM will record the code for the associated problem or problems.


    The following is a chart of the Trouble Codes and an abbreviated description of the problems each Code indicates. Use this as a starting point. The Factory Shop Manual will have a more complete description and instructions for repairing the vehicle.

    Trouble Code Name of Code and Remedy
    12 Engine is off, Diagnostic system is working.
    This code will display at the beginning and end of the scan cycle when the computer is functioning normally.
    If this code will not display, check the power and ground to the ECM.
    13 Oxygen Sensor - Open or Grounded Circuit
    Indicates a problem with the reading reported by the Oxygen Sensor.
    Check the wires and connectors between the ECM and the Oxygen Sensor.
    Check the Oxygen Sensor itself. The easiest way to check if the sensor is functioning is with the Tech-1 scan tool. Start the engine, set the scan tool to indicate if the computer is in Open Loop or Closed Loop mode. Allow the engine to warm up for 5 minutes. If the sensor is functioning, the computer will got from Open Loop to Closed Loop operation mode when the water temperature and exhaust manifold reach normal operating temperatures.
    14 A Coolant Temperature Sensor Circuit - High Temperature Indicated
    Indicates that the ECM believes the CTS has failed and is reporting a high temperature reading regardless of actual temperature.
    Check the wires and connectors between the ECM and the Coolant Temperature Sensor.
    Check the resistance of the sensor with a multi-meter and verify that it is showing high resistance when it is cold, or bring up the CTS reading on the Tech-1 scan tool and compare the temperature reading to what would be expected for the ambient temperature and engine temperature.
    14 B Coolant Temperature Sensor Circuit - Low Temperature Indicated
    Indicates that the ECM believes the CTS has failed and is reporting a low temperature reading regardless of actual temperature.
    Check the wires and connectors between the ECM and the Coolant Temperature Sensor.
    Check the resistance of the sensor with a multi-meter and verify that it is showing low resistance when it is warm, or bring up the CTS reading on the Tech-1 scan tool and compare the temperature reading to what would be expected for the ambient temperature and engine temperature.
    21 A Throttle Position Sensor Circuit - Voltage Signal High
    Indicates that the ECM believes the TPS has failed and is reporting a high voltage reading regardless of the actual throttle position.
    Check the wires and connectors between the ECM and the Throttle Position Sensor.
    Checking the sensor requires the use of the Tech-1 scan tool. Set the scan tool to show TPS voltage and position. Move the gas pedal and/or throttle arm on the throttle body, and verify that the sensor reading changes with the throttle position.
    21 B Throttle Position Sensor Circuit - Voltage Signal Low
    Indicates that the ECM believes the TPS has failed and is reporting a low voltage reading regardless of the actual throttle position.
    Check the wires and connectors between the ECM and the Throttle Position Sensor.
    Checking the sensor requires the use of the Tech-1 scan tool. Set the scan tool to show TPS voltage and position. Move the gas pedal and/or throttle arm on the throttle body, and verify that the sensor reading changes with the throttle position.
    23 A Intake (Manifold) Air Temperature Sensor Circuit - Low Temperature Indicated
    Indicates that the ECM believes the IAT Thermistor has failed and is reporting a high temperature reading regardless of actual temperature.
    Check the wires and connectors between the ECM and the Intake Air Temperature sensor.
    Check the resistance of the sensor with a multi-meter and verify that it is showing high resistance when it is cold, or bring up the IAT reading on the Tech-1 scan tool and compare the temperature reading to what would be expected for the ambient temperature and engine temperature.
    23 B Intake (Manifold) Air Temperature Sensor Circuit - High Temperature Indicated
    Indicates that the ECM believes the IAT Thermistor has failed and is reporting a low temperature reading regardless of actual temperature.
    Check the wires and connectors between the ECM and the Intake Air Temperature sensor.
    Check the resistance of the sensor with a multi-meter and verify that it is showing low resistance when it is warm, or bring up the IAT reading on the Tech-1 scan tool and compare the temperature reading to what would be expected for the ambient temperature and engine temperature.
    24 Vehicle Speed Sensor Circuit
    This Trouble Code will register when the ECM notices the engine is running above idle speed for an extended period of time, and the VSS (located in the speedometer mechanism) is not registering any vehicle speed.
    Check the wires and connectors between the instrument cluster (Vehicle Speed Sensor) and the ECM. Check the speedometer cable connection at the transmission and at the speedometer. Make sure the cable itself is not broken.
    This Trouble Code is most commonly seen if the vehicle is operated with a broken or disconnected speedometer cable.
    32 Exhaust Gas Recirculation Circuit - California Emissions
    Indicates a problem with the reading reported by the temperature sensor installed in the EGR valve for vehicles equipped with California Emissions.
    The computer wants to see a different temperature reading when the EGR valve is open, and when the EGR valve is closed. This would indicate when exhaust gas is flowing through the valve and the valve is functioning properly.
    Check the function of the valve. Check the function of the transducers in the vacuum system. Check for water accumulated in the vacuum lines and/or rust inside the EGR diaphragm. Check for blockage of the EGR pipe and/or passages in the intake manifold behind the EGR valve. Check the sensor connector, wires, and output readings.
    This Trouble Code will often display when there is no problem with the EGR system. When the intake manifold and EGR valve assembly reach operating temperature, the temperature sensor will start reporting a steady reading, the temperature of the manifold and valve assembly itself, and not the temperature of air passing through the valve assembly. This is due to normal heat absorption of the metal assembly (heat soak).
    33 A Manifold Absolute Pressure Sensor Circuit - Signal Voltage High - Low Vacuum
    Indicates that the ECM believes the MAP sensor has failed and is reporting a low vacuum reading regardless of actual pressure.
    Check the hose and hose connections between the Manifold Absolute Pressure sensor and the intake manifold.
    Check the wire and connecters between the Manifold Absolute Pressure sensor and the ECM.
    Checking the sensor requires the use of the Tech-1 scan tool. Set the scan tool to show MAP voltage and reading. Check the reading before starting the engine and make sure it is close to the local barometric pressure. Start the engine, rev the engine lightly, and make sure the sensor reading shows high vacuum at idle with the engine running, and when the throttle is opened, that the vacuum drops almost to the reading before the engine was started.
    33 B Manifold Absolute Pressure Sensor Circuit - Signal Voltage Low - High Vacuum
    Indicates that the ECM believes the MAP sensor has failed and is reporting a high vacuum reading regardless of actual pressure.
    Check the hose and hose connections between the Manifold Absolute Pressure sensor and the intake manifold.
    Check the wire and connecters between the Manifold Absolute Pressure sensor and the ECM.
    Checking the sensor requires the use of the Tech-1 scan tool. Set the scan tool to show MAP voltage and reading. Check the reading before starting the engine and make sure it is close to the local barometric pressure. Start the engine, rev the engine lightly, and make sure the sensor reading shows high vacuum at idle with the engine running, and when the throttle is opened, that the vacuum drops almost to the reading before the engine was started.
    42 Electronic Spark Timing Circuit
    This code appears specific to 1992-1993 models only.
    Indicates that the ECM believes the Ignition Module has failed.
    Check the wires and connectors between the Distributor and the ECM.
    44 Oxygen Sensor Circuit - Lean Exhaust Indicated
    Indicates that the ECM believes the O2 sensor has failed and is reporting a lean fuel mixture reading regardless of actual condition.
    Check the wires and connectors between the ECM and the Oxygen Sensor.
    Checking the sensor requires the use of the Tech-1 scan tool. Bring the vehicle up to operating temperature and wait for the ECM to go into Closed Loop mode. Set the scan tool to read the O2 sensor reading. The normal reading will jump to the extremes of rich and lean repeatedly (100-900 ohm). This will indicate that the computer is constantly adjusting the gasoline flowing through the injectors, and that the sensor is constantly reading the adjustments. If the sensor reading is constantly at one end of the range or the other, a problem is indicated. Usually a sensor failure and the sensor is no longer able to read the full range of measurement.
    45 Oxygen Sensor Circuit - Rich Exhaust Indicated
    Indicates that the ECM believes the O2 sensor has failed and is reporting a rich fuel mixture reading regardless of actual condition.
    Check the wires and connectors between the ECM and the Oxygen Sensor.
    Checking the sensor requires the use of the Tech-1 scan tool. Bring the vehicle up to operating temperature and wait for the ECM to go into Closed Loop mode. Set the scan tool to read the O2 sensor reading. The normal reading will jump to the extremes of rich and lean repeatedly (100-900 ohm). This will indicate that the computer is constantly adjusting the gasoline flowing through the injectors, and that the sensor is constantly reading the adjustments. If the sensor reading is constantly at one end of the range or the other, a problem is indicated. Usually a sensor failure and the sensor is no longer able to read the full range of measurement.
    51 ECM Failure
    Indicates that the ECM has failed, or the EPROM within the ECM has failed.
    Check the electrical connections for the ECM.
    If code persists, the ECM requires replacement.


    Clearing the Trouble Codes, or Resetting the ECM
     Once the repair has been executed, the Trouble Codes should be cleared.
    This can be accomplished by two methods:
    • Disconnect the power from the ECM for 30 seconds. This can be done by disconnecting the battery for 30 seconds (easiest method), removing the ECM fuse from the fuse box for 30 seconds, or unplugging the ECM for 30 seconds (most difficult method).
      One thing to remember with this method is that this will also clear all the settings that the ECM has recorded, and when the engine is started again after the power has been restored, the ECM will go into learning mode, and operate at high idle and rich fuel mixture, until it has reset all of its learned settings.
    • Access the Trouble Code functions of the Tech-1, and select to clear the Trouble Codes from the ECM.
      This method will not throw the ECM back into learning mode, because it will retain all of the learned settings and will operate based on the measurements retained before the codes were cleared.


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