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.
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HOMESERVICE MANUALSGEO1993PRIZM L4-1600CC 1.6L DOHCREPAIR AND DIAGNOSISPOWERTRAIN MANAGEMENTCOMPUTERS AND CONTROL SYSTEMSDESCRIPTION AND OPERATIONSELF DIAGNOSTIC FUNCTIONS
1993 Geo Prizm L4-1600cc 1.6L DOHC
Self Diagnostic Functions
1993 Geo Prizm L4-1600cc 1.6L DOHCSECTION Self Diagnostic Functions
SELF-DIAGNOSIS FUNCTION
The ECM/PCM also controls the diagnostic function of the fuel control system. It can recognize operational problems, alert the driver through the malfunction indicator lamp (MIL), and store a diagnostic trouble code (DTC) which identifies the problem areas to aid the technician in making the necessary repairs to the following circuits:
^ Oxygen Sensor (02S).
^ Engine Coolant Temperature (ECT)
^ Intake Air Temperature (IAT) Sensor
^ Manifold Absolute Pressure (MAP) Sensor
^ Ignition Signal
^ Vehicle Speed Sensor (VSS)
^ Knock Sensor (KS)
^ Exhaust Gas Recirculation (EGR) System
^ Central Processing Unit (CPU) of ECM/PCM
The ECM/PCM and MIL operate as follows:
The MIL illuminates when the ignition switch is turned "ON" (but the engine is stopped) with the diagnostic request terminals ungrounded regardless of the condition"of the electronic engine control system. This is known as an instrument panel bulb check and is only to check the MIL and its circuit.
If no malfunctions in the electronic engine control system exist after the engine starts, the MIL goes out. If a malfunction exists in the electronic engine control system, the MIL may turn off, but only until the ECM/PCM detects the malfunction.
If a malfunction is detected in the electronic engine control system while the engine is running, the ECM/PCM lights the MIL to alert the driver. At the same time, the ECM/PCM stores a diagnostic trouble code (DTC) in the ECM/PCM memory. The DTC is stored in the ECM/PCM memory even if the malfunction was temporary and is no longer present. The DTC remains stored in the ECM/PCM memory until the power to the ECM/PCM is interrupted for 10 seconds or longer.
The ECM/PCM also indicates DTCs in its memory by means of flashing the MIL at the time of inspection (i.e., when diagnostic request terminals are grounded and the ignition switch is in the "ON" position).
The ECM/PCM also controls the diagnostic function of the fuel control system. It can recognize operational problems, alert the driver through the malfunction indicator lamp (MIL), and store a diagnostic trouble code (DTC) which identifies the problem areas to aid the technician in making the necessary repairs to the following circuits:
^ Oxygen Sensor (02S).
^ Engine Coolant Temperature (ECT)
^ Intake Air Temperature (IAT) Sensor
^ Manifold Absolute Pressure (MAP) Sensor
^ Ignition Signal
^ Vehicle Speed Sensor (VSS)
^ Knock Sensor (KS)
^ Exhaust Gas Recirculation (EGR) System
^ Central Processing Unit (CPU) of ECM/PCM
The ECM/PCM and MIL operate as follows:
The MIL illuminates when the ignition switch is turned "ON" (but the engine is stopped) with the diagnostic request terminals ungrounded regardless of the condition"of the electronic engine control system. This is known as an instrument panel bulb check and is only to check the MIL and its circuit.
If no malfunctions in the electronic engine control system exist after the engine starts, the MIL goes out. If a malfunction exists in the electronic engine control system, the MIL may turn off, but only until the ECM/PCM detects the malfunction.
If a malfunction is detected in the electronic engine control system while the engine is running, the ECM/PCM lights the MIL to alert the driver. At the same time, the ECM/PCM stores a diagnostic trouble code (DTC) in the ECM/PCM memory. The DTC is stored in the ECM/PCM memory even if the malfunction was temporary and is no longer present. The DTC remains stored in the ECM/PCM memory until the power to the ECM/PCM is interrupted for 10 seconds or longer.
The ECM/PCM also indicates DTCs in its memory by means of flashing the MIL at the time of inspection (i.e., when diagnostic request terminals are grounded and the ignition switch is in the "ON" position).
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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.