The self-diagnostic system used on all fuel injected engines is capable of detecting specific system malfunctions and storing trouble codes. All codes are available for examination unless codes have been erased, or ignition has been cycled on/off 50 times since the malfunction last occurred. There are two variations of the this system. The 1984 E15ET engine and all 1987-90 fuel injected engines, use a dual LED type ECM. This ECM has 5 diagnostic modes. The LED is used for diagnostics and troubleshooting. A malfunction is displayed by both the red and green Light Emitting Diodes (LED's), if so equipped.
All 1991-96 vehicles use an Engine Control Module (ECM) with a single LED located on the side. The ECM lamp will blink simultaneously with the "Check Engine'' lamp located on the instrument panel. These ECMs only have 2 modes.
Modes will not switch while the engine is running. The ECM automatically switches to MODE I when the ignition is switchedOFF.1984-90 Vehicles With Dual LED System
The self-diagnostic system has 5 modes. To switch modes, switch the ignition ON and use a screwdriver to turn the mode select switch through the access port on the ECM. The inspection lights will begin to flash. When the desired mode has been entered, turn the screwdriver fully counterclockwise. The red LED and green LED perform different functions in different modes.Mode I-
This is the normal vehicle operating mode. The green LED will indicate loop status. If the green LED is not blinking when the engine is running, the vehicle is in open loop, or the HO 2 S, O 2 S, or sensor circuit has a malfunction. If the green LED is blinking, the vehicle is in closed loop. If a malfunction takes place, the red LED and the MIL will glow, indicating a malfunction has occurred.Mode II-
Both the red LED and green LED are used to monitor mixture ratio feedback control. The green LED will function as described in MODE I. If vehicle is in open loop, the red LED and green LED will remain on or off. In order for results to be valid, vehicle must be in closed loop. In closed loop, the red LED remains off if the system is running rich, comes on if the system is running lean, and blinks at the same time with the green LED if system is within 5% of the exact air/fuel ratio.Mode III
-When this mode is accessed, the codes stored in the ECM's memory will be flashed by the green LED and red LED on the side of the ECM. The red LED will light first. This is the first digit of the trouble code; the green LED will flash next, showing the second digit of the code. For example: 2 flashes of the red LED followed by 1 flash of the green LED corresponds to Code 21.Mode IV-
This mode is used for checking the throttle plate (TP) switch, starter, and vehicle speed switch status. When the TP switch or starter switch is activated, the red LED will briefly flash as the status changes. For VSS status, the green LED will remain off when vehicle speed is less than 12 mph (19 km/h) and come on when vehicle speed is greater than 12 mph (19 km/h).Mode V-
This mode represents a real-time diagnostic test of the crank angle sensor, ignition signal and the mass airflow signal. The malfunction code will be displayed only when the malfunction occurs and will not be stored in memory. The output code will appear as a single or short string of pulses. Count the number of pulses for the malfunction code.
1991-96 Engines With Single LED System
See Figures 1, 2, 3, 4, and 5
Modes will not switch while the engine is running. The ECM automatically switches to MODE I when the ignition is switchedOFF.
To switch modes, turn the ignition on and use a screwdriver to turn the mode select switch through the access port on the ECM. Turn the mode select switch clockwise and hold for 2 seconds before turning the screwdriver counterclockwise.Mode I (Normal/Malfunction Warning)
-On California models, if a malfunction occurs with the engine running, the red LED and the MIL will glow. This indicates an ECM malfunction has occurred, and a code has been stored. On Federal models, codes are stored, and the red LED will glow only when the CPU in the ECM malfunctions.Mode II (Self-Diagnostics)
-When mode is accessed (the engine OFF), ECM memory codes will be flashed by the MIL and the red LED on the side of the ECM. Long flashes (0.6 second) represent the first digit of the code. Short flashes (0.3 second) represent the second digit of the code. For example: 4 long flashes followed by 3 short flashes of the MIL (or the red LED) would indicate a Code 43.
Mode II (HO2S or O2S monitor) -When this mode is used (while the engine is running), both the red LED and the MIL are used to monitor air/fuel mixture feedback control. For accurate results, the vehicle must be in closed loop. In closed loop, the red LED will indicate if system is running rich (light off), lean (light on) or at ideal air/fuel ratio (blinking at 5 times in 10 seconds).
See Figure 6Dual LED System
Switch from diagnostic Mode III to Mode IV with the engine off by turning the diagnostic mode selector clockwise. Turning the test selector fully clockwise will cause the diagnostic modes to cycle until a mode is selected. Also, disconnecting the battery for no less than 24 hours will clear codes.Single LED System
Switch from diagnostic Mode II to Mode I with the engine off by turning the diagnostic mode selector fully clockwise. The diagnostic mode selector is located on the side of the engine control module (ECM). Wait for 2 seconds before turning the mode selector counterclockwise. Also, disconnecting the battery for at least 24 hours will clear codes.