Reading and Clearing Diagnostic Trouble Codes

Fig. 36 Engine diagnosis flow chart.:

Fig. 37 Interpreting diagnostic codes.:

Fig. 38 ``Check Engine'' lamp & diagnostic switch:

Fig. 39 Reading ``Check Engine'' lamp codes.:

On these models, the ECM continually monitors operating conditions of their controlled systems for possible malfunctions. When a problem is detected, a problem is detected, a two-digit numerical ``trouble-code'' is stored in the computer memory. These codes can be displayed as an aid in system repair. Any malfunctions which the vehicle operator should be aware of will illuminate the ``Check Engine'' lamp on the instrument panel.
The computer controlled emission control system incorporates a self diagnostic code function which lights the ``Check Engine'' lamp on the instrument panel in the event of a system malfunction. As long as the emission control system is operating properly, the ``Check Engine'' lamp will light along with the other indicator lamps when the ignition switch is turned on. After the engine is started, the lamp goes out while the engine is running. If there is trouble in the system, the ``Check Engine'' lamp remains ``On'' and the trouble code is stored in the ECM trouble code memory.
The system has a diagnosis switch which is usually set in the ``Off'' position, but when turned on for diagnosis, it indicates the trouble code in the ECM memory by flashing the ``Check Engine'' lamp in a diagnostic code to indicate the area where the malfunctioning devices can be detected.
If the ``Check Engine'' lamp is ``On'' when the engine is running, a system malfunction is indicated. Refer to Figs. 36 and 37 to determine diagnostic codes.
When the ignition switch and the diagnosis switch, Fig. 38, are turned ``On,'' the ``Check Engine'' lamp indicates code 12 three times, then the trouble codes three times each, and continues to cycle between code 12 and the trouble code (13) as shown (A), Fig. 39. Code 12 consists of one flash, a short pause, then two flashes, which indicates the system is functioning properly. The code following code 12 is a trouble code, which means that trouble has occurred in the system to which the particular code corresponds.
If two or more trouble codes are stored in trouble code memory (trouble has occurred in more than two systems), the ``Check Engine'' lamp indicates code 12 three times, the each of the trouble codes in ascending order. Fig. 39, (B) illustrates codes 12, 13 and 21.