The Engine Control Module (ECM) monitors the output signals (some signals at all times and the others under specified conditions) from various sensors. When it is noticed that a signal irregularity has continued for a specified time or longer, the ECM judges that an irregularity has occurred, memorizes the diagnostic trouble code, and outputs the signal to the diagnostic output terminals.
There are 16 (for Pick-up trucks) or 18 (for Monteros) diagnostic items. Since memorization of the diagnostic trouble codes is backed up directly by the battery, the diagnostic results are memorized even if the ignition key is turned OFF. The diagnostic trouble codes will, however, be erased when the battery terminal or the ECM connector is disconnected. The diagnostic trouble codes are also erased by setting the ignition switch to the ON position and then disconnecting the negative (-) terminal of the battery for ten seconds or longer.
The 16 or 18 diagnostic items are listed in the Diagnostic Trouble Charts and are all indicated sequentially from the smallest code number to the largest.
NOTE: The ignition timing adjustment signal diagnostic trouble code is output when the ignition timing adjustment terminal line is short-circuited to ground. This is not a malfunction.
To read out the trouble codes, a voltmeter will be needed. The trouble codes are output as a series of voltage spikes of two different lengths. The different lengths are 1.5 seconds long for the large spike and 0.5 seconds long for the short spike. Each long spike is equal to ten and each short spike is equal to one, therefore if a trouble code reads out as two long spikes followed by four short spikes, this would be trouble code No. 24.