Self-Diagnosis Function

ECM diagnoses troubles which may occur in the areas including the following parts when the ignition switch is ON or the engine is running, and indicates the result by turning on or flashing malfunction indicator lamp ("CHECK ENGINE" light).
^ Heated oxygen sensor
^ Intake air temp. sensor
^ Engine coolant temp. sensor
^ Throttle position sensor (including idle switch)
^ Vehicle speed sensor
^ Mass air flow sensor
^ Ignition fail-safe signal
^ Camshaft position sensor
^ EGR system including EGRT sensor (for 1994 model USA spec. and 1995 model vehicle only)
^ CPU (Central Processing Unit) of ECM
ECM and malfunction indicator lamp ("CHECK ENGINE" light) operate as follows.
^ Malfunction indicator lamp ("CHECK ENGINE" light) lights when the ignition switch is turned ON (but the engine at stop) with the diagnosis switch terminal ungrounded regardless of the condition of Electronic Fuel Injection system. This is only to check the malfunction indicator lamp ("CHECK ENGINE" light) bulb and its circuit.
^ If the above areas of Electronic Fuel Injection system is free from any trouble after the engine start (while engine is running), malfunction indicator lamp ("CHECK ENGINE" light) turns OFF.
^ When ECM detects a trouble which has occurred in the above areas, it makes malfunction indicator lamp ("CHECK ENGINE" light) turn ON while the engine is running to warn the driver of such occurrence of trouble and at the same time it stores the exact trouble area in ECM back-up memory.
(The memory is kept as it is even if the trouble was only temporary and disappeared immediately. And it is not erased unless the power to ECM is shut off for 20 seconds or longer.)
ECM also indicates trouble area in memory by means of flashing of malfunction indicator lamp ("CHECK ENGINE" light) at the time of inspection (i.e. when diagnosis switch terminal is grounded and ignition switch is turned ON).
^ Only federal spec. vehicle except California emission spec. and Canada spec. vehicle has mileage sensor and cancel switch in its malfunction indicator lamp ("CHECK ENGINE light) circuit.
When mileage reaches 50,000, 80,000 and 100,000 miles respectively (i.e. mileage sensor turns ON) malfunction indicator lamp ("CHECK ENGINE" light) turns ON even while engine is running. This is to warn the driver that it is time for periodical inspection. In this case, turn OFF malfunction indicator lamp ("CHECK ENGINE" light) with its cancel switch upon completion of inspection.

NOTE:
Only ignition circuit trouble (code No. 41) among the above areas is not stored in back-up memory of ECM. (In other words, even if ECM has detected a trouble in ignition circuit, once ignition switch is turned OFF, code No. 41 will not be indicated even when diagnosis switch terminal is grounded and ignition switch is turned ON). Therefore, to check diagnostic trouble code when engine fails to start, crank engine and then ground diagnostic switch terminal with ignition switch ON.
Even when a trouble occurs in Camshaft Position (CMP) Sensor or idle switch circuit (circuit open), ECM does not indicate it (or activate malfunction indicator lamp ("CHECK ENGINE" light)) while engine is running.
And when that troubled circuit regains good condition, the memory of defective area will be erased automatically even if the power circuit to ECM is not opened as described above.