Computer Diagnostics

OBD I and Prior Check Engine Light

The emissions malfunction indicator on pre OBD II vehicles (most models prior to 1996) is known on some models as the "Check Engine", "Power Loss", "Service Engine Now", or "Service Engine Soon" light. This light is intended to alert the operator when there is a failure in the system that may cause an increase of harmful emissions.

The light illuminates when the ignition key is in the ON position and the engine is OFF; this is to functionally test the system and check the bulb. When the light turns ON during engine operation, even momentarily, a system diagnosis is necessary to determine the fault.

When the light is ON steady it means there is a fault currently detected. If the light illuminates and then turns off it can mean that the fault is intermittent or that the fault is only being detected intermittently. In either case, if the light is intermittent it usually means the technician will have to try to recreate the operating conditions under which the light illuminates in order to diagnose the problem.

Diagnosis of an intermittent problem is more difficult, sometimes a hit or miss situation, and sometimes requires bringing the vehicle in several times before the fault is located. Although this warning lights purpose is to warn of increased emissions, in most cases if the system is not promptly repaired, damage to other components can occur.

OBD II System
OBD II Malfunction Indicator Lamp (MIL)

When the ignition switch is initially turned on and the engine is not running, the malfunction indicator lamp lights for a bulb check. While the engine is running, the MIL will light only if there is an emissions-related concern.

The on board diagnostic (OBD) generation two (II) system continuously monitors all engine and transmission sensors and actuators looking for electrical faults, as well as values that do not logically (rationally) fit with other powertrain data. When certain operating conditions are met and a comprehensive monitor detects a failure that will result in emissions exceeding a predetermined level, the computer stores a diagnostic trouble code, and illuminates the MIL.

The OBD II system also actively tests some systems for proper operation while the vehicle is being driven. Fuel control and engine misfire are checked continuously, catalyst efficiency, exhaust gas recirculation operation, evaporative system integrity, oxygen sensor response, and the oxygen sensor heaters are tested once per trip when prerequisite operating conditions are met. The computer will illuminate the MIL if during these prerequisite operating conditions the system detects a failure that will result in emissions exceeding a predetermined level.

Whenever an engine misfire severe enough to damage the catalytic converter is detected, the MIL will blink on and off.

Once lit, the MIL will remain on until the vehicle has completed three consecutive good trips (three trips in which the fault is not detected). The MIL is also turned OFF when stored diagnostic trouble codes are cleared. However, the MIL will only remain OFF if the fault is successfully repaired.

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