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.