On Board Diagnostic Systems
On Board Diagnostic System Topics
  1. Introduction
  2. OBD II Systems
  3. OBD I and Prior Check Engine Lights
Malfunction Indicator Light (also called the Check Engine LightAutomobile manufacturers started using electronic monitoring of engine functions during the 1970's and early 1980's, primarily to help meet EPA emission standards. The monitors were initially proprietary and called Check Engine lights or similar names. Eventually, a standard was developed by the Society of Automotive Engineers (SAE) and adopted by the Enviromental Protection Agency (EPA). The standard was called On Board Diagnostics (OBD). In the mid-90's a more sophisticated standard called On Board Diagnostics II (OBD II) was developed by SAE and adopted by the EPA. The earlier OBD system was then dubbed OBD I to differentiate it from the new OBD II.

The On Board Diagnostic (OBD) generation II system continuously monitors all engine and transmission sensors and actuators for electrical faults and other out of bounds parameters, primarily to detect emission control failures. When a fault is detected, the computer stores a diagnostic trouble code, and illuminates the Malfunction Indicator Light (MIL). Most people call the MIL the Check Engine light.

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OBD II Systems
The OBD II system has three different types of signals.

In all cases the OBD system computer stores a trouble code for diagnostic use by the automotive technician. The trouble code indicates the type of failure.

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OBD I and Prior Check Engine Lights
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.

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