Computer Controlled Engine Management
& Emission System Failures
The job of the engine is to convert the energy stored in gasoline into mechanical energy to propel the car down the road. The job of the computer control system is to analyze sensor input information and decide which components to activate for the best possible combination of performance, fuel economy and reduced emissions under any given driving condition.

Engine Control Modules are programmed to maintain engine efficiency and to perform diagnostic functions. When sensors or actuators fail , the ECM will set a diagnostic trouble code and activate the malfunction indicator lamp. From 1978 to 1996 the computer diagnostics were referred to as OBD I. The number one goal of these diagnostic programs is to let the driver know that the emission levels are not being maintained and repair action must be taken to correct the malfunction.
What Turns On The Malfunction Indicator Lamp On OBD I Systems?
OBD I systems are required to monitor the following :
  • EGR System
  • Fuel Metering
  • Five Major Sensor Input
  • Exhaust Oxygen Content
  • Engine RPM
  • Throttle Position
  • Air & Coolant Temperature
  • Engine Load
What Goes Wrong?
Each of these inputs can affect emissions directly, because they affect the air fuel mixture being supplied to the cylinders. If there is a problem with one of these sensors it's likely that the emission levels will rise above the federal regulations.
If any of the systems or sensors being monitored fail due to the circuit shorting, goes open or if the voltage signal is out of a specified range. The ECM will set a 'Trouble Code' and activate a light on the dash to indicate a system failure. When the ECM's diagnostic mode is activated by a technical, the stored trouble codes will be displayed. Some systems flash the 'Check Engine' light on the dash while others must be accessed with a volt meter or scan tool. The trouble code number indicates what circuit has malfunctioned.