Car Sensors Connecting to Malfunction Indicator

Education posted on 12/12/18

An illuminated or flashing “Check Engine” light — more formally known as the Malfunction Indicator Lamp (MIL) — is a signal from the car’s engine computer that something is wrong. The difficulty lies in determining exactly what has caused the light to appear. The original idea was for a system that would continuously monitor a car’s emissions system over the life of the car. However, that simple idea has evolved to a much more complicated system, especially since the Federal government started regulating emissions.

It all began with the Federal emissions regulations Clean Air Act of 1963, and has continued to become more and more complicated, up to the Amendments of 1990 which started the compulsory monitoring of engine performance called “On Board Diagnostics” (OBD).

Emission control or emission related components that OBD must monitor:

  • Catalyst efficiency
  • Oxygen sensor response and heater
  • Exhaust gas recirculation (EGR) system
  • Fuel delivery system
  • Engine misfire
  • Evaporative system
  • Secondary air system
  • Air conditioning system if using R-12
  • Powertrain control module (PCM)
  • All other electronic inputs/outputs to the PCM

The following gives a summary of the legal requirements in the USA and in Europe:

Detection of electrical failures of emission related components.

Detection of plausibility failures of emission related components.

Actuator faults and detection of degradation of emission related components and subsystems:

  • Catalytic converter (HC, CO and NOx)
  • 02 sensors
  • Particulate filters (In Europe)
  • Fuel system
  • Misfire
  • Evaporative system leakage detection (In USA)
  • Evaporative system electrical diagnostic (In Europe)
  • Crankcase ventilation (In USA)
  • A/C refrigerant loss
  • Secondary air system (In USA)
  • Coolant thermostat (In USA)

Starting with model-year 1996, automakers have had to standardize their systems under a protocol called OBD-II, which stipulates a standardized list of diagnostic trouble codes (DTC) and mandates that all cars provide a universal connector to access this information.

If you are concerned with basic engine functions, the following are the “essential” or “major” computer sensors- the first sensors to be checked:

  • Manifold Absolute Pressure sensor
  • Mass Airflow Sensor (if so equipped)
  • Engine Block Temperature Sensor
  • Crankshaft/Distributor Position Sensor

Even the basic sensors can be complicated. For example, there are two types of oxygen sensor trouble codes: O2 heater circuit codes and O2 sensor codes. There are various kinds of oxygen sensors, conventional zirconia, titania and wide-range.

At this time, the various sensors included in the OBD-II include:

  • Coolant sensors
  • MAP sensors
  • BARO sensors
  • Throttle position sensors (TPS)
  • Mass airflow sensors (MAF)
  • Vane airflow sensors (VAF)
  • Karmen vortex airflow sensors
  • Knock sensors
  • Air temperature sensors
  • EGR valve position sensors
  • Crankshaft position sensors
  • Camshaft position sensors
  • Vehicle speed sensors (VSS)
  • Wheel speed sensors (WSS) on ABS systems
  • Steering angle sensors
  • G-Force sensors
  • Airbag crash sensors

The asTech® Remote Diagnostic Device helps shop side technicians diagnose and repair vehicles when the “check engine light” is illuminated. For more information, visit

Other Articles

Want to Stay Up-to-Date?

Sign up for the asTech newsletter