Check engine lights can seem mysterious. Understanding how a check engine light occurs greatly increases the likelihood of repairing the issue. Generally speaking, it’s all just voltage.


Battery voltage enters the engine control unit. Battery voltage will be about 12 volts, but may be higher or lower. In order for the computer to work accurately, it needs a steady and constant voltage supply. The computer therefore takes battery voltage and reduces it to reference voltage. Reference voltage will be much lower than battery voltage, and commonly is 5 volts

The reference voltage then enters a sensor of some kind. Based on what the sensor is seeing, it will have a varying level of resistance. A temperature sensor may increase in resistance as the temperature increases. The amount of resistance combined with the known reference voltage allows the car to calculate the sensor’s environment (temperature, throttle position, etc)

If 5 volts enter a sensor that has 2 volts of resistance, then the computer will see 3 volts as the signal voltage. The car then compares what the sensor is reading to what it expects to see. If the voltage is not within the accepted levels, a check engine light results. The process by which a car sends reference voltage to a sensor and the sensor returns a signal voltage for the computer to read is known as closed loop diagnostics. The voltage is sent out, and returned, completing a loop.

So far, it’s fairly simple. The difficulty occurs when there are multiple components working together in a loop.

Let’s consider a EGR system that uses an electronically opened valve, and the MAP sensor to check EGR flow. When the system works correctly, the computer opens the EGR valve, and can measure how far the valve is open, based on the pintle position sensor inside the EGR valve. As exhaust gas enters the intake manifold, the absolute manifold pressure changes, and the MAP sensor registers a change. The computer knows that when the pintle position changes form point A to pint B, there will be X amount of change seen by the MAP.

Potential failures:

  1. Valve does not open, or is sticky and opens or closes slowly
  2. Plugged ports from exhaust to valve, or valve to intake
  3. Pintle position sensor not reading correctly
  4. MAP sensor not reading correctly
  5. Faulty vacuum supply to the MAP sensor
  6. Faulty wiring / improper voltage reference
  7. Faulty ECU

Any of the problems, alone or in tandem could cause one or many EGR related problems. The number of potential problems quickly becomes time consuming to properly diagnose. Each item must be isolated and tested. This is where many mechanics begin to have trouble. Rather than taking the time to inspect, check, document, test or repair, re-inspect and re-document, they call a part bad and order a new one.

Although “known good part” is one of the potential diagnostic procedures, it is time consuming in it’s own way, and is not cost effective. Taking the time to understand each component of a diagnostic loop generally yields the best results. It can be frustrating, but the effort is generally worth it in the long run.