A modern vehicle is a network of control systems. The engine computer, transmission controller, ABS/ESC module, body controller, climate-control module and driver-assistance systems all need information about what the vehicle is doing. Sensors provide that information. A sensor does not usually ‘decide’ anything by itself. It converts a physical condition such as temperature, pressure, position, speed, oxygen content or acceleration into an electrical value that a control module can interpret.
The important beginner idea is that a sensor is only one part of a measuring chain. There is the physical condition being measured, the sensing element, the wiring and connector, the power and ground supplies, the signal circuit, the control-module input and finally the software that converts the signal into a useful value. A fault anywhere in that chain can make the scan-tool value look wrong. Replacing the sensor without proving which part of the chain failed is one of the most common causes of wasted parts.
Some sensors create their own signal. A variable-reluctance wheel-speed sensor, for example, can generate an AC waveform as a toothed ring passes the sensor. Other sensors need a supply from the vehicle, commonly 5 V or 12 V, and return a changing signal. Still others communicate digitally over a data bus. Understanding which type you are dealing with determines what testing is safe and meaningful.
Throughout this book, use the same habit: identify what the sensor measures, identify its power/ground/signal arrangement, compare the reported value with reality, and then test the circuit before condemning the component. That method works far better than memorizing a list of fault codes.
Figure: What an Automotive Sensor Actually Does
Diagnostic habit: Compare the sensor reading with reality and with at least one related measurement before buying a part.