Chapter 25 Computer System Service 411
Sensor Service
Sensor service involves testing and, if needed,
replacing computer system sensors. Since sensor
designs vary and some can be damaged by incorrect
testing methods, it is important for you to know the
most common ways of checking sensor values.
For testing purposes, you can classify sensors into
one of two categories: passive sensors or active sen-
sors. This will help you select a testing method.
Testing Passive Sensors
Since passive sensors do not generate their own
voltage, the computer must feed them a reference
voltage. A passive sensor can change its internal resis-
tance with a change in system or vehicle condition
or operation. This resistance change modifi es the ref-
erence voltage, which is interpreted and used by the
computer to control the engine’s various systems.
A B C
D E F
Figure 25-2. The scan tool will usually direct you to the area of the vehicle causing the problem. A—“Read DTC Options” allows
you to select stored codes, pending codes (codes that are almost out of normal operating parameters), and manufacturer data.
B—The scan tool denotes the vehicle has tripped two trouble codes. C—The scan tool readout explains that the mass or volume
airfl ow sensor circuit is showing low voltage or current input to the computer. D—The scan tool readout shows there is no engine
misfi re problem, the fuel injection system is working normally, and the catalytic converter is working. E—No signal from the heated
catalyst may be due to the engine not warming up completely. The evaporative emission control system is working normally. Air
conditioning has no signal because it might not be turned on. F—The scan tool indicates both O2 sensors are working normally.
The EGR system may not have a signal because the engine has not warmed fully. (Snap-on Tool Corp.)
Figure 25-3. Always inspect the engine compartment for signs
of trouble. A loose or corroded connection or a vacuum leak
may be setting trouble codes or tricking the computer system.