After studying this chapter, you will be able to:
❚ Perform a visual inspection of the computer,
its sensors, actuators, and the systems they
monitor and control.
❚ Test sensors and their circuits.
❚ Remove and replace sensors.
❚ Test and replace actuators.
❚ Remove and replace a computer.
❚ Program an EEPROM.
❚ Demonstrate safe working practices when
servicing automotive computers.
❚ Correctly answer ASE certifi cation test questions
on servicing computer system components.
Since almost all vehicle systems now use computer
components, you must have a thorough knowledge
of computer circuit service methods. Otherwise, you
will not fully understand the chapters on the service
and repair of fuel injection, ignition, and emission
This chapter will briefl y summarize how to test com-
puter components and circuits and help you develop
the skills needed to verify where problems are.
After you have checked the computer for trouble
codes, you can fi nd the exact source of the problem
by doing pinpoint tests. Pinpoint tests are more
specifi c tests of individual components. The service
manual will normally explain how to do each pin-
point test. It will show specifi c tests, as well as pro-
vide component electrical values and other critical
Remember that trouble codes only indicate the
area of trouble and sometimes the type of problem,
not what part or circuit is at fault. It is therefore
imperative that you know how to do basic electrical
tests on individual components.
Preliminary Visual Inspection
A preliminary visual inspection involves look-
ing for signs of obvious trouble: loose wires, leak-
ing vacuum hoses, part damage, etc. For example,
if the trouble code says there is something wrong in
the coolant temperature sensor circuit, you should
check the sensor resistance and the wiring going to
the sensor. You should also check the coolant level
and the thermostat. A low coolant level or engine
overheating could also set this code.
KISS is an acronym that could help you fi nd the
source of performance problems on a computer-
controlled vehicle. KISS stands for “keep it simple,
stupid”. This means you should start your trouble-
shooting with the simple checks and tests. Then, as
the common causes are eliminated as the source of
the problem, you will move to more complex tests.
It is easy for the untrained person to instantly think
“computer problem” when an engine runs rough,
fails to start properly, or exhibits some other perfor-
For example, contaminated engine oil can trigger
a computer trouble code, Figure 25-1. Fumes from
the contaminated oil can be drawn into the engine’s
intake manifold from the crankcase. If these fumes
are excessively strong, the oxygen sensor could be
tricked into signaling a rich air-fuel mixture. The
computer would then lean the mixture to compen-
sate for the crankcase fumes. An oxygen sensor trou-
ble code may be produced and, in some cases, an
engine performance problem could result.
As this points out, it is critical that you check for
conventional or simple problems fi rst. Start check-
ing for computer problems only after all the conven-
tional causes have been ruled out.