How do you know when a sensor needs to be
The vehicle will usually exhibit a driveability
problem (hard starting, stalling, hesitation, poor mileage, high emissions,
etc.) and/or an illuminated check engine light. Many things other than
a bad sensor can cause driveability problems, but a check engine light
is a good indication that the problem is in the electronics.
Mileage is another consideration. The oxygen
sensor should go 50,000 miles or more, but some fail in as little as
30,000 miles. Other sensors should last the life of the vehicle. All
are covered under the vehicle manufacturer's five year/50,000 miles
Troubleshooting sensor problems requires
checking the on-board diagnostics to see if the computer has set a trouble
code (see chart) corresponding to one of the sensor circuits. This is
done by either putting the computer into a special diagnostic mode and
counting check engine "flashes" or special diagnostic LEDs
on the computer itself (many import applications), or by plugging a
special scan tool into the diagnostic connector to access on-board diagnostics.
The latter is the preferred technique because
it also allows you to read sensor voltages and inputs directly on most
GM and some Ford and Chrysler systems.
A trouble code does not necessarily mean
a sensor is bad. However. It only means a problem has been detected
in a particular sensor circuit. It could be the sensor, the wiring,
or a connector somewhere in the wiring harness.
To isolate the fault, a series of diagnostic
tests usually have to be performed, following a step-by-step procedure.
Tests may require the use of a breakout box that allows individual circuits
to be tested. By checking continuity, resistance and/or voltage readings,
the faulty component can be isolated.
Another approach is to use a tester that
simulates voltage, resistance or frequency inputs from various sensors.
The tester is used in place of a sensor to produce a substitute signal.
If the on-board computer then responds properly, the sensor is assumed
to be faulty.
Intermittent faults are the hardest to find, and some
sensor problems may not generate a trouble code at all. A technician
may have to test drive the car with a portable "flight recorder"
plugged into the on-board computer system in order to locate the problem.
When the problem occurs, pressing a button records the various sensor
readings for later analysis.