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How Do You Know When A Sensor Has to Be Replaced?

The vehicle will usually have some kind of driveability problem (hard starting, stalling, hesitation, poor mileage, high emissions, etc.) and/or a glowing "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 do fail in as little as 30,000 miles. Other sensors should last the life of the vehicle. All are covered under the vehicle manufacturers five year/50,000 miles emissions warranty.

Troubleshooting sensor problems requires checking the on-board diagnostics to see if the computer has set a "trouble code" 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. To the problem in these situations, a technician may have to test drive the car with a portable "flight recorder" plugged into the on-board computer system. When the problem occurs, pressing a button records the various sensor readings for later analysis.

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