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Why does my vehicle need all these different sensors?

When Theresa learned that her car needed a new oxygen sensor, she recalled that not long ago, the mass airflow (MAF) sensor had been replaced. And not very long before that, the coolant temperature sensor (CTS) needed replacement.

When her brother suggested that she might need to have her throttle position sensor (TPS) checked, she found herself wondering, "What on earth are all these sensors for, and why would they need to be replaced? Why can't I just keep driving with the original ones?"

This is a logical question, especially when you consider that the vehicles manufactured just a few decades ago seemed to get along fine without any sensors at all! To find the answer, we need to look at how today's high-tech vehicles differ from their low-tech predecessors.

Any car or light truck built since the early 1980s has a computer-controlled engine that's run by an electronic control module (ECM). In fact, some of the newest vehicles can easily have six or more different ECMs that control the engine, transmission, antilock brakes, cruise control, electronic suspension, climate control, etc. The primary onboard computer is now called the power-train control module, or PCM, on many of these vehicles.

Once your engine is warmed up, the PCM monitors signals from various electronic sensors and uses that information to continually adjust critical functions like ignition timing, air-to-fuel ratio, cooling fan operation, alternator output and the emission control system.

These sensor inputs are constantly changing as you accelerate, slow down, climb hills and raise or lower the engine's operating temperature. As the inputs change, the PCM continually "tweaks" the various engine management functions to optimize performance and fuel economy while minimizing the release of harmful pollutants into the atmosphere.

To do this effectively, the PCM must continually be updated with fresh information. So the sensors feed it a steady stream of data such as vehicle speed, engine speed, throttle position, engine operating temperature, exhaust gas content and so forth. In essence, the sensors serve as the eyes and ears of the computerized control modules.

This is how the system is supposed to work when everything is functioning smoothly. If one of the sensors should send a false or abnormal signal (or no signal at all) to the PCM, however, the control module may find itself flying blind without sufficient information about how to do its job properly. At this point, it may start making bad decisions. Or it may switch to an "open loop" or "limp home" mode so the engine can continue to run without the benefit of the sensor data.

Either way, the engine is likely to exhibit driveability problems such as rough idle, hesitation, reduced fuel economy or excessive (and smelly) exhaust emissions. In many cases, the PCM will trigger the malfunction indicator lamp (MIL) on the instrument panel, and store a diagnostic trouble code (DTC) in its memory. The symptoms are likely to continue until the cause of the bad sensor signal is found and corrected.

The problem could lie within the sensor itself. Some sensors may fail if they get damaged or corrode with carbon, get fried by a high-voltage spike, or simply wear out. The presence of a faulty signal does not always mean that the sensor itself has failed, however.

Various other conditions such as vacuum leaks, clogged fuel injectors, a defective EGR valve, fouled spark plugs, low fuel pressure or low alternator output can produce symptoms that might indicate a bad sensor. The source of the problem may just as easily be a bad spark plug, loose wiring or a faulty fuel pump.

Your Wheeling Auto Center technician may need to run a series of diagnostic tests to determine the cause of the malfunction. Only then can the defective component be identified and repaired or replaced.

You may have heard that some vehicles have a computer that can detect things like a bad spark plug or a gas cap that hasn't been tightened properly. Sound farfetched? Well, it’s true! That's because most vehicles built since 1996 have a more sophisticated onboard computer system (OBD) that can detect a variety of malfunctions such as fuel tank leaks, ignition misfires and failing catalytic converters.

Sensors are the eyes and ears of your vehicle's electronic control module (ECM)

Oxygen (O2) Sensor: Measures the amount of unburned oxygen in the exhaust so the electronic control module (ECM) can determine if the air/fuel mixture is too rich or too lean. It may become less responsive with age or when contaminated by coolant, sulfur or motor oil. Failure can produce a rough idle and poor fuel economy. O2 sensors usually have replacement intervals of 30,000 to 50,000 miles on older vehicles, and 60,000 to 100,000 miles on newer vehicles.

Coolant Temperature Sensor (CTS): Reports the temperature of the engine coolant so the ECM will know when the engine has warmed up. This sensor may degrade with age. Failure can result in excessive fuel consumption and high levels of carbon monoxide emissions.

Knock Sensor: Detects engine vibrations that can indicate a "knock" or premature detonation of the air/fuel mixture in the cylinders. The ECM can then adjust the timing to eliminate the knock. Sensor failure can cause engine damage if the knocking persists. 

Vehicle Speed Sensor (VSS): Monitors vehicle speed so the ECM can regulate shifting and torque converter lockup. Sensor failure can cause erratic shifting and also disable cruise control.

Manifold Absolute Pressure (MAP) Sensor: Measures the load on the engine by reading the amount of vacuum pressure in the intake manifold. The ECM can then adjust ignition timing to compensate for increased load. MAP sensors may drift out of calibration, which can trigger the MIL light and cause the engine to hesitate and run poorly. Various conditions such as a plugged or loose vacuum hose also can cause erratic MAP readings.

Mass Airflow (MAF) Sensor: On some fuel-injected vehicles, this sensor measures the speed and density of the air entering the engine. If the sensing element becomes contaminated, it can cause stalling, hesitation and hard starting. It eliminates the need for a MAP sensor.

Manifold Air Temperature (MAT) Sensor: Measures temperature of the incoming air so the ECM can adjust fuel mixture according to air density. Failure can result in the engine running too "rich" or too "lean."

Throttle Position (TPS) Sensor: Tells the ECM the engine is at idle, at wide-open throttle or somewhere in between. The computer can then adjust fuel mixture and ignition timing accordingly. If theTPS fails, the engine should still run, but is likely to experience surging and misfiring.

Crankshaft Position (CPS) Sensor: Tells the ECM which cylinder is going to fire next and also reports engine speed. Failure of this sensor will normally cause the engine to shut down because the ECM cannot function without this data and it will not activate the fuel pump.

If all of this seems confusing, no need to worry. This is precisely why you bring your car, light or medium truck to Wheeling Auto Center, where highly trained and fully certified technicians with access to state-of-the-art diagnostic equipment and computer-based technical reference data are ready to meet your needs.

If you have additional questions about preventive maintenance, diagnostic systems, sensors or anything related to the safe and economic operation of your passenger car or light/medium truck, visit our Ask the Expert page.