A DTC does NOT mean a bad sensor

The PCM tries to suggest the problem it has found, by storing a diagnostic trouble code or DTC. A DTC is data, not information. An example could be a sensor in the intake manifold called the MAP sensor. It reads the vacuum in the manifold. When the EGR valve opens, the vacuum should fall. If the PCM commands EGR operation, and does not see a reduction in the vacuum, it could show a MAP sensor fault. The cause might be a plugged EGR passage, a bad EGR valve, a vacuum hose off or several things other than the sensor. Proper diagnosis requires an understanding of the system and an ability to test the components. Replacing the sensor is not only a waste of money, but it often produces other problems.

A new part is NOT a "known-good" part

They build the various sensors that come with a vehicle to a very high standard. Many will last the life of the vehicle. Replacement parts, are often not of the same quality. A new aftermarket sensor, is often not as good as the original equipment manufacturer (OEM) part, even after 100,000 miles or more. Many fail to work properly, right out of the box. A well-meaning enthusiast, may replace a good OEM sensor with an inferior replacement.

Swapping parts is NOT diagnosis

In the example above, someone may replace the MAP sensor with an aftermarket part from a part store. The light is still on and the code still shows the sensor out of range. In frustration, they take the vehicle to a shop. The shop must now isolate the original problem and the defective sensor. This complicates the diagnosis and is a waste of time and money. This client bought an inferior sensor, they did not need, then pays to learn it is bad. The shop had to install a new OEM sensor, to learn more problems exist. Repair costs are several times more than a proper diagnosis from the start.

No universal method exists for testing sensors

Engineers design each type of sensor for a specific purpose. Even the same sensor, on different models of the same vehicle may be different. For example, a Toyota may have an oxygen sensor or an air/fuel sensor, in the same position. They appear similar, but function in totally different ways. Diagnosis requires manufacturers’ service-data and specialized equipment for each sensor tested.

Because so many designs exist, we need several pieces of equipment for testing. Using a digital lab scope is common, along with a temperature probe, vacuum pump, a logic probe and several other devices may be needed to verify a single sensor.

A few engine sensors

Vehicles have hundreds of sensors that test thousands of parameters. A few of the more common engine sensors include the following:

Mass air flow (MAF) Measures air entering the engine. Fuel injectors mix fuel with the air at a precise ratio, depending on the information from this and several other sensors.

Intake air temperature (IAT) informs the power control module or PCM of how cool or warm the air entering the engine is. Cool air is more dense, so we need less in the mixture. Warm air is less dense. The PCM considers this in calculating air/fuel mixture.

The throttle position sensor (TPS) reports how wide the throttle is open. This corresponds with power request from the driver that the accelerator-pedal-position (APP) sensor determines. The TPS makes sure the throttle is in the requested position and informs the transmission of power demands.

Manifold absolute pressure (MAP) shows the vacuum in the intake manifold. As the throttle opens, intake vacuum drops. It can also determine exhaust-gas-recycle (EGR) flow, by sensing the rise in manifold pressure. Low intake-vacuum relates to high power demand so the transmission can use MAP data to decide the ratio need.

Engine coolant temperature (ECT) determines how rich or lean the fuel/air mixture should be, the proper engine idle speed and lets the PCM know if this is a cold start or a restart of the engine. Engine temperature may also determine what ratios the transmission may use. When coolant temperature rises above a preset limit, the PCM will command a cool-down mode. This will turn off non essential accessories and shut down alternating cylinders.

Engine oil pressure (EOP) determines variable camshaft timing and may shut the engine down if it falls below a specified limit. A crankshaft position sensor (CPS) reports the position of the piston in the cylinder. The PCM uses crankshaft position to decide if the engine is running and helps with ignition, camshaft and fuel injector timing on many engines.

Camshaft position sensors (CMP) inform the PCM how far we advance or retard the camshaft, in relation to the crankshaft. Most engines use variable camshaft timing to increase power and efficiency. Data from the camshaft position sensor allows consistent ignition and fuel injector timing.

A knock sensor (KS) determines when pre ignition, detonation or spark knock is occurring. The PCM retards ignition and camshaft timing and varies fuel/air ratios to help prevent damage. The computer may also enable EGR flow to help cool the combustion chamber temperature.

A heated oxygen sensor (HO2S) reports how much oxygen remains in the exhaust, after combustion. They relate to this to how fully the fuel and air burn in the engine. A rich fuel/air mixture consumes nearly all of the oxygen in the exhaust. An oxygen sensor increases its voltage when less oxygen remains. Pulse-width of the injectors decreases in response.

When too much oxygen remains in the exhaust, the voltage from the sensor will drop. We increase pulse-width of the injector and the fuel/air mixture becomes more rich. The oxygen sensors help the PCM optimize fuel/air mixture and can override the MAF sensor. Oxygen sensors also protect the catalytic converters and monitor their condition.

Newer vehicles use air/fuel sensors that look much like an oxygen sensor, but operate differently. An air/fuel sensor changes amperage flow in relation to the oxygen in the exhaust. Much closer measurements are possible, reducing emissions and optimizing efficiency. Opposite of the oxygen sensor, voltage output increases as the mixture becomes leaner. Air/fuel sensors also operate at a much higher temperature than oxygen sensors, close to 1,200 degrees Fahrenheit.

Vehicle makers interrelate every major component of a vehicle to the others. Hundreds of sensors report information to more than seventy computers on the average car. The use of sensors will only increase as vehicles become even more sophisticated. Learning a bit about how they operate will help us better understand our vehicles.

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