Car Engine Sensors [A Complete Guide]

Do you know about all car engine sensors? Alright, I’ll explain each one, their function, how they work, their position, and what happens if they break. So, this is a comprehensive article on vehicle engine sensors, and I will provide helpful information about 16 different types of sensors. Now the engine and ECU do need a lot of sensors to do their job correctly. But many of the sensors work similarly.

5 Categories of Car Engine Sensors

To make it easier for you to understand the engine sensors in your car, I have grouped them into five categories as follows:

  1. position
  2. airflow
  3. pressure
  4. temperature
  5. air-fuel ratios, emissions, etc.

Position Sensors

Crankshaft Position Sensor

This sensor will tell the ECU the position of the crankshaft. The reason is that the position of the piston is fixed concerning the position of the crankshaft. So, by knowing the position of the crankshaft, the ECU also knows the location of the piston.

After that, it can initiate fuel injection and spark ignition events at the appropriate time. The crankshaft position sensors are often either Hall effect, three wires, or VR (variable reluctance), which is two wired. Although they work slightly differently, they both rely on the basic principles of electromagnetism to read a trigger wheel made of ferrous metal. The trigger wheel will lose one or more teeth.


Every time the missing gear passes in front of the sensor, it will change its signal to the ECU. Now the missing gear position concerning the sensor will be fixed to the specified crankshaft position. So, by quickly viewing the signal changes, you can tell precisely where the crankshaft is at that moment.

Now the ECU is also configured to the total number of teeth on the trigger wheel. Knowing the number of teeth and measuring how often a missing tooth passes in front of the ECU sensor can calculate engine speed or RPM. Every car engine has one crankshaft, so it only needs one crankshaft position sensor.

The crankshaft position sensor to read the trigger wheel must be very close. Also, the trigger wheel must be mounted on something that rotates at the same speed as the crankshaft, and that’s usually the crankshaft pulley, the timing gear to which the timing belt or timing chain is attached, or the flywheel. So, one of these three locations is where you’ll find your crankshaft position sensor. It can also be on the side of your engine block if it read triggers on the balancing shaft gear or if the timing gear is on the block.

What Happen if the crankshaft position sensor fails?

Usually, you will get error code P0335, and your car will have problems like:

  1. No start
  2. Stall after start
  3. Poor mileage
  4. Bad acceleration
  5. Rough idle
  6. Misfiring

Camshaft Position Sensor

The camshaft position sensor does the same thing as the crankshaft position sensor, but for the camshaft. The engine can run with the crank sensor alone. But adding a pickup position sensor provides a much more complete picture for the ECU. In addition, it is a verification layer for the information coming from the crankshaft position sensor.


Camshaft position sensors let the ECU know what each cylinder is doing and when to do it; this is why it can be used for cylinder-selected NOx control, sequential injection, and other cylinder-selective systems. The cam sensor works on the same principle as the crank sensor.

The only difference is that it reads a much smaller trigger wheel with fewer teeth due to the size of the camshaft. How to read the position? The camshaft position sensor must be near the cam. Usually, you’ll find it somewhere on the cam cover, either in front of or behind the camshaft or less often somewhere along the camshaft axis. Also, usually, you can find it somewhere on the cam cover, on the back, or front of the camshaft.

Symptoms of a camshaft position failure are usually the same or very similar to those of the crankshaft position sensor, whether it is not starting or the engine is running poorly.

Throttle Position Sensor

The throttle position sensor measures the position of the throttle plate. When you operate the throttle pedal, you operate the throttle plate, which is typically located at the intake manifold entrance. When you do, the throttle plate fully opens, allowing maximum air into the engine for maximum acceleration.

By knowing the position of the throttle plate, the ECU can determine the load placed on the engine. Thus can vary the timing of injection and ignition accordingly. The throttle position sensor works by relying on a variable resistor.


A variable resistor is an electrical component that changes its electrical resistance output based on its position. It contains movable parts, and different positions of these movable parts will have different resistance outputs.

In the case of the throttle position sensor, the shaft of the throttle plate is directly connected to the variable resistor. So different positions of the throttle plate will have different output resistance, which is then measured and converted into a valuable signal for the ECU by the onboard electronics of the throttle position sensor.

Today modern throttle position sensors rely entirely on non-contact position measurement and use the Hall effect in induction or magneto-resistance to do the same job as a variable resistor.

How to measure the throttle plate position? The throttle position sensor has to be located on the throttle body itself, and this is where you always find the throttle position sensor.

Symptoms of throttle sensor fail.

Generally, you will get error codes P0122, P0125.

  1. Your car accelerates unpredictably
  2. Car hard to start
  3. Idle too high or too low

Car Engine Sensors based on Air Flow

Mass Air Flow Sensors

in addition to knowing the position of the piston to inject fuel easily, you also have to know how much fuel is injected. So, to know how much fuel is injected, you must know how much air is entering the engine. So it can inject the appropriate amount of fuel, which is what the MAF or mass air-fuel sensor for ECU does. It tells the ECU how much air is getting into the engine.

Now, most MAF sensors are hot wire or hot film, and they are slightly different. The heat-film sensors are more advanced, but they both rely on the same basic principle of very high-temperature resistance coefficient of certain metals like platinum or tungsten.


The very high-temperature coefficient of resistance of these metals means that even a minimal change in the temperature of these metals will change their resistance. So when air flows over these metals, the temperature changes.

The more air flows, the more resistance changes, and the onboard electronics of the MAF sensor measure this and then convert the signal into valuable data for the ECU. Now it’s clear this is just a basic working principle, and MAP sensors are more complicated and exciting than this.

You will usually find the MAF right after the air purifier houses the air filter housing. Otherwise, it will be somewhere after the air filter housing but before the throttle body. If the MAF fails miserably, the engine will not start.

How do you know if the mass airflow sensor is not working?

Usually, it generates error codes P0101, P01012.

  • Engine hesitation and stumbling
  • Car won’t start
  • Poor mileage
  • Poor acceleration

Air Flow Meter Sensors

AFM or airflow meters or vane airflow meters, as they are also known, do the same thing as MAF. It measures how much air gets into the engine but does it differently and is usually present in older vehicles.

The airflow meter has a flap or vane inside. The incoming air pushes this flap. The more air it gets, the more it will push and open the flap. Now that vane or flap is connected to a variable resistor like the throttle blade shaft of the throttle position sensor.

Different vane positions will produce different resistances, and then the onboard electronics will measure this and convert it into a helpful signal to the ECU. So the different resistance outputs will tell the ECU how much the blades are open and how much air is getting into the engine.

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Like the MAF sensor, the airflow meter will always be near or right after the air filter.

Symptoms of fail airflow meter sensors

AFM does pretty much the same thing as a mass airflow sensor, so the symptoms of an airflow meter failure will be similar to or the same as a mass airflow sensor.

Pressure Sensors

Manifold absolute pressure sensor

Like the MAF for AFM, the MAP or manifold absolute pressure sensor measures how much air is getting into the engine.

But unlike AFM or MAF, it does not measure the mass of incoming air directly. Instead, it measures the pressure inside the engine’s intake manifold and calculates air mass based on that.

The logic is that the more air there is in an enclosed space, the more pressure the air will exert against that cold room’s walls, which MAP uses. By measuring the pressure, it can calculate how much air enters the engine.


The MAP sensor measures the intake air pressure by relying on a tiny micromachined silicon chip. The silicon chip consists of a silicon membrane and a small piezoelectric material that reacts to the bending of silicon membranes under pressure by changing their electric charge.

The more pressure, the more the circuit membrane flexes, the more the charge changes and the onboard electronics measure this and then convert it into a helpful signal to the ECU.

If you are building a performance engine or significantly increasing your engine boost level, then the stock MAP sensor is no longer adequate and needs to be upgraded.

The MAP sensor must measure the pressure directly in the intake manifold so that it is always located somewhere in the intake manifold.

Symptoms of a poor manifold absolute pressure sensor

Since it is ultimately trying to measure the mass of incoming air, its failure symptoms will be similar to or the same as MAF or AFM.

Oil Pressure Sensors

The oil pressure sensor measures the oil pressure in the engine. It may be effortless, but it is one of the engine’s most essential sensors. Without sufficient oil pressure, your engine will suffer severe damage very quickly.

Like many other pressure sensors in cars, the oil pressure sensor is essentially a piezoresistive pressure transducer, meaning that it works on pretty much the same basic principle as the MAP sensor.

It is simply calibrated for the expected different values โ€‹โ€‹of engine oil pressure. In some cases, the engine oil pressure sensor is not a sensor at all but just a switch. The most basic version contains a diaphragm that, when bent, only causes an electrical circuit and reports sufficient oil pressure.

If there is insufficient oil pressure, the diaphragm will not be flexible enough to cause an electrical circuit, and the oil pressure lamp will flash, warning that you must stop the engine to prevent damage.

The oil pressure sensor is usually located somewhere in the engine block. It is often near the oil filter but could be anywhere in the engine block where this sensor can be tapped directly into the engine oil line to read its pressure accurately.

What happens when the oil pressure sensor fails?

Lack of oil pressure is one of the most dangerous situations for an engine. So if the oil pressure sensor is not working and incorrectly reports all oil pressure to the ECU, the ECU will incorrectly trigger and refuse to start the engine.

Usually, it generates codes P0520, P0521, P0522.

  • Failure does not affect how the engine works unless you have low oil pressure.
  • Incorrect low oil pressure check may trigger the latest cars into limp home mode.
Fuel Pressure Sensor

The fuel pressure sensor measures the fuel pressure; that’s what it does. So why is it important to correctly measure fuel pressure? By knowing the fuel’s fuel pressure in your rail, the ECU can tell how long it takes to open the injectors to ensure the correct amount of fuel is injected into the engine.

The fuel pressure sensor is always located somewhere directly on the fuel rail to measure the fuel pressure right inside the fuel rail.

Symptoms of faulty fuel pressure sensors

Codes P0193, P0190.

  • Hard starting often happen in cold engine condition.
  • Poor mileage
  • Poor acceleration

Car Engine Sensors based on Temperature

Intake Air Temperature Sensor

The IAT sensor measures the intake air temperature. It is essential because the density of air is affected by its temperature. The hotter the air, the less dense it is, and the fewer air molecules there will be in a given space.

Although, in theory, an engine can run without an air intake timing sensor, it is still an excellent idea to have one, and the engine will perform better with it than without it. The IAT sensor works in conjunction with the MAF, MAP, or AFM to give the ECU a complete picture of the air entering the engine, helping to reduce emissions and increase power and efficiency.

Like almost any other temperature sensor in a car, the IAT sensor is a thermistor, meaning that its electrical resistance will change in response to changes in its temperature. As usual, the resistance is measured and converted into a valuable signal to the ECU.

The IAT sensor is often integrated into the MAP or AFM. You can usually find it somewhere in the intake duct, usually near the throttle body if you don’t have one.

Intake Air Temperature Sensor Failure

Codes P0113.

  • Very mild if sensor just a bit dirty
  • Rough running
  • Stumbling and Stalling
  • Power surges

Coolant Temperature Sensors

The coolant temperature sensor is vital to the ECU, which tells how warm the engine is. Cold engines require a different amount of fuel when compared to hot engines. The coolant temperature sensor is also needed to trigger various actions, such as turning on the radiator fan or overheating the engine.

It can tell the ECU to trigger limp home mode or refuse to start the engine until it cools down again. Like almost all temperature sensors, the coolant temperature sensor must be in direct contact with the liquid whose temperature must be measured. Typically, the position of the coolant temperature sensor is at or near the thermostat.

Some cars have more than once, and you can find them anywhere on the coolant pipe or anywhere the coolant passes through.

What happens if the coolant temperature sensor fails?

Cold engines require more fuel than hot engines. The problem is that if the coolant temperature sensor doesn’t work and tells the ECU that the engine is cold even though it’s hot, the ECU will inject extra fuel, resulting in rich working conditions. As a result, you will get poor mileage or even black smoke from the exhaust in severe cases.

The opposite scenario is also possible on these car engine sensors. You can think that the engine is hot even though it is cold. In this case, it will inject too little fuel, which will cause the engine to misfire or even NOx until it heats up.

Fuel Temperature Sensor

The fuel temp must be measured because its density and the density of air are affected by temperature.

Hot fuel is less dense and more burned, so a little more must be injected. The fuel temperature sensor is often located somewhere in the fuel tank as part of the fuel pump assembly. Less often found elsewhere along the fuel line. Many cars have more than one fuel temperature sensor, one in the tank and the other in the engine bay.

The information derived from the fuel temperature sensor is only used to improve injection accuracy further to meet stringent emission standards.

The engine may run quite well with a faulty fuel temperature sensor, and symptoms may not be visible in some cases.

If the fault is more severe or the sensor fails, the vehicle may fail the emissions test or get slightly worse mileage and performance than usual.

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Fuel Temperature Sensor failure

Error code P0183.

  • Symptoms can be barely noticeable.
  • Failing emission test
  • Decreased mileage and performance

Oil Temperature Sensor

Although many cars only have an oil pressure sensor and don’t measure the oil temperature, measuring the oil temperature can be an additional functional layer of protection.

If the oil is too hot, this will harm its viscosity, and the oil will not be able to do its job, leading to engine failure.

The ECU can activate limp mode or refuse to start the engine before the oil temperature returns to normal by looking at the oil temperature.

In addition to relaying information to the ECU, the oil temperature sensor can also convey information to the gauge or display, visible to the driver. Then the driver can take precautions and stop the engine if the oil temperature is too high.

How to measure the oil temperature? The sensor must be in direct contact with the oil in the engine, as is often found on the engine block or the cylinder head. In newer cars, it can be found integrated into the oil level sensor.

Oil Temperature Sensor Fails

Error code P0196, P0197, P0198

  • It does not impact how the engine runs
  • If it falsely reports too high temp, ECU can falsely trigger limp home mode.
  • If it is failed and is ignored, it can’t save your engine

A faulty oil temperature sensor is straightforward to spot if there is a gauge or display.

Sensors of Air Fuel Ratios, Emission, etc

Oxygen Sensor

The oxygen sensor measures the amount of O2 present in the exhaust gases. 

The amount of O2 present in the exhaust gas is directly proportional to the fuel burned in the combustion chamber.


By measuring the oxygen, the oxygen sensor can tell the ECU the air-fuel ratio the engine is running. Now the air-fuel ratio in the combustion chamber is critical to engine performance and emissions.

So, it has to be constantly monitored to ensure that it is where it is needed.

Type of O2 Sensor

You have two types of O2 sensors, narrowband and wideband. Narrowband sensors can tell if an engine is running rich or lean, but they can’t tell exactly how rich or how lean an engine is running.

On the other hand, the wideband sensor covers a much more comprehensive range of air-fuel ratios, and you can tell the ECU exactly how rich or lean the engine is running. Although small, the oxygen sensor is quite a complicated device.

It involves zirconium lamps and various coatings, and whatnot. The amount of oxygen measured at the sensor’s tip corresponds to the voltage generated by the sensor. So, for example, 0.5 volts will be close to the ideal air-fuel ratio, and at the other end of the spectrum, you can find it too rich and too lean.

So the goal of these car engine sensors is to increase or decrease the amount of fuel injected to achieve the ideal target air-fuel ratio.

Now, most of the catalytic converters in your car will have at least two oxygen sensors: one going before and one after the catalytic converter.

The one before the catalytic converter is called the upstream oxygen sensor. It is usually directly on the exhaust manifold or directly behind it. It is used to see the exact ratio of airflow to where the engine is running.

The one after the catalytic converter sensor is called a downstream sensor and is used to verify that the catalytic converter is doing its job correctly.

Oxygen Sensor Fails

Error code P0131, P0132, P0133, P0135, P0138, P0420, P0141, P2238

  • Symptoms can be mild on older cars but more noticeable on newer cars
  • Emission test failure
  • rough or stumble engine
  • poor mileage
  • loss power

Don’t ignore oxygen sensor failure because it will damage your catalytic converter, typically more expensive.

Exhaust Gas Temperature Sensor

The EGT sensor measures the exhaust gas temperature, and the probe must be present directly in the exhaust gas stream to measure the gas temperature. By measuring the exhaust temperature, you can infer information about the air-fuel ratio because changes are directly correlated with changes in exhaust gas temperature.

The EGT probe is not often used in gasoline engines, but it can help protect turbochargers and catalytic converters from thermal overholding.

The EGT probe is widely used in turbo diesel and diesel engines to verify that the DPF or diesel particulate filter has reached a sufficiently high temperature for regeneration. They are also helpful for protecting SCR or selective catalyst reduction, LNT (lean NOx trap), and other NOx absorbing systems.

The EGT probe can still be a helpful tool that provides valuable insight into what’s going on inside the engine. How to get the most accurate exhaust temperature readings? The EGT probe should be as close to the exhaust valve as possible, and this is why you will always find it in the exhaust manifold very close to the cylinder head. Another location is after the diesel particulate filter to verify if the DPF is doing its job.

Symptom of Exhaust Gas Temperature Sensor Failure 

Error codes P2032, P0546, P0405.

  • Decreased performance and mileage in diesel
  • More frequent and longer-lasting regeneration mode in diesel

Unlike other car engine sensors, EGT probes are very rare from manufacturers and gasoline engines. So there isn’t a prevalent symptom to discuss when it comes to the gas engine.

Nitrogen Oxide Sensor

The NOx sensor measures the nitrogen oxides in the exhaust gases. They are usually only present on diesel vehicles and briefly present on some stratified gasoline engines of the past. Their primary role is to verify the correct operation of the SCR or selective catalyst reduction system. This active emission system in diesel vehicles injects ammonia or diesel emission liquid directly into the exhaust gases.


After that, the nitrogen oxides will turn into nitrogen, water, and a minimum amount of CO2. There are usually two sensors in a diesel vehicle, one before and one after the selective catalyst reduction system. The first sensor will measure how much nitrogen is coming in, and the next one is measuring how much NOx is coming out to ensure that the SCR system is doing its job.

NOx is abnormal combustion in an engine. If it is strong enough and if it lasts long enough, it can cause fatal engine damage. The NOx sensor is a microphone tuned to listen to a specific frequency in the engine.

So, this frequency depends on the engine but mostly depends on the bore and stroke. If the NOx sensor detects NOx, it will relay this information to the ECU. It will respond by slowing the ignition timing or increasing the fuel injected into the engine to prevent NOx from occurring again.

The NOx sensor will always be in the engine block to hear the NOx resonance throughout the engine block. Many engines with six or more cylinders have two or even more NOx sensors in some cases. In most cases, the failure of the NOx sensor will not affect the way the engine runs.

You may get a check engine light, but the engine will run normally. The scenario is slightly different on more modern cars, where the ECU will often trigger limp mode when it detects a NOx sensor failure and will normalize once you replace the NOx sensor.

NOx Sensor Failure

Error code P0235.

  • Only check engine light on older cars
  • It will trigger limp home mode on newer cars

Is what I describe here really all sensors? Most likely not, because the number of sensors on modern engines is growing every day. So while I was writing this article about car engine sensors, someone out there might have discovered something new. So if I missed one, please let me know in the comments. Also, let me know if you want a more in-depth discussion of one of the sensors on the car.

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