Fuel Tank Pressure Sensor

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By Reinier (Contact Me)
Last Updated 2022-10-22
Automobile Repair Shop Owner

What Does the Fuel Tank Pressure Sensor Do?

As its name suggests, the FTP (Fuel Tank Pressure) sensor measures the pressure in the fuel tank that is generated by fuel vapors as a result of fuel sloshing around in the fuel tank, or when fuel is heated by high ambient temperatures, such as might happen when a vehicle is parked in full sunlight for long periods.

Note though, that the FTP sensor must not be confused with the FRP (Fuel Rail Pressure) sensor, which is used to monitor the actual pressure of liquid fuel in the fuel rail(s). While this function is critically important to ensure optimal engine performance, the pressure of liquid fuel is not in any way related to the vapor pressure in the fuel tank.

Why is the Fuel Tank Pressure Sensor Needed?

All OBD II-compliant vehicles use some form of EVAP (Evaporative Emissions Control) system that has two functions. The first is to trap and contain fuel vapors that are generated as a natural consequence of fuel moving around in the fuel tank, and the second is to convey said trapped fuel vapors to the engine via dedicated vacuum lines to be combusted along with the regular air/fuel mixture.

Note, though, that collected fuel vapors are only purged from the EVAP system when a) engine operating conditions allow, or b) when the vapor pressure in the tank exceeds a maximum allowable threshold.

We need not delve into the technical details of how EVAP systems are implemented by different car manufacturers, but suffice it to say that the FTP sensor is the only source of input data a PCMs (Powertrain Control Module) has available to monitor the vapor pressure in the fuel tank derives from the FTP sensor. Thus, as a practical matter, the FTP sensor is a critical component in the EVAP system, without which the EVAP system cannot function.

In addition to the above, the FTP sensor also plays a critically important role in detecting leaks in the EVAP system. Again, we need not delve into the technical details of how EVAP leak detection systems are implemented in different vehicles, beyond saying that on most vehicles, the fuel tank is either pressurized or partly evacuated when the PCM runs a series of diagnostic tests to detect leaks in the EVAP system. Here is the short version of how this works-

If the fuel tank I pressurized by a dedicated air pump, the FTP monitors the pressure in the fuel tank, and if a leak is present, the FTP will report the rate at which the pressure in the tank decreases to the PCM. Based on the rate of the pressure drop, the PCM will determine the size of the leak and set one or more appropriate fault codes based on this determination.

If the fuel tank is evacuated by the engine vacuum during a diagnostic test, the FTP sensor will monitor the negative pressure (vacuum) in the fuel tank. If a leak is present, the vacuum will decay as atmospheric air enters the EVAP system through the leak. The FTP sensor will report the rate of decay to the PCM, which will determine the size of the leak based on the rate at which the vacuum decays, and set one or more appropriate fault codes based on this determination.

In both cases, the FTP sensor plays a crucial role in detecting leaks in the EVAP system, but it should be noted that leak detection in EVAP systems is more than a nice-to-have functionality. It is, in fact, a legally mandated requirement on all OBD II-compliant vehicles, and without a properly functioning Fuel Tank Pressure sensor, this legal requirement cannot be satisfied.

How Does the Fuel Tank Pressure Sensor Work?

In most designs, FTP sensors contain an element that reacts to changes in pressure. In practice, though, the PCM supplies the sensor with a (typically 5V) reference voltage that forms the cornerstone of the sensor’s operation.

Depending on the actual application, the reference voltage may increase or decrease in response to the electrical resistance of the sensing element increasing or decreasing in response to fuel vapor pressure changes in the fuel tank. We can put this differently by saying that the wire carrying the reference voltage from the PCM attaches to one side of the sensing element, while the other side of the sensing element attaches to a wire that leads back to the PCM, which wire carries the actual signal created by the sensing element.

Let us use a generic example of how this arrangement works in practice in an FTP sensor whose electrical resistance increases as the vapor pressure in the fuel tank rises. In this case, the increasing electrical resistance will cause a decrease in the voltage that flows through the sensing element; if we assume that the voltage decreases to say, 3V when it leaves the sensing element, the PCM will interpret the 2V difference as an increase in the fuel vapor pressure in the fuel tank.

Note that regardless of whether the electrical resistance of the sensing element increases or decreases in response to increases in the fuel vapor pressure in the fuel tank, the PCM will, in all cases, interpret the changes in the reference voltage as changes in the fuel vapor pressure.

Where is the Fuel Tank Pressure Sensor Located on the Engine?

FTP sensors are typically located on the top of the fuel tank, such as in the example shown here, to prevent them from coming into contact with liquid fuel, which if it happens, could affect the operation of the sensor. In this example, which shows the FTP sensor on a Toyota Corolla, the FTP sensor is circled in yellow.

It should be noted, though, that accessing the FTP sensor on some applications is sometimes challenging, even for professional mechanics because in many cases, the fuel tank has to be removed from the vehicle to access the FTS sensor.

Therefore, due to the high risk of spilling fuel, or worse, causing a fuel fire, we do not recommend that non-professional mechanics attempt the removal of the fuel tank from any vehicle. If you suspect that The FTP sensor on your vehicle might be defective or malfunctioning, the wiser option would be to seek professional assistance with diagnosing and replacing the sensor.

What Does the Fuel Tank Pressure Sensor Look Like?

This image shows an example of a typical fuel tank pressure sensor. Note, however, that the actual appearance and design of FTP sensors vary greatly between vehicle makes and models, and that on many vehicles, there may also be other sensors located on, or near the fuel tank. Note that in this example, which is suitable for use on some Toyota vehicles, the red arrow indicates the rubber grommet that seals the hole in the tank that the sensor plugs into, while the yellow arrow indicates the sensor’s electrical connector.

In many cases, other sensors that are located on or near the fuel tank often resemble the FTP sensor, so we strongly recommend that reliable service information for the affected vehicle be consulted to locate and identify the FTP sensor correctly. Testing, removing, or replacing the wrong sensor could lead to confusion, misdiagnoses, and possibly, the unnecessary replacement of parts and components.

What are the Symptoms that the Fuel Tank Pressure Sensor is Bad?

Although the technical implementation of EVAP systems varies between vehicle makes and models, all EVAP systems perform the same function, which is to purge fuel vapors from the fuel tank when operating conditions allow. Therefore, the symptoms of a failed or malfunctioning FTP sensor are largely similar, if not always identical, across all applications. Nonetheless, typical symptoms of a malfunctioning FTP sensor could include one or more of the following-

  • One or more, or even multiple fault codes could be present, and on some vehicles, the “CHECK ENGINE” light may also be illuminated.
  • Typical (stored) fault codes could include those relating to sensor performance, open or short circuits, and/or implausible input data
  • The EVAP readiness monitor may not initiate, or may not run to completion
  • The vehicle will not pass a mandatory emissions test*

Note that mandated emissions tests are not designed to measure a vehicle’s emissions: these tests are designed to check whether (or not, as the case may be) the OBD II system on the vehicle can detect faults that have the potential to affect exhaust emissions negatively. Thus, since a failed FTP sensor will cause the EVAP readiness monitor not to run or complete, the vehicle will not pass an emissions test because a defective component in the EVAP system (the FTP sensor) has the potential to affect the vehicle’s overall emissions negatively.

How do you test the Fuel Tank Pressure Sensor?

Although FTP sensors are designed to be sensitive to pressure, the actual fuel vapor pressures in fuel tanks are so low that there are no practical ways to simulate the conditions in fuel tanks accurately.

Thus, assuming that there is no damaged, burnt, shorted, disconnected, or corroded wiring or connectors in the FTP sensor’s harness, the only practical (and reliable) way to test an FTP sensor is to measure the electrical resistance of the sensing element.

This can usually be done with a high-quality digital multimeter, but it should be noted that there is no single resistance value that applies to all FTP sensors. In practice, all sensors, including FTP sensors, usually have electrical resistances that fall into a wide range of acceptable values, which means that obtained test readings must be compared with OEM service information to arrive at valid diagnostic conclusions.

Put differently, the above means that a suspect FTP pressure sensor should not be condemned out of hand even if its electrical resistance seems to be too high or too low. The only way to be sure whether or (not) an FTP sensor is defective is to compare its electrical resistance with the range of values that are specified by the vehicle’s manufacturer.

How do you replace the Fuel Tank Pressure Sensor?

As stated elsewhere in this guide, the FTP sensor is usually located at the top of the fuel tank, but in this context, “top of the fuel tank” means that the sensor is usually located in the top part of the fuel pump assembly.

In practice, this means that while the sensor might sometimes be accessible without removing the fuel pump assembly from the tank, in other cases, the FTP sensor is located on the fuel pump assembly, but inside the fuel tank, meaning that the pump assembly has to be removed from the fuel tank to access the  FTP sensor.

In passenger vehicles, the fuel pump assembly can usually (but not always) be accessed by removing the rear seat, and then removing a cover plate to expose the mechanism that seals the point where the pump assembly fits into the tank. If the FTP sensor is located on the outside of the tank, removing and replacing the sensor is as easy as disconnecting its connector, and removing it from the pump assembly. Installation of the new sensor is done in the exact reverse order of removal.

If, however, the FTP sensor is located inside the tank, you may need a special, and often, an application-specific tool to remove the large nut that holds the pump assembly in place inside the fuel tank. While these kinds of special tools are available in the aftermarket, we would at this point recommend that you seek professional assistance with replacing the FTP sensor to avoid the very real possibility of creating a fuel leak when you reassemble and reinstall the fuel pump assembly.

Note also, that the fuel pump assembly is typically not accessible on trucks unless the fuel tank is removed from the vehicle. Be aware that removing the fuel tank from a truck can be extremely challenging for non-professional mechanics, so we recommend that you seek professional assistance with diagnosing and/or replacing a suspect FTP sensor.