P2539 – Fuel low pressure sensor – circuit malfunction

By Reinier (Contact Me)
Last Updated 2018-01-16
Automobile Repair Shop Owner

Table of Contents

1. What Does Code P2539 Mean?
2. Where is the P2539 sensor located?
3. What are the common causes of code P2539?
4. Get Help with P2539

What Does Code P2539 Mean?

SPECIAL NOTES: Be aware that while this code is generally displayed as “P2539- Fuel low pressure sensor – circuit malfunction”, on most generic code readers, many vehicle and make specific scanners may display the code for this issue as a code that is specific to that manufacturer. One example of this is VAG group-specific equipment that instead of P2539, may display codes P310B/01255- “Low Fuel Pressure Regulation: Fuel Pressure Outside Specification”.

There are other examples, such as code 29F3 –“Low Fuel Pressure Sensor” on many BMW applications, so always refer to the manual for the affected application for details on the exact definition of any codes that pertain to the fuel low-pressure system on that particular application. END OF SPECIAL NOTES. 

OBD II fault code P2539 is a generic code that is defined as “Fuel low pressure sensor – circuit malfunction”, and is set when the PCM (Powertrain Control Module) detects a general failure of, or malfunction in the control circuit of the low-pressure side of the fuel system, or in the fuel low-pressure sensor itself. Note that some code readers will display this code as “P2539- Fuel low pressure sensor- range/performance”, since most manufacturers have in recent years elected to no longer use the term “malfunction” in trouble code definitions.

Note that-

• Most modern pulse width modulated returnless fuel delivery systems are highly complex, and typically consist of both a high-pressure, and a low-pressure side. Since code P2539 refers to a specific problem in the control circuit of the low pressure fuel system side, this code should NOT be confused with other, general low fuel pressure issues.
• Non-professional mechanics are strongly urged to determine whether or not the affected application has a low-pressure fuel system, and if it has, to read the section in the manual that describes the fuel system thoroughly before attempting a diagnosis of code P2539. Failure to gain at least a working knowledge of a PWM (Pulse Width Modulated) returnless fuel system will result in confusion, a misdiagnosis, the unnecessary replacement of parts and components, and very likely in additional damage to the application’s fuel and electrical systems.

The purpose of the low fuel pressure system on many older diesel applications is to deliver a steady supply of somewhat pressurized fuel to the high-pressure injection pump. However, on gasoline engines that use returnless direct fuel injection systems, the low-pressure system is used to calculate fuel demand and delivery strategies.

On older designs, fuel pumps worked at 100% of their rated capacities, and all excess fuel pressure was bled off via a system that returned excess fuel to the tank. While these systems worked well in practice, there were several serious drawbacks to using fuel delivery systems that depended on a pressure regulator to maintain fuel pressure. Examples of disadvantages include frequent fuel pressure regulator failures, excessive heating, and agitation of the fuel that sometimes made it difficult for the EVAP system to cope with the resulting large volumes of fuel vapors, but more importantly, fuel pumps often failed prematurely.

Newer, returnless fuel delivery systems eliminate all of these issues since on these systems, both the pressure and volume of fuel delivered to the injectors can be regulated by varying the rotational speed of the fuel pump. Common means of varying the speed of the fuel pump include adjusting the duty cycle of the fuel pump (turning the pump motor on and off in rapid succession), or simply by varying the fuel pump motor’s supply voltage.

In terms of operation, the PCM sends a fuel demand request to the fuel pump control module via the CAN (Controller Area Network) bus system, and depending on the method of fuel pump control used by the application, the fuel demand request dictates the set point value of the requested rate of fuel delivery (typically in litres per hour), based on input data obtained from the low fuel pressure sensor.

However, since PWM (Pulse Width Modulated) fuel pumps cannot pressurize the fuel system when the ignition is turned on, most PWM fuel systems incorporate a low-pressure fuel pump to pressurize only a part of fuel system when the ignition is switched on. This pressure is monitored by a dedicated sensor known as the “fuel low pressure sensor”, and in a fully functional fuel system, the input data from this sensor is used by the PCM (and sometimes, other control modules) to calculate the fuel demand from start up, right through to WOT (wide Open Throttle) settings.

Thus, and at the risk of over simplifying the case, if the fuel low-pressure sensor and/or its control circuit fails, or if the pressure in the  low pressure fuel system falls below  a predefined minimum allowable threshold, the PCM cannot control the high pressure fuel pump’s duty cycle or required input voltage. When this happens, the PCM (or other control module(s)) will set code P2539, illuminate a warning light, and may or may not initiate a failsafe or limp mode as a safety precaution. Note that if the PCM does initiate a limp mode, the condition will remain until the fault is corrected.

Where is the P2539 sensor located?

The image above shows the typical location of the low-pressure fuel sensor on a BMW 5-Series application. Note though that while this sensor is most commonly located near the high-pressure injection pump, the actual location and appearance of the sensor varies greatly between applications. For example, on applications that have the high-pressure fuel pump in the fuel tank, the fuel low-pressure sensor is often also located in the fuel tank, typically on the fuel pump canister.

For this reason, it is critically important to refer to the manual for the affected application to locate and identify the fuel low-pressure sensor correctly. Failure to do this will almost certainly result in confusion, a misdiagnosis, and the unnecessary replacement of expensive parts and components.

What are the common causes of code P2539?

NOTE: While several additional fuel system, misfire, and engine management related codes frequently appear with P2539, these codes almost always set as a result of code P2539, as opposed to P2539 setting as a result of one or more other codes. Therefore, code P2539 must be resolved first, since doing so will almost always resolve all other codes following on P2539 as well.

Nonetheless, some common causes of P2539 could include the following-

• Damaged, burnt, shorted, disconnected, or corrode wiring and/or connectors
• Defective fuel low pressure sensor
• Note that a lack of fuel is a relatively common cause of P2539. Therefore, the actual fuel level in the tank must be verified independently of the fuel level gauge even if the fuel gauge appears to be working correctly
• Defective PCM or fuel control module. Note that these are rare events, and the fault must therefore be sought elsewhere before any control module is replaced

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