P2509 – Engine control module (ECM) – supply voltage, intermittent

Avatar photo
By Reinier (Contact Me)
Last Updated 2017-02-06
Automobile Repair Shop Owner
CodeFault LocationProbable Cause
P2509 Engine control module (ECM) - supply voltage, intermittent
(Buy Part On Amazon)
Wiring, fuses, engine control (EC) relay

We recommend Torque Pro

Table of Contents

  1. What Does Code P2509 Mean?
  2. What are the common causes of code P2509?
  3. What are the symptoms of code P2509?
  4. How do you troubleshoot code P2509?
  5. Codes Related to P2509
  6. Get Help with P2509

What Does Code P2509 Mean?

SPECIAL NOTES: Since code P2509 relates to the quality of the power supply of the PCM (Powertrain Control Module), non-professional mechanics are strongly urged NOT to attempt a diagnosis of this code without the aid of a repair manual for application, and/or a wiring diagram for the application in which the color of all wiring is clearly indicated. The diagram should also include clear images of the PCM connector(s), on which the connector terminals/pins are clearly numbered and/or described to ensure that the correct wires and circuits are tested for continuity and/or resistance issues.   

Additionally, non-professional mechanics are urged NOT to commence a diagnostic procedure for this code before they have read, and understand the section in the manual that describes the PCM’s power supply and power requirements. Failure to gain at least a basic understanding of the PCM’s power requirements (and the system(s) that deliver that power to the PCM), will almost certainly result in damage to the vehicles’ electrical system in general, and possibly fatal damage to the PCM, in particular.

Moreover, it should be noted that diagnostic/repair procedures for this code vary between applications, and particularly between applications that are fitted with single batteries, and those that are fitted dual-battery systems as stock equipment. There are also major differences between the diagnostic procedures for this code between applications that use alternators with built-in voltage regulators, and those on which the PCM controls the alternator’s output via a dedicated charging system control module. 

For these reasons, this guide cannot provide detailed diagnostic/repair information for this code that will be valid for all applications under all possible conditions. Therefore, the information provided here is intended for general informational purposes only, and as such, the information provided here should NOT be used in ANY diagnostic procedure(s) for this code without making proper reference to the manual for the application being worked on.

However, the generic diagnostic/repair information provided here should enable most non-professional mechanics to diagnose and resolve code P2509 on most applications. END OF SPECIAL NOTES.

OBD II fault code P2509 is a generic code that is defined as “Engine control module (ECM) – supply voltage, intermittent”, and is set when the PCM (Powertrain Control Module) detects intermittent power supply issues to itself. Note that it is not necessary for the power supply to the PCM to fail completely before this code sets. On almost all applications, this code will set when the quality of the PCM’s power supply fluctuates, is erratic, or falls outside of high or low limits set by the manufacturer.

Also, note that the acronyms ECM (Engine Control Module), ECU (Engine Control Unit), ECU (Electronic Control Unit), PCM (Powertrain Control Module), ICU (Injection Control Unit), and/or DME/DDE (Digital Motor Electronics / Digital Diesel Electronics) all refer to the controller that ultimately controls the operation of the engine on a modern vehicle via data obtained from subordinate control modules and sensors. However, to avoid confusion, only the term “PCM” (Powertrain Control Module) will be used in this guide.

Like PC’s and laptop computers, the PCM’s on all applications require a steady voltage to operate efficiently, and like other computers, a PCM can lose programming and/or memory when its power supply fluctuates, exceeds or falls below design limits, or is interrupted; therefore, vehicle manufacturers go to great lengths to design the electrical systems of their products in such a way that the PCM is always supplied with a stable current. While design specifics of these measures differ between applications, the end result of these design features on all applications is that the PCM’s power supply remains stable, despite the continual electrical load changes that occur in the electrical system of any application both during normal operation of the vehicle, and when routine maintenance tasks and repairs are performed in accordance with methods that are approved by the manufacturer.

However, real-world operating conditions and adverse environmental factors such as extreme temperatures, vibration, and humidity place great stresses and strains on wiring, connectors, and electrical components. Over time, any of the thousands of connections that link the various wiring harnesses and control modules that make up the electrical system of a vehicle can develop resistance or continuity issues due to partial or complete loss of contact, or corrosion of wiring and/or wiring terminations in connectors.

Thus, when continuity or resistance issues occur in the circuits that feed power to the PCM, the variations in the current that reaches the PCM can have unpredictable results and/or symptoms. In some cases, the PCM can deactivate itself as a protective measure, while on others the only symptom may be an illuminated warning light and a stored trouble code, until the fluctuating power supply damages the PCM to the point where the vehicle may be immobilized.

From the above it should be obvious that this code must NOT be ignored, or allowed to persist until driveability issues appear- at which point the PCM is already likely to be damaged.

The image below shows one typical problem that can cause fluctuating, or intermittent breaks in power to the PCM. Note that battery terminals that are as corroded as shown in this image can also affect the electrical system in other ways, such as causing erratic failures of (among other systems) the security/anti-theft and/or central locking systems.


What are the common causes of code P2509?

Some common causes of code P2509 could include the following-

  • Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors
  • Poor contact between battery terminals and battery posts caused by corrosion or acid build-up
  • Defects in the charging system that can cause either a high or low system voltage condition
  • Poor battery ground connections
  • On many GM applications, a short to ground in the fan clutch can set code P2509
  • Defective batteries
  • While PCM failure is seldom the cause of this code, PCM damage can result from this code

What are the symptoms of code P2509?

Some common symptoms of code P2509 could include the following-

  • Stored trouble code and illuminated warning light
  • Engine may crank, but not start
  • Engine may not crank, depending on the application
  • Instruments could behave erratically, or freeze in position
  • Idling speed may fluctuate
  • Additional codes may be present, which may or may not cause driveability issues. Note that this depends on the application as much as it does on the particular codes that are present along with P2509

How do you troubleshoot code P2509?

SPECIAL NOTES: While it may be necessary to disconnect the battery terminals or wiring connectors during the diagnostic procedure for code P2590, non-professional mechanics are advised NOT to disconnect ANY wiring unless an approved memory saving device is installed to keep critical systems powered up while either the battery or other wiring is disconnected. Failure to install a memory saving device can (and almost certainly will) cause the PCM to lose vital memory or programming. Also, note that if this happens, the PCM will require reprogramming, which can sometimes only be performed by the dealer. END OF SPECIAL NOTES.

WARNING: All manufacturers have very specific testing requirements/procedures when it comes to testing the integrity of the PCM’s power supply system on their products. For instance, on some applications, the voltage that enters the PCM cannot be tested between the terminal where it enters the PCM and battery negative, or ground. In many cases, the voltage must be verified between the “live” terminal in the connector and another terminal in the connector, so make absolutely certain that the pin-out chart you have available applies to the PCM connector on the specific model being worked on, and that you have a complete understanding of the circuit(s) you are testing at any given point. 

Placing a probe into the wrong terminal can cause extensive damage to the PCM as well as to other parts of the electrical system, so exercise extreme care when testing for any electrical value in the PCM connector. 

Step 1

Record all fault codes present, as well as all available freeze frame data. This information can be of use should it be necessary to trace the cause of an intermittent fault.

Step 2

If there are other codes present along with P2509, refer to the manual for the application to be sure which of the codes could have contributed to the setting of P2509, and which codes are the likely to be the result of P2509.

For instance, if some additional codes refer to the charging system or system voltages in general, it is likely that one or more of these codes are the cause(s) of P2509, and resolving these codes first will very likely resolve P2509 as well. Codes that refer to the anti/theft and or central locking systems, or misfire related codes are often the result of P2509, and these types of codes almost always resolve themselves when P2509 is resolved.

NOTE #1: Be sure to check for the presence of pending codes, and especially for pending codes that relate to the charging system, since an over -, or undercharging condition on many GM applications can set P2509.

NOTE #2: If ANY corrosion is present on either battery terminal, regardless of the application, do NOT disconnect the terminals without first installing a memory saving device. Once such a device is installed, follow the directions in the manual for the application on the correct order in which to remove and refit the terminals if the terminals have to be removed to remove corrosion and acid accumulations from the battery.

Step 3

Assuming then that there are no other codes present, consult the manual to locate and identify ALL wiring and components that supply power the PCM before disconnecting and or testing ANYTHING. It is important to have a thorough understanding of this part of the electrical system to avoid causing damage where there was none before.

Step 4

When all relevant wiring is identified and fully accessible, make sure that the ignition is in the “OFF” position, and wait until all controllers that supply accessories and /or systems with delayed power (power windows, headlights, etc.) have entered “sleep” mode. In many cases, delayed power to these systems is routed through the PCM, and testing the PCM’s power supply before all relevant controllers are deactivated could result in misleading, incorrect, or confusing readings.

NOTE: Bear in mind that even with the ignition “OFF” and all controllers deactivated, there will be a continual current drain. This is normal since the both the PCM and the radio require a constant current to keep their memories alive. However, this drain is measured in mere tens of milliamps, and it should never exceed the value stated in the manual. If the accepted value is exceeded, the cause is almost always a short circuit to Bat+, and this must be resolved before continuing with the diagnostic procedure for P2509.

Step 5

Assuming that the system voltage agrees with the value stated in the manual, that the charging system is fully functional, and that the battery is serviceable and in good condition, start the diagnostic procedure by inspecting the ground connections between the battery and the engine, as well as between and battery and the bodywork for evidence of corrosion, or poor contact. Make repairs as required, but at the risk of overstating the case, do NOT disconnect any wiring/cabling unless a memory saving device is installed.

NOTE: If the application is fitted with two batteries as standard equipment, follow the directions provided in the manual on the correct procedure to test both batteries. In some cases, code P2509 can be set when one battery is weaker than the other is, or when one or more battery cable connections/terminations is not as perfect as it should be.

WARNING: Do NOT short out batteries across terminals, or by touching positive and negative cables together in an effort to “gauge”, or “test” the condition of a battery, and especially not on applications that use two batteries. Doing this could cause one, or both batteries to explode, which can cause serious personal injuries, or even death as a result of acid burns. 

Step 6

If it is certain that the battery, or batteries, and all positive/ground connections are in perfect condition, consult the manual to determine the starting point of the PCM’s power supply circuit. This is usually at the ignition switch, so use a digital multimeter check the voltage in this wire or terminal by following the directions in the manual. Compare the obtained reading with the value stated in the manual, and make repairs as required if a deviation is found. Repeat the test after repairs are complete to ensure that all electrical values fall within the manufacturer’s specifications.

NOTE #1: This voltage should remain stable when the key is wiggled in the switch; however, do not turn the ignition “ON”. If the voltage leaving the ignition switch varies when the key is wiggled about, the switch is almost certainly defective, and replacing it should resolve the problem.

NOTE #2:  Be aware that replacing an ignition switch on some applications can be a tricky affair, so do NOT attempt this if you are not perfectly comfortable with the idea of partially dismantling the steering column and/or dashboard.

Step 7

If the code persists but the power supply up to, and from, the ignition switch falls within the specified range, follow the supply to the fuse box, and then on to the PCM connector.

However, on some applications, the power passes through a fuse or fusible link, while on others, it energises a relay that draws power directly from the battery. Refer to the manual and/or wiring diagram to trace the exact path, and test for continuity and resistance across all connectors, fuses, and/or fusible links and relays between the ignition switch and the PCM connector. Be sure however not to disconnect anything to perform these checks, and always to insert the multimeters’ probes into connectors from the back, aka “back probing” to prevent accidental short circuits.

Compare all obtained readings with the values stated in the manual. If deviations are found, make repairs as required to ensure that all electrical values fall within the manufacturer’s specifications.

NOTE #1: If relays are present in the circuit, be sure to test them as per the instructions provided in the manual. Replace all defective or suspect relays with OEM parts to ensure that the replacement(s) perform(s) as expected.

NOTE #2: On some applications, it may be necessary to switch the ignition “ON” to test for continuity in some parts of the PCM’s power supply circuit. Be sure to consult the manual for the application being worked on for detailed information on the correct testing procedure(s) to follow during this step.

Step 8

The steps outlined above will resolve code P2509 nine times out of every ten, but in stubborn cases where the cause is not obvious, it is likely that an intermittent fault is present, or that the PCM is already damaged.

If either condition is suspected, the wiser option is to refer the vehicle to a specialist repair shop for professional diagnosis and repair, since in some cases the repair could involve reprogramming of the PCM.

  • P2505 – Relates to “ECM/PCM Power Input Signal”
  • P2506 – Relates to “ECM/PCM Power Input Signal Range/Performance”
  • P2507 – Relates to “ECM/PCM Power Input Signal Low”
  • P2508 – Relates to “ECM/PCM Power Input Signal High”

Help Us Help You

Please comment below describing your issue as well as the specifics of your vehicle (make, model, year, miles, and engine). To get a detailed, expedited response from a mechanic, please make a $9.99 donation via the payment button below.