P2509 The engine computer's power supply is dropping out on and off

ECM/PCM power input signal intermittent

P2509 in short

P2509 means the engine computer's power supply is coming and going rather than failing completely. Loose or corroded battery terminals, bad grounds, wiring faults or a charging problem are the usual causes. The engine may stall or not start, so have it checked soon.

The engine computer needs a steady supply of battery voltage to run. P2509 sets when that supply is unstable, with the voltage dipping or fluctuating outside the limits the maker sets, even though power is not lost entirely.

Because the supply passes through the battery, its terminals, fuses, relays and grounds, a weak link anywhere along the way can cause it. TroubleCodes.net lists corroded battery terminals, charging faults, weak grounds and a defective battery, and notes that some GM vehicles have a short in the fan clutch.

An owner of a 2015 Ram 3500 reported P2509 stored with other module codes after the truck shut down on the road. That fits the picture of a power interruption, but a complaint does not show the cause.

Most likely causes

These are general causes for this code across vehicles. None is specific to one make or model.

  1. Corroded or loose battery terminals, or a weak ground

    Common general cause
    Why it ranks here
    TroubleCodes.net lists poor battery terminal contact and weak ground connections, and they are the cheapest things to check.
    What would change this
    Rises if terminals are loose or crusted, or voltage drops when the engine is cranked or the wiring is moved. Falls if connections are clean and tight and voltage holds.

  2. Damaged, corroded or loose wiring and connectors in the computer power feed

    Common general cause
    Why it ranks here
    Both TroubleCodes.net and the diagnosis it describes follow the feed from the ignition switch through the fuse box to the computer connector, checking continuity at each connector and relay.
    What would change this
    Rises if a voltage drop test finds a loss in one part of the feed. Falls if the feed holds steady through the whole run.
    How to test the underhood fuse block (engine compartment fuse box)

  3. Charging system fault or a defective battery

    Common general cause
    Why it ranks here
    TroubleCodes.net lists charging system defects causing high or low voltage and a defective battery as causes, and tells you to check for related charging codes.
    What would change this
    Rises if the battery tests weak, charging voltage is out of range or charging codes are stored. Falls if both test good.

  4. Short in the fan clutch (some GM vehicles)

    Possible — needs testing
    Why it ranks here
    TroubleCodes.net names this for some GM vehicles only. It is one source and make-specific, so it ranks last.
    What would change this
    Rises on a GM vehicle if the code appears when the fan clutch engages. Falls if it does not.

  5. Engine computer itself (rare)

    Possible — needs testing
    Why it ranks here
    A computer fault is possible, but TroubleCodes.net and the testing it describes point to the supply first.
    What would change this
    Rises only after the power feed and grounds test good. Falls if any fault is found in the feed.
    How to test the engine control module (ecm / pcm)

What to test first

Start at step 1. Each result tells you what it means and where to go next.

  1. Record all codes and freeze-frame data, and note any charging system or other module power codes stored with P2509.

    Low riskTool: OBD-II scan tool

  2. With the engine off, look at the battery terminals and the main ground connections for corrosion or looseness.

    Low riskTool: flashlight, wire brush

  3. Have a shop test the voltage and continuity of the computer power feed from the ignition switch through the fuse box and relays to the computer connector, and test the battery and charging output.

    Best left to a shopTool: multimeter, wiring diagram

    Warning:Wrong probe placement on the computer connector can damage the computer or the wiring, which TroubleCodes.net flags. Work from the wiring diagram.

Symptoms

  • Check engine light
  • Engine cranks but does not start, or does not crank Gets more serious: stop and have it checked.
  • Engine stalls intermittently Gets more serious: stop and have it checked.
  • Gauges freeze or behave erratically

How to fix

  1. With the engine off, check that the battery terminals are tight and free of corrosion, and clean them if they are not.

    Warning:Disconnecting the battery can reset radio and learned settings; take the key out first and reconnect negative last.

Parts and cost context

  • Engine control module (ECM / PCM):Replace the computer last, after the power feed and grounds test good. Labour varies; get a quote.

Labor varies by vehicle and shop; get a quote before you approve a repair.

Compare used part prices on PartPriceGuide

On your vehicle

Ram

  • 2015 · 3500

    Possible — needs testing

    One 2015 owner reports P2509 stored with other module codes after a roadside shutdown. A complaint shows the code appeared, not what caused it.

  • P0562System voltage low

    Same system Possible — needs testing

    TroubleCodes.net tells you to check for related charging system codes such as low system voltage when P2509 is stored.

All Engine computer (ECM/PCM) codes

Sources and review

  1. P2509 trouble code page — TroubleCodes.net
  2. P2509 OBD-II code page — Kelley Blue Book
  3. NHTSA complaint ODI 11658341 — NHTSA Office of Defects Investigation
Last reviewed
Written by
Saurabh K.
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