A weak or failed battery, or one bad battery in a dual-battery truck (OBD-Codes.com, Ram service manual, RepairPal). Loose, damaged or corroded battery cables and connections (OBD-Codes.com, Ram service manual). An alternator that overcharges or undercharges, or a leaking alternator diode draining the battery (OBD-Codes.com). An open in the fused battery power feed or the ground circuits to the engine computer (Ram service manual, OBD-Codes.com). A battery power wire shorted to ground or to other circuits in the engine harness (Ram service manual, OBD-Codes.com). A bad connection at the engine computer (OBD-Codes.com). On Dodge/Ram diesels, an electric fan clutch shorted to ground (OBD-Codes.com).

Clean, tighten or replace corroded or loose battery cables and grounds (Ram service manual, OBD-Codes.com). Recharge or replace a weak battery, or both batteries if needed (Ram service manual, OBD-Codes.com). Replace an alternator that overcharges, undercharges or has a leaking diode (OBD-Codes.com). Repair an open or shorted power or ground wire to the engine computer (Ram service manual). Reprogram the computer if a service bulletin applies, such as Dodge/Ram TSB 18-001-07 (OBD-Codes.com).

P2509🔧 Engine computer power supply (battery, charging and ECM power/ground)Stop and fix promptly

P2509: ECM/PCM Power Input Signal Intermittent (Meaning, Causes, Fixes)

Q: What does P2509 mean?

P2509 means the engine computer (ECM or PCM) saw its power input drop out briefly. RepairPal says the computer relies on a constant power supply from the battery and sets this code when that supply isn't sufficient. The most common causes are weak batteries, loose or corroded battery cables, charging system problems and open or shorted power and ground wires to the computer. OBD-Codes.com notes the code is most common on Dodge and Ram Cummins diesel trucks. The engine computer gets constant battery power through wires connected straight to the battery, plus a switched power feed when the key is on. P2509 is the generic code for that power input being intermittent. The sources describe the trigger a little differently. RepairPal simply says the code sets when the computer doesn't have sufficient power supply. OBD-Codes.com describes it as an intermittent power signal, not constant and within limits with the key off, because battery voltage is low, being drained, or a connection is bad. The 2008-2009 Ram 2500/3500 6.7L Cummins service manual is the most specific: it monitors continuously with the key on or off, and sets P2509 when supply voltage to the computer momentarily falls below 6.2 volts, or when the computer isn't allowed to power down correctly by keeping battery voltage for 30 seconds after key off. That manual says the check engine light comes on immediately when the test fails and turns off after four drive cycles in a row with the test passing. It also warns that stored fault and trip information may be inaccurate after a power loss. Check the factory procedure for your vehicle, since the exact thresholds are make-specific.

Q: Can I keep driving with P2509?

Stop and fix promptly. Get it checked soon. RepairPal rates P2509 as Urgent. The engine computer is losing its power supply for moments at a time, and both OBD-Codes.com and the Ram 6.7L Cummins service manual say this can cause hard starting or the engine dying, including stalling while driving. If the battery or charging warning shows low voltage, have it diagnosed before relying on the vehicle.

Reviewed 2026-10-24 · Based on 3 cited source(s)

What P2509 means

The engine computer gets constant battery power through wires connected straight to the battery, plus a switched power feed when the key is on. P2509 is the generic code for that power input being intermittent. The sources describe the trigger a little differently. RepairPal simply says the code sets when the computer doesn't have sufficient power supply. OBD-Codes.com describes it as an intermittent power signal, not constant and within limits with the key off, because battery voltage is low, being drained, or a connection is bad. The 2008-2009 Ram 2500/3500 6.7L Cummins service manual is the most specific: it monitors continuously with the key on or off, and sets P2509 when supply voltage to the computer momentarily falls below 6.2 volts, or when the computer isn't allowed to power down correctly by keeping battery voltage for 30 seconds after key off. That manual says the check engine light comes on immediately when the test fails and turns off after four drive cycles in a row with the test passing. It also warns that stored fault and trip information may be inaccurate after a power loss. Check the factory procedure for your vehicle, since the exact thresholds are make-specific.

Severity: can I keep driving?

Stop and fix promptly

Get it checked soon. RepairPal rates P2509 as Urgent. The engine computer is losing its power supply for moments at a time, and both OBD-Codes.com and the Ram 6.7L Cummins service manual say this can cause hard starting or the engine dying, including stalling while driving. If the battery or charging warning shows low voltage, have it diagnosed before relying on the vehicle.

Common symptoms

Check engine light on
Hard starting or no start (OBD-Codes.com, Ram service manual)
Engine dying or stalling, including while driving (OBD-Codes.com, Ram service manual)
On Dodge/Ram diesels, the check gauges light with the voltmeter at 11 volts or less, and a cold idle that wanders between about 500 and 700 rpm (OBD-Codes.com)
Charging system codes such as P2502 or P2503 stored alongside it (OBD-Codes.com)

Likely causes

Roughly most to least common, based on the sources below. Your vehicle may differ.

A weak or failed battery, or one bad battery in a dual-battery truck (OBD-Codes.com, Ram service manual, RepairPal)
Loose, damaged or corroded battery cables and connections (OBD-Codes.com, Ram service manual)
An alternator that overcharges or undercharges, or a leaking alternator diode draining the battery (OBD-Codes.com)
An open in the fused battery power feed or the ground circuits to the engine computer (Ram service manual, OBD-Codes.com)
A battery power wire shorted to ground or to other circuits in the engine harness (Ram service manual, OBD-Codes.com)
A bad connection at the engine computer (OBD-Codes.com)
On Dodge/Ram diesels, an electric fan clutch shorted to ground (OBD-Codes.com)

Diagnostic steps you can do

⚠ Before you start

Work on a cool engine, parked on level ground with the parking brake set. Never work under a car supported only by a jack.

Read all stored codes. Note any charging system codes like P2502 or P2503, since a charging problem can be the root cause.
Check for technical service bulletins. OBD-Codes.com points 2006-and-older Dodge/Ram Cummins owners to TSB 18-001-07, whose fix is a computer reprogram at the dealer.
Inspect both ends of every battery cable and the main grounds. Clean corrosion and make sure each connection is tight. This is the first step in the Ram service manual.
Charge and load-test the battery, or both batteries on a dual-battery truck. OBD-Codes.com says a fully charged battery reads about 12.75 volts, and the Ram manual has both batteries tested with a battery tester.
With the engine running, check charging voltage at the battery. OBD-Codes.com says it should be between 13.5 and 15 volts.
If the batteries keep running down, have a parasitic draw test done. OBD-Codes.com says under 30 milliamps after everything goes to sleep is normal.
With factory wiring diagrams, check the computer's power and ground wires for opens and shorts, as the Ram manual does.
Clear the code after the repair and watch whether it returns.

Typical fixes

Clean, tighten or replace corroded or loose battery cables and grounds (Ram service manual, OBD-Codes.com)
Recharge or replace a weak battery, or both batteries if needed (Ram service manual, OBD-Codes.com)
Replace an alternator that overcharges, undercharges or has a leaking diode (OBD-Codes.com)
Repair an open or shorted power or ground wire to the engine computer (Ram service manual)
Reprogram the computer if a service bulletin applies, such as Dodge/Ram TSB 18-001-07 (OBD-Codes.com)

Repair costs vary widely by vehicle, region, parts brand and shop, so we don't quote prices. Get a written estimate after diagnosis.

🛠 When to call a pro

The engine stalls while driving or won't start reliably
The batteries and cables test good but the code keeps returning
You need a parasitic draw test or wiring checks at the computer's connectors
A service bulletin calls for a computer reprogram

Where to buy

Plain search links. Always match parts to your exact year, make, model and engine.

OBD2 scanners on Amazon Parts at Advance Auto Parts catalog at RockAuto

FAQs

Is P2509 serious?

It can be. RepairPal rates it Urgent, and OBD-Codes.com and the Ram service manual both say the engine can be hard to start or die. Most of the time the cause is the battery, its cables or the charging system, which are usually straightforward fixes.

Can a bad battery cause P2509?

Yes. A weak or failed battery is one of the main causes in all three sources. On trucks with two batteries, OBD-Codes.com says one battery reading noticeably lower than the other is probably bad.

Why is P2509 so common on Dodge Ram Cummins trucks?

OBD-Codes.com says it shows up most on Dodge and Ram diesel trucks and that Cummins service bulletins link it to overcharging or undercharging. There is also TSB 18-001-07 for 2006-and-older trucks, fixed by reprogramming the computer.

Will P2509 clear itself?

On the 2008-2009 Ram 6.7L, the service manual says the light turns off after the test passes on four drive cycles in a row. The code stays in history, so fix the cause first.

How much does it cost to fix P2509?

None of these sources give a repair cost. It ranges from cleaning a battery cable to replacing batteries or an alternator, so get a written estimate after diagnosis.

Sources

Last reviewed 2026-10-24. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.