P2509 on 2018-2020 Ram 2500 Cummins: Causes and Fixes for Intermittent PCM Power

What's Unique About the 2018-2020 Ram 2500

The Ram 2500 with the 6.7L Cummins engine uses a dual-battery setup. This is a common point of failure leading to the P2509 code. If one battery becomes weak, it can draw power from the healthy battery, causing the overall system voltage to drop intermittently and trigger the code. Many owners are unaware that the batteries must be tested separately and are often the root cause, even if they seem to hold a charge or pass a simple voltage test at a parts store. A proper load test on each isolated battery is critical for an accurate diagnosis.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Generation note: This year range covers the end of the fourth generation (2018) and the beginning of the fifth generation (2019-2020) Ram HD trucks. While the fundamental causes like battery and cable issues are the same, the physical location of wiring, grounds, and the fuse box (TIPM) may differ slightly between the two generations. The underlying principle of ensuring stable voltage to the PCM remains identical.

Symptoms You May Notice

• Engine may stall intermittently while driving.
• Engine cranks but fails to start, or won't crank at all.
• Check Engine Light is illuminated (though sometimes it may not be, and the code is stored as 'permanent').
• Multiple warning lights on the dashboard may flicker, including the airbag light.
• Message on the dash such as "Service Electronic Throttle Control".
• Erratic transmission shifting behavior.
• In some cases, the code may be stored with no noticeable symptoms, but can prevent passing an emissions test.
• Tachometer needle jumps erratically at idle.
• Truck experiences a 'no-start, just click' condition that resolves after jump-starting.

Most Likely Causes

1. Weak or Failing Batteries 🔴 High Probability The dual-battery system can mask a single failing battery. One bad battery will drain the good one, causing low voltage conditions that trigger the P2509 code. Even new batteries can be faulty, and parts store tests are not always reliable.
   How to confirm: Disconnect both batteries from each other and the truck to isolate them. Use a digital multimeter to check the static voltage of each (a healthy battery is ~12.6V). More importantly, use a carbon pile or electronic load tester to test each battery individually to see if it can hold a charge under load. A simple voltage check is insufficient. 🎬 See how to properly test and fix ECM power signals.
   Typical fix: Replace both batteries at the same time with high-quality AGM units. It is not recommended to replace only one, as the older battery can degrade the new one. Many owners have resolved the code by replacing batteries that had previously 'tested good'.
   Est. part cost: $350-$600 for two high-quality AGM batteries.
2. Loose or Corroded Battery Terminals and Cables 🔴 High Probability Vibration and exposure to the elements can lead to loose connections or a buildup of corrosion on the terminals, crossover cable, and ground points, creating intermittent electrical contact. This includes the main grounds from the battery to the chassis and engine block.
   How to confirm: Visually inspect all battery terminals, positive and negative cables, and the crossover cable connecting the two batteries. Attempt to wiggle the terminals by hand; if they move, they are too loose. Check the main ground straps from the batteries to the chassis and engine block for tightness and corrosion. Corrosion can hide under the insulation at the cable ends.
   Typical fix: Disconnect, thoroughly clean all terminals, cable ends, and grounding points with a wire brush, and re-tighten securely. Apply a dielectric grease or anti-corrosion spray to prevent future buildup.
   Est. part cost: $5-$20 for cleaning supplies.
3. Failing Alternator ⚪ Low Probability
   How to confirm: With the engine running, use a multimeter to measure the voltage at the battery terminals. A healthy alternator should output between approximately 13.5 and 14.5 volts. If the voltage is too low, too high, or fluctuates wildly, the alternator or its voltage regulator is likely faulty.
   Typical fix: Replace the alternator.
   Est. part cost: $300-$700

Rare But Worth Checking

• Loose Fuses or Relays in the TIPM (Fuse Box): On Ram trucks, fuses and relays in the Totally Integrated Power Module (TIPM) can sometimes work themselves loose over time, causing intermittent power to various modules, including the PCM. It's worth taking a few minutes to press down firmly on all fuses and relays in the under-hood fuse box. Corrosion on the underside connectors of the TIPM has also been noted as a cause.
• Damaged PCM Power Wire or Ground: A wire in the harness that supplies power or ground to the PCM could be chafed or broken. This is more common on older models but can still occur. Wires can rub against the firewall or other components, causing an intermittent short or open circuit. A thorough visual inspection of the harness near the PCM is required.
• Faulty Powertrain Control Module (PCM): While possible, the PCM itself is rarely the cause of this code. The PCM is robust, and all other power and ground supply issues should be exhaustively ruled out before considering PCM replacement. A faulty PCM might smell burnt.
• PCM Software Update Needed: In some cases, particularly on older generations but still relevant, a PCM software reflash can resolve the issue. TSB 18-061-20 for 2018 models involves a PCM software update that addresses a list of codes including P2509, which can fix false triggers.

Diagnosis Steps

1. Scan for all stored trouble codes. Note any communication (U-codes) or other module codes that appear alongside P2509.
2. Visually inspect the terminals on both batteries for any signs of corrosion or looseness. Clean and tighten as necessary. Check under the red plastic covers and at the cable ends.
3. Inspect the main power and ground cables, including the crossover cable between the batteries and the ground straps to the engine block and chassis. Ensure they are clean, tight, and undamaged.
4. Disconnect the batteries from each other to prevent a bad battery from affecting the good one during testing.
5. Test each battery individually with a quality load tester. A simple voltage check is not sufficient. A healthy, fully charged battery should read around 12.6V and hold voltage under load. Replace both if one fails.
6. If batteries and connections are good, start the engine. Use a multimeter to test the alternator's output voltage at the battery posts. It should be stable, between 13.5V and 14.5V.
7. Open the under-hood fuse box (TIPM) and firmly press down on all fuses and relays to ensure they are fully seated.
8. If the problem persists, perform a parasitic draw test to check if a component is draining the batteries while the truck is off. A draw over 30-50 milliamps after the modules go to sleep (approx. 15-30 mins) indicates a problem.
9. As a final step, inspect the wiring harness leading to the PCM for any signs of chafing, melting, or damage, paying close attention to where it might rub against the firewall 🎬 Watch a mechanic perform advanced electrical testing for this code. or engine components.
10. Check with a dealer for any applicable software updates, such as TSB 18-061-20, which may address false triggers for the P2509 code.

Parts You'll Likely Need

• AGM Battery (OEM #Mopar BBAH700L or similar Group H7/94R) — This is the most common cause. One or both of the original batteries fail to hold a stable voltage, triggering the code. It is critical to replace them as a pair.
  Trusted brands: Mopar, Interstate, Odyssey, NorthStar

Related Codes That Often Appear With This One

• P0868 — This code for low transmission line pressure can be triggered when the PCM momentarily loses power and reboots, causing a temporary communication loss with the transmission controller. (Bulletin #18-061-20 REV A)
• P229F — An aftertreatment NOx sensor performance code that can be set spuriously when the PCM power is interrupted. (Bulletin #18-061-20 REV A)
• P207F — A reductant quality performance code that, like others, can be falsely triggered by a PCM reboot. (Bulletin #18-061-20 REV A)
• U-prefix codes (e.g., U0100) — Communication codes are common companions to P2509. If the PCM loses power, it cannot communicate with other modules on the vehicle's network, logging communication fault codes.
• P2271 — Oxygen sensor codes can appear alongside P2509. A weak battery or unstable voltage can cause issues with sensor heater circuits, leading to O2 sensor fault codes.

Technical Service Bulletins (TSBs) & Recalls

• TSB 18-061-20 REV A (October 17, 2020): Involves reprogramming the PCM with updated software. P2509 is listed as one of many DTCs that this flash can address, suggesting it can be triggered spuriously by software logic. This update also provides other enhancements like improved shifting and misfire detection.
• TSB 18-080-22 (April 12, 2022): Also lists P2509 among a group of DTCs related to a service bulletin, reinforcing that P2509 often appears with other codes.
• TSB 18-061-20 (August 07, 2020): The original version of the PCM flash bulletin that includes P2509.

Platform-Specific Known Issues

• The dual-battery system is a known weak point for causing this specific code. Forum discussions frequently point to owners replacing batteries, even if they test 'good' at a parts store, as the ultimate fix.
• Multiple Technical Service Bulletins (TSBs) for 2018-2020 Ram trucks list P2509 as a potential code that may appear alongside other powertrain issues, indicating it can be a symptom of a broader software issue that can be fixed with a PCM reflash (TSB 18-061-20 REV A).
• A Reddit user with a 2021 Ram 3500 experienced a no-start condition with code P2509. After the batteries tested 'fine' at Autozone, they ultimately solved the problem by replacing both batteries anyway, highlighting the inadequacy of simple tests.

Mechanic-Grade Diagnostic Values

• PCM Power Supply Voltage (Key On, Engine Off) — expected: 12.4V - 12.8V. Failure: Voltage dips below 9V, or is unstable.
• PCM Power Supply Voltage (Engine Running) — expected: 13.8V - 14.4V. Failure: Voltage is below 13.5V, above 14.5V, or fluctuates wildly.
• PCM Power Supply Voltage Drop Threshold — expected: Must remain above 6.2 volts. Failure: Voltage momentarily drops below 6.2 volts, triggering the code.
• Resistance between Positive Battery Post and ECM Supply Circuits — expected: Less than 10.0 Ohms. Failure: Resistance is greater than 10.0 Ohms, indicating an open circuit in the harness.
• Resistance between ECM B+ Supply Circuits and Ground — expected: Greater than 100k Ohms. Failure: Resistance is less than 100k Ohms, indicating a short to ground.

Hidden / Shadow Codes Worth Checking

• P2509 (Permanent): The code can be stored as a 'permanent' fault, meaning it will not clear with a standard scan tool reset. The vehicle's computer must run and pass its internal diagnostic for the specific monitor (in this case, continuous voltage monitoring) over several drive cycles before the code will clear itself. This can prevent passing an emissions test even if the Check Engine Light is off. (see via A standard OBD-II scanner will show this code, but may not explicitly label it 'permanent'. Smog testing equipment will identify it as a permanent code preventing a 'pass' result.)

Wiring & Ground Locations

• G114 (example) — On the engine block, where multiple ECM ground wires converge into a single ground point.. A corroded or loose main ECM ground is a common, but often overlooked, cause for intermittent power issues that trigger P2509. Verifying this ground is clean and tight is a critical diagnostic step.
• Battery-to-Chassis Grounds — Main negative battery cables connect directly to the chassis/body on both the driver and passenger side, often near the battery tray or on the inner fender.. These are the primary ground paths for the entire electrical system. Corrosion or looseness at these points can create system-wide voltage instability, directly causing the P2509 code.
• PCM Power Feed Wires — Within the main engine wiring harness, running from the under-hood fuse box (TIPM/PDC) to the large connectors on the PCM itself.. These wires provide constant and switched power to the PCM. They can chafe against engine components or the firewall, causing an intermittent short or open that sets the code.

Real Owner Repair Stories

• Dodge Diesel Forum user (2003-2009 Ram Cummins (principles apply to 2018-2020)) — Engine would die for a split second and then come back to normal, tachometer and speedometer drop to zero momentarily. Code P2509 was present.
  ❌ Tried (didn't work) Cleaning all battery terminals., Replacing one of the two batteries., Having both batteries load tested at a parts store (they passed).
  ✅ What actually fixed it The user noted that many aftermarket batteries have slightly smaller post diameters than OEM batteries. The cable clamps could not be tightened sufficiently, causing a poor connection even though they felt tight. The user had to modify the clamps to allow them to close further, which secured the connection and resolved the issue.
• Reddit user r/ram_trucks (2019 Ram 3500) — No drivability issues or check engine light, but a permanent P2509 code was present and preventing the truck from passing a California smog test.
  ❌ Tried (didn't work) Driving the truck extensively to complete drive cycles., Replacing both batteries.
  ✅ What actually fixed it The user was advised by others to meticulously check all wiring harness connections for looseness or chafing and to ensure all sensor plugs around the engine were clean and fully seated, as a poor connection anywhere could contribute to the intermittent voltage fault. The final resolution was not posted, but the focus shifted from batteries to wiring integrity.

Model Year Variations Within This Range

• 2019-2020 (5th Gen): The 5th generation trucks have a different interior fuse box and under-hood Power Distribution Center (PDC) compared to the 4th generation (2018) TIPM. For example, on the 2019+ models, fuse F94 (10A) powers the Shift-By-Wire module and Trailer TPM module, while on the older trucks these may be routed differently. While the root cause of P2509 is still power-related, tracing the specific circuits requires the correct diagram for the generation.