P0604 on 2018-2025 Jeep Wrangler: Internal RAM Error Causes and Fixes

What's Unique About the 2018-2025 Jeep WRANGLER

For the 2018-2025 Jeep Wrangler (JL generation), this code is often linked to software issues rather than outright hardware failure. A Technical Service Bulletin (TSB #18-044-26) specifically lists P0604 as a potential internal PCM error that can be stored. 🎬 Watch: Understanding the causes and fixes for the P0604 code. On modern Jeep platforms, it's frequently seen that software updates, or "reflashes," performed by a dealership are the correct fix for internal module errors, suggesting the factory software can be prone to glitches. Furthermore, the JL Wrangler's dual-battery system (main and auxiliary for the Engine Stop/Start system) adds complexity. A failing auxiliary battery or issues with the charging system logic can cause voltage fluctuations that trigger memory-related codes like P0604.

Diagnostic Flowchart

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

Generation note: The 2018 model year was a transition period. This guide applies specifically to the Jeep Wrangler JL generation (2018-2025). Some 2018 models were the previous 'JK' generation, which may have different diagnostic considerations, though the root causes are often similar.

Symptoms You May Notice

• Check Engine Light is on
• Engine may crank but not start
• Intermittent stalling or sudden loss of power, especially after stopping
• Erratic engine performance, like hesitation or misfiring
• Harsh or erratic automatic transmission shifting
• Other warning lights, such as the ABS or Traction Control light, may illuminate
• Failure to pass an emissions test
• Vehicle enters 'limp mode' with reduced power

Most Likely Causes

1. PCM Software Glitch 🔴 High Probability Modern Jeep platforms have a history of TSBs that address internal module errors via software updates, indicating that the factory programming can have correctable faults. TSBs explicitly state that reprogramming the PCM is the fix for various performance-related DTCs.
   How to confirm: A Jeep dealership or a properly equipped independent shop can use a scan tool (wiTECH) to check if a newer software calibration is available for the Powertrain Control Module.
   Typical fix: Reprogramming (reflashing) the PCM with the latest manufacturer software. This typically costs one hour of shop labor.
   Est. part cost: $0
2. Low Battery Voltage or Poor Connections 🟡 Medium Probability → Shop Vehicle Battery The complex electronics and dual-battery system on the JL Wrangler are sensitive to voltage fluctuations. A weak main or auxiliary battery, or corroded/loose terminals can cause a 'brown-out' condition for the PCM, corrupting its memory. Owners on forums frequently trace electrical gremlins back to the factory batteries, especially the auxiliary one.
   How to confirm: Test both the main and auxiliary batteries to ensure they hold a charge above 12.4 volts at rest. Visually inspect and physically check all battery terminals, cables, and the main PCM ground connections for tightness and corrosion. Voltage should remain above 9V during cranking.
   Typical fix: Cleaning battery terminals, tightening loose connections, or replacing a weak battery. Often, both batteries are replaced at the same time.
   Est. part cost: $0-$450
3. Faulty Powertrain Control Module (PCM) ⚪ Low Probability → Shop Engine Control Module (ECM) While less common than software or power issues, the internal hardware of the PCM can fail from heat, vibration, or internal defects like cracked solder joints.
   How to confirm: This is a diagnosis of exclusion. If the batteries, connections, and software are all confirmed to be good, but the P0604 code persists or returns immediately after being cleared, the PCM itself is the likely culprit.
   Typical fix: Replacing the PCM and programming the new unit to the vehicle's VIN. This can be done with a new Mopar unit or a remanufactured one from a reputable supplier.
   Est. part cost: $800-$1500

Rare But Worth Checking

• Faulty PCM Power or Ground Circuit: A damaged wire or corroded connection in the PCM's dedicated power or ground circuit can mimic the symptoms of a failed PCM. A thorough inspection of the harness is required before condemning the module.
• Blown PCM Fuse: A simple blown fuse can cut power to the PCM, leading to this code. Always check the fuse box before proceeding with more complex diagnostics.
• Failing Alternator: → Shop Alternator An alternator providing unstable voltage (too low or too high) can stress the PCM and cause memory errors. The charging system should be tested to ensure it's operating correctly, typically between 13.5 and 14.5 volts when running.

Diagnosis Steps

1. Scan for all stored trouble codes. Note if P0604 appears with other internal PCM codes like P062B or P060C.
2. Check the battery system. Test both the main and auxiliary batteries. Ensure they are fully charged and pass a load test. A voltage below 12.4V at rest is a concern.
3. Inspect all battery terminals and main power/ground cables. The JL/JT platform is known for having ground issues. Clean any corrosion and ensure all connections are tight.
4. Inspect the wiring harness and connectors at the PCM for any signs of damage, corrosion, or loose pins.
5. Check for any blown fuses related to the PCM or engine controls in the under-hood fuse box (Power Distribution Center).
6. Using a dealer-level scan tool (wiTECH), check for available software updates (reflashes) for the PCM. TSBs for PCM flashes require a stable voltage between 13.2 and 13.5 volts from a battery charger during the procedure. If an update is available, perform it.
7. If all power/ground circuits are good and the software is up to date, but the code returns immediately after being cleared, the PCM is likely faulty and requires replacement.

Parts You'll Likely Need

• Powertrain Control Module (PCM) (OEM #68434927AC (example for 2022 2.0L, varies by year/engine)) — This is the part that is replaced if the RAM error is due to an internal hardware failure that cannot be fixed with a software update.
  Trusted brands: Mopar, WranglerFix, Flagship One Inc.
  OEM price range: $1000-$1500
  Aftermarket price range: $600-$1000
• AGM Main and Auxiliary Batteries — The dual-battery system is a frequent source of electrical issues on the JL. A weak or failing battery is a common trigger for voltage-sensitive codes like P0604.
  Trusted brands: Mopar, Odyssey, Northstar
  OEM price range: $200-$300 each
  Aftermarket price range: $180-$280 each

Related Codes That Often Appear With This One

• P062B — This is an 'Internal Control Module Fuel Injector Control Performance' code. Its appearance with P0604 reinforces that the PCM is experiencing internal processing errors, as cited in TSB 18-044-26.
• P060C — This code for 'Internal Control Module Main Processor Performance' also points directly to a fault within the PCM itself. TSB 18-044-26 groups this with P0604 as a related internal error.
• U-series codes (e.g., U0100) — Communication codes often appear alongside P0604, indicating the PCM is failing to communicate properly with other modules on the vehicle's network.

Technical Service Bulletins (TSBs) & Recalls

• 18-044-26: Addresses multiple potential internal PCM performance codes, including P0604.
• Multiple other TSBs (e.g., 18-026-23, 18-044-19) confirm that flashing the PCM is a standard repair procedure for a wide range of DTCs on Jeep vehicles, reinforcing the 'software first' diagnostic approach.

Platform-Specific Known Issues

• Technical Service Bulletin #18-044-26 mentions P0604-00 as a potential 'Internal Control Module Random Access Memory Ram Error' that can be set in Active or Stored status on 2018 models.
• The dual-battery system for Engine Stop/Start (ESS) is a known weak point. A failing auxiliary battery can cause widespread, seemingly unrelated electrical problems, including triggering PCM codes.

Mechanic-Grade Diagnostic Values

• PCM Fused B+ Circuit Voltage — expected: Battery voltage (approx. 12.6V with engine off). Failure: Voltage significantly below battery voltage or 0V indicates an open circuit or blown fuse.
• PCM Fused Ignition Circuit Voltage — expected: Battery voltage when ignition is ON; 0V when OFF.. Failure: No voltage when the key is on indicates a fault in the ignition switch circuit or related fuse.
• PCM Ground Circuit Integrity — expected: Less than 5.0 Ohms resistance between the PCM ground pins and a known good chassis ground.. Failure: High or infinite resistance indicates a broken or corroded ground connection, which can cause floating voltages and memory errors.
• Alternator Output Voltage — expected: 13.5V to 14.5V when the engine is running.. Failure: Voltage that is too low (under-charging) or too high (over-charging) can corrupt the PCM's Keep Alive Memory (KAM).

Scan Tool Commands That Help

• wiTECH 2.0: Flash PCM — Used to reprogram the PCM with updated software from FCA. This is the first step when a TSB indicates a software fix is available for P0604 or related symptoms.
• wiTECH 2.0: PCM Replaced — This function is used after installing a new or used PCM. It allows the technician to write the vehicle's VIN into the module, which is required for it to communicate with other modules and for the engine to start.
• wiTECH 2.0 / AlfaOBD: Learn ETC (Electronic Throttle Control) — After a PCM replacement or reflash, the throttle body's learned values must be reset and relearned. Failure to do this can cause idle or performance issues.
• wiTECH 2.0 / AlfaOBD: Check PCM Odometer — When installing a new PCM, the vehicle's mileage must be written to the module. This function is part of the replacement procedure.

Wiring & Ground Locations

• PCM Location — On the driver's side of the engine compartment, mounted to the inner fender/firewall area.. Physical access is needed to inspect connectors for corrosion or damage and to perform pin-out testing for power and ground.
• G905A / G906A — These are primary dash and instrument panel ground points, typically located behind the kick panels or under the dash on the driver's and passenger's side.. The PCM relies on clean, tight ground connections. A loose or corroded dash ground can cause intermittent power loss or voltage fluctuations to the module, triggering memory errors like P0604.
• PCM C1 Connector — This is one of the main multi-pin connectors plugging into the PCM.. Technicians must probe specific pins on this connector (or a breakout box connected to it) to verify the PCM is receiving correct power and ground signals before condemning the unit.
• CAN Bus STAR Connector (Gateway) — Located behind the glove box area on JL Wranglers.. While not a direct cause of P0604, a faulty STAR connector can cause widespread communication failures across the vehicle network, leading to multiple error codes and symptoms that can mimic a failed PCM.

Real Owner Repair Stories

• jlwranglerforums.com user (2018 Jeep Wrangler JL Unlimited Sport S (3.6L) with 83k miles) — Engine would not start, or would only run for a few seconds. Multiple random high voltage codes for 5V sensors (P0108, P0198), a generator field sense code (P0622), and various communication codes.
  ❌ Tried (didn't work) Replaced a faulty coolant temperature sensor., Verified all fuses and relays were seated., Checked and cleaned all ground connections., Verified all harness connectors were secure.
  ✅ What actually fixed it The user suspected a PCM failure but noted that most communication codes appeared after the battery voltage got low. The final fix that resolved the widespread electrical issues was replacing both the main and auxiliary batteries.
• Reddit user r/MechanicAdvice (2013 Jeep Wrangler Sahara Unlimited (JK generation, but relevant principle)) — Sudden onset of bizarre electrical issues, including dash lights flickering and warnings.
  ❌ Tried (didn't work) Initial diagnosis was unclear due to the variety of electronic symptoms.
  ✅ What actually fixed it The problem was resolved by replacing a 'really bad battery.' This highlights how a single failing battery can cause numerous, seemingly unrelated electronic gremlins that could be misdiagnosed as a module failure.

Model Year Variations Within This Range

• 2024-2025: The 2024 model year introduced a mid-cycle refresh for the Wrangler JL. The most significant electronic change was the replacement of the analog instrument cluster with a new digital display and the introduction of the Uconnect 5 infotainment system on a larger 12.3-inch touchscreen. While the PCM's core function remains, these changes to the vehicle's network architecture could present different diagnostic symptoms or require updated procedures in the wiTECH software.