U0594 on 2020-2021 Ram 1500: Invalid Hybrid Data Causes and Fixes

What's Unique About the 2020-2021 Ram 1500

This code is exclusive to Ram 1500 models equipped with the eTorque mild-hybrid system. Unlike a conventional truck, the eTorque system uses a Motor Generator Unit (MGU) in place of a traditional alternator to assist the engine and manage the start-stop system. Code U0594 is a direct result of a communication breakdown or internal failure within this specific hybrid architecture, an issue that does not exist on non-eTorque Rams.

Diagnostic Flowchart

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

Symptoms You May Notice

• Check Engine Light (Malfunction Indicator Lamp) is on
• Vehicle enters limp mode (may not exceed 20 MPH)
• Engine may stall while driving and fail to restart
• Loss of 12V battery charging, leading to a dead battery
• Auto start-stop system may become disabled, often with a 'Service Stop/Start System' message
• Reduced power and weak acceleration
• Low battery light may illuminate
• A distinct whining noise from the engine bay, which may get progressively worse and precede the failure
🎬 Watch: Listen to the specific whining noise of a failing MGU bearing.

Most Likely Causes

1. Faulty Motor Generator Unit (MGU) 🔴 High Probability A known issue addressed by Ram TSB 08-014-21 for 2020-2021 models with the 5.7L eTorque engine built between June 11, 2020, and November 14, 2020. The MGU can suffer from internal bearing failure or electronic faults, causing it to send implausible data. The issue is prevalent enough that the part is often on backorder.
   How to confirm: A dealership can check if the vehicle's VIN is covered under TSB 08-014-21. Diagnosis requires a wiTECH scan tool to confirm the code and related symptoms. Owners should also insist on checking coverage under the 8-year/80,000-mile Federal Emissions Warranty, which lists the 'Belt Driven Hybrid Starter Generator' (MGU).
   Typical fix: Replace the Motor Generator Unit (MGU). This repair is covered under the vehicle's basic warranty, powertrain warranty, or the Federal Emissions Warranty if applicable.
   Est. part cost: $1500-$2400
2. Software Glitch / Corrupted Firmware 🟡 Medium Probability Complex vehicle networks are susceptible to software conflicts. A failed update or random glitch can cause modules like the HCP or BPCM to send invalid data. Ram has issued other TSBs, like 08-032-24 for the 2021 model's BPCM, to correct software-related DTCs in the eTorque system.
   How to confirm: A dealer or qualified shop uses a scan tool to check the current software version of the Hybrid Control Processor (HCP) and Battery Pack Control Module (BPCM) against the latest versions available from the manufacturer.
   Typical fix: Reprogram or 'flash' the affected module(s) with the latest software. This is a dealer-only procedure.
   Est. part cost: $0
3. Wiring Harness or Connector Issue ⚪ Low Probability Vibrations and exposure to the elements can cause issues with the complex wiring harnesses connecting the MGU, BPCM, and PCM. An earlier customer satisfaction campaign (V87) for 2019 models involved an MGU flex cable not being properly latched to the circuit board, showing a precedent for connection issues.
   How to confirm: Visually inspect the wiring and connectors between the Powertrain Control Module (PCM), the Battery Pack Control Module (BPCM), and the Motor Generator Unit (MGU) for any signs of damage, corrosion, or loose pins.
   Typical fix: Repair the damaged section of the wiring harness or clean/reseat the affected connector.
   Est. part cost: $20-$200

Rare But Worth Checking

• Failed Hybrid Control Processor (HCP): While less common than an MGU failure or software issue, the HCP (also called the HPCM) itself can fail internally, causing it to send out corrupt data.

Diagnosis Steps

1. Check for any applicable Technical Service Bulletins (TSBs), especially TSB 08-014-21, by providing your VIN to a Ram dealership. Note the specific build dates of June 11, 2020, to November 14, 2020.
2. Use a dealer-level scan tool (like wiTECH) to confirm the presence of U0594 and check for any other accompanying DTCs in all vehicle modules.
3. Listen for a persistent whining noise from the front of the engine, which is a strong indicator of a failing MGU bearing.
4. Verify warranty coverage. Insist the service advisor checks the 8-year/80,000-mile Federal Emissions Warranty, which covers the 'Belt Driven Hybrid Starter Generator Assembly' (MGU), as this is often overlooked.
5. If TSB 08-014-21 applies, follow the repair procedure which involves replacing the Motor Generator Unit (MGU).
6. If no TSB applies, inspect the wiring harness and connectors between the MGU, the Battery Pack Control Module (BPCM), and the Powertrain Control Module (PCM) for any visible damage, corrosion, or loose connections.
7. Check the software versions of the relevant control modules (HCP, BPCM, PCM) and perform a software update or 'flash' if a newer version is available, referencing TSBs like 08-032-24 as a procedural example.
8. If the MGU is found to be faulty and replaced, also test the 12V AGM battery. It is frequently damaged from being deeply discharged and may also need replacement.

Parts You'll Likely Need

• Motor Generator Unit (MGU) (OEM #68438054AB, superseded by 68623194AC) — This is the most common cause of U0594 on these trucks, as identified by Ram's own Technical Service Bulletin 08-014-21.
  Trusted brands: Mopar
  OEM price range: $1500-$2400
  Aftermarket price range: N/A
• 12V AGM Battery — Collateral damage. The failed MGU can no longer charge the 12V system, causing the battery to be completely drained. This deep discharge often permanently damages the battery, requiring replacement. Owners report this is typically not covered by the MGU warranty repair.
  Trusted brands: Mopar, Interstate, Odyssey
  OEM price range: $300-$400
  Aftermarket price range: $200-$350

Related Codes That Often Appear With This One

• P1B03-00 — Resolver Signal / MCPA Rationality - This code is listed alongside U0594 in TSB 08-014-21 and relates to the MGU's internal position sensors.
• P0A3F-00 — Drive Motor A Position Sensor 1 Circuit - Also listed in TSB 08-014-21, this points directly to a fault within the MGU.
• P0607-00 — ECU Internal Performance - This code can be triggered when a critical control module like the HCP receives implausible data from a subsystem.
• P0A92-00 — Hybrid Generator Performance - This code, also from TSB 08-014-21, indicates a problem with the MGU's ability to generate power.
• P1C64-00 — Hybrid Perf - Engine Speed Below Target During Autostart - Also listed in TSB 08-014-21, indicating a failure of the MGU to perform its engine starting function correctly.

Technical Service Bulletins (TSBs) & Recalls

• 08-014-21 (also RRT 21-019): Addresses MIL illumination with DTC U0594 (and others) due to a faulty Motor Generator Unit (MGU) on specific 2020-2021 Ram 1500 eTorque models (build dates June 11, 2020 - Nov 14, 2020), instructing dealers to replace the MGU.
• 08-032-24: While not for U0594, this TSB for 2021 models involves updating the Battery Pack Control Module (BPCM) software to fix other electrical DTCs, demonstrating a pattern of software fixes for the eTorque control system.

Platform-Specific Known Issues

• TSB 08-014-21 was issued for 2020-2021 Ram 1500s with the 5.7L eTorque engine, specifically for vehicles built between June 11, 2020 (MDH 0611XX) and November 14, 2020 (MDH 1114XX), identifying a faulty MGU as the cause for this code.
• Many owners report significant delays and backorders for the replacement MGU part, sometimes leaving the truck at the dealership for weeks or months.
• There is widespread confusion at the dealership level regarding warranty coverage. Many owners have had to proactively inform service advisors about the 8-year/80,000-mile Federal Emissions Warranty that covers the MGU to avoid being incorrectly charged for the repair.

Mechanic-Grade Diagnostic Values

• MGU Mounting Bolt Torque — expected: Upper M10 bolts: 55 N·m (41 Ft. Lbs.). Lower/Side M8 bolts: 25 N·m (18 Ft. Lbs.).. Failure: Improper torque can lead to misalignment or vibration.
• Battery Voltage During Module Flashing — expected: Maintain between 13.2 and 13.5 volts.. Failure: Voltage dropping too low or climbing too high can interrupt or corrupt the software flash.

Scan Tool Commands That Help

• wiTECH: Flash ECU / Reprogram Module — Required after installing a new MGU to load vehicle-specific software and calibrations. Also used to update existing modules (PCM, BPCM, HCP) to resolve software-related faults.
• wiTECH: MGU Programming — After physical replacement of the MGU, the new unit must be programmed. The technician will need to know the vehicle's axle ratio to complete the programming.
• wiTECH: Clear All DTCs — This function is automatically presented after a successful module flash and should be run to clear any codes set in other modules during the reprogramming process.

Wiring & Ground Locations

• Cabin Ground Points — Behind trim in the driver's side instrument panel and between the front seats or under the front row center seat section.. A poor ground for any of the network modules can introduce electrical noise, corrupting CAN bus communication and potentially triggering data-related codes like U0594.
• Engine Bay Ground Stud — A dedicated ground stud is often located in the engine bay for jump-starting or charging, separate from the battery negative terminal. On some models, a ground strap location is on the passenger side wheel well near the alternator.. Verifying main chassis and engine grounds is a key step in diagnosing any network communication issue, as the MGU, PCM, and BPCM rely on solid grounds to communicate reliably.
• Rear Frame Ground — A ground terminal is located at the rear of the vehicle on the frame, on the driver's side just rearward of the stabilizer link mount.. This ground is critical for components located at the rear of the vehicle, including the Battery Pack Control Module (BPCM) which is part of the eTorque system.

Real Owner Repair Stories

• RamForum.com user (Ram 1500 eTorque (year not specified)) — Bad bearing in the MGU, likely causing a whining noise and eventual failure codes.
  ❌ Tried (didn't work) Waiting for a backordered MGU from the dealership for warranty work.
  ✅ What actually fixed it The owner disassembled the MGU and replaced the faulty internal bearing themselves. They found a replacement bearing at NAPA for $42 and performed the labor in a few hours, resolving the issue without replacing the entire MGU.

OEM Part Supersession History

• 68438054AB / AC / AD / AE / AF → 68623194AA / AB / AC — Revisions are common to address failure points, improve durability, or change suppliers. The progression from the 68438 series to the 68623 series likely indicates a significant design or component update to address the widespread failures.
  Heads up: While later revisions are intended as direct replacements, always confirm the latest part number with a dealer using the vehicle's VIN. Using an older, non-superseded part number from new-old-stock is not recommended.

Model Year Variations Within This Range

• 2019: A known issue for this year was an internal MGU flex cable not being properly latched to the circuit board, addressed by Customer Satisfaction Notification V87. This is a specific manufacturing defect.
• 2020 (specifically builds from June 11 to Nov 14): These vehicles are covered by TSB 08-014-21, which points to a high rate of MGU failure (likely due to internal bearing or electronics issues) and instructs dealers to replace the entire unit.