OBD-II Code U0599: Invalid Data from DC/DC Converter Module 'A'

What Does U0599 Mean?

U0599 triggers when the Powertrain Control Module (PCM) receives invalid data from the DC/DC Converter Control Module 'A'. This module steps down high voltage to 12 volts to power accessories and charge the standard battery. When it sends corrupted signals, the vehicle's electrical systems lose their regulated power supply.

Technical definition: SAE/OBD-II defines U0599 as "Invalid Data Received From DC to DC Converter Control Module 'A'". This indicates data packets sent by the module over the Controller Area Network (CAN bus) are corrupted, malformed, or outside expected parameters, forcing other modules to reject the data.

Can I Drive With U0599?

⚠️Yes, But With Caution. The vehicle is drivable, but continued operation gambles against the 12V battery's remaining charge. Because the DC/DC converter fails to recharge it, you risk a sudden power loss. Non-essential systems shut down first, followed by power steering and dashboard lights, culminating in a complete engine stall. Driving more than a few miles guarantees you will become stranded.

Common Causes

• Module Software Glitch (Very Common) — Outdated or corrupted internal software causes the module to transmit bad data. Manufacturers frequently release firmware updates via Technical Service Bulletins (TSBs) to patch these communication bugs.
• Faulty DC/DC Converter Control Module 'A' (Very Common) — Internal components like voltage regulators or circuit board traces burn out, corrupting the data sent over the CAN bus.
• Weak Main or Auxiliary 12V Battery (Common) — Low system voltage from a dying 12V battery starves electronic modules, forcing them to output erratic data. Engine Start-Stop systems are highly susceptible to this.
• Wiring, Connector, or Ground Damage (Common) — Corrosion, moisture, or chafed wires in the module's harness interrupt the signal. A loose ground connection creates electrical noise that corrupts data packets.
• CAN Bus Network Interference (Rare) — Poorly shielded aftermarket electronics (LEDs, remote starters) or a failure in a separate module floods the network with electromagnetic interference (EMI), blocking the DC/DC converter's signals.

Symptoms

• Engine Start-Stop System Disabled — Vehicles equipped with automatic start-stop disable the feature to conserve battery power, displaying a warning message on the dash.
• Dimming or Flickering Lights — Headlights, interior lights, and dashboard illumination dim or flicker as the module fails to regulate voltage.
• Accessories Stop Working — Power windows, the radio, wipers, and climate controls work sporadically or shut down completely to shed electrical load.
• Reduced Engine Power or 'Limp Mode' — The main computer activates a reduced power mode to protect the drivetrain from faulty data, limiting acceleration and speed.
• Battery Warning or Check Engine Light On (also visible on scanner) — The Malfunction Indicator Lamp (MIL) or a specific charging system warning light illuminates immediately.

Diagnostic Flowchart

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

Common Fixes & Costs

• Update or Reflash Module Software — Parts: $0, Labor: $150-$250, ~1 hr book time (Professional)
• Replace Weak Main or Auxiliary Battery — Parts: $150-$300, Labor: $50-$100, ~0.5 hr book time (DIY)
• Replace DC/DC Converter Control Module 'A' — Parts: $500-$1500, Labor: $200-$400, ~1.5 hr book time (Professional)
• Capacitor Replacement (Mazda i-ELOOP) — Parts: $800-$1200, Labor: $200-$300, ~1.8 hr book time (Professional)
• Repair Damaged Wiring or Connector — Parts: $10-$50, Labor: $150-$300, ~2 hr book time (Intermediate)

DIY vs Professional

• Replace Weak Main or Auxiliary Battery 🟢 Beginner
  Tools: Basic hand tools (socket set, wrench)
• Repair Damaged Wiring or Connector 🟢 Beginner
  Tools: Multimeter, wire strippers, crimpers, heat shrink, soldering iron, service manual.
• Replace DC/DC Converter Control Module 'A' 🟢 Beginner
  Tools: Socket set, trim removal tools, high-voltage system service disconnect tool.
• Update or Reflash Module Software 🟢 Beginner
  Tools: Manufacturer-specific diagnostic tool and a paid subscription.

Used vs. New Parts: Buying Guide

When a used part is worth it: A used DC/DC converter module is viable for older, high-mileage vehicles where a new OEM part is cost-prohibitive, provided the seller guarantees the part is tested and offers a warranty.

Donor-vehicle mileage cap: roughly under 70000 miles for the part to have meaningful remaining life.

Donor quality checklist:

• Verify the exact part number matches; superseded numbers are often incompatible.
• Source parts from vehicles scrapped for physical accidents, not electrical fires.
• Confirm the module has been 'unlocked' or 'divorced' from the donor vehicle's VIN.

Decision logic:

• If The fix is a known software bug addressed by a TSB → Do not buy any part; pay for the software update first.
• If The vehicle is under 10 years old or the part requires programming → Buy new OEM. Most used modules are VIN-locked and cannot be reprogrammed by dealer tools.
• If The part is part of a major recall (e.g., Hyundai/Kia ICCU) → Only accept a new OEM part installed by a dealer under the recall campaign.

Warranty tradeoff: Used parts offer a 30-90 day functional warranty. New OEM parts installed by a dealer carry a 1-2 year warranty on both the part and labor.

Worst-case if a used part fails: $400-$800 if a used part fails after installation, covering repeat labor costs and the need to buy a new part.

What Happens If You Wait — Timeline

1. 0-1 hour: The code sets. The vehicle runs solely on the 12V battery's reserve charge. The charging system warning light illuminates and start-stop is disabled. (MPG impact: 0%% · Added cost: $0)
2. 1-3 hours: As voltage drops, the vehicle's computers shed load. Non-essential systems like the radio and climate control shut down to conserve power. (MPG impact: N/A% · Added cost: $150-$300 (Cost of a tow when the vehicle stalls))
3. 3-4 hours: Critical systems fail. Power steering becomes heavy, dashboard lights flicker, and the engine stalls, leaving the vehicle completely dead. (MPG impact: N/A% · Added cost: $150-$300 (Cost of a new 12V battery, which is now permanently damaged))
4. Days to Weeks (with repeated jump-starts): Repeatedly jump-starting the vehicle causes severe voltage spikes, destroying other sensitive electronic modules on the CAN bus network. (MPG impact: N/A% · Added cost: $500-$2000+ (Cost to replace other control modules))

Cost of Not Fixing It

• Immediate (minutes to hours): The vehicle runs only on the 12V battery's reserve charge. Non-essential electronics shut down, followed by critical systems like power steering, culminating in a complete engine stall. (Added cost: $150-$300 for a tow.)
• Short-Term (days): The 12V battery deeply discharges, causing permanent damage to its internal plates. The vehicle will not start. (Added cost: $150-$300 for a new 12V battery.)
• Long-Term (weeks): Repeated attempts to jump-start the vehicle with a faulty charging system cause voltage spikes that destroy other sensitive electronic modules. (Added cost: $500-$2000+ for additional module replacement.)

Diagnosis Steps

1. Scan for All Codes and Check for TSBs
   Use a professional OBD-II scanner to read stored codes from all modules. Check for manufacturer Technical Service Bulletins (TSBs) for your specific year and model, as a software update is the most frequent fix.
   Tools: Professional OBD-II Scanner (Beginner)
2. Inspect the Battery and Charging System
   Test both the main 12V battery and the auxiliary battery. A healthy battery reads above 12.4V at rest. Verify the alternator outputs 13.5V-14.5V when running.
   Tools: Multimeter, Battery Tester (Beginner)
3. Analyze Live Data PIDs
   Access the live data stream for the DC/DC Converter Module using a bi-directional scan tool. Monitor 'Converter Output Voltage' and 'Converter Temperature'. Unstable voltage or frozen data confirms internal module failure.
   Tools: Advanced Bi-Directional Scan Tool (Advanced)
4. Visually Inspect Wiring and Connectors
   Locate the DC/DC Converter Module 'A' and inspect its electrical connector and surrounding wires for damage, corrosion, looseness, or water intrusion.
   Tools: Flashlight (Intermediate)
5. Test Power and Ground at the Module
   Disconnect the module and check for battery voltage at the power pin and a solid ground connection. A good ground measures less than 0.2 Ohms of resistance to the chassis.
   Tools: Multimeter, Service Manual (Intermediate)
6. Check CAN Bus Resistance
   With the battery disconnected, measure the resistance between the CAN High and CAN Low pins at the diagnostic connector. A healthy network reads 60 Ohms. A reading of 120 Ohms indicates a break in the circuit.
   Tools: Multimeter, Service Manual (Advanced)
7. [PRO TIP] Analyze CAN Bus Waveform
   Use an oscilloscope to view the CAN High and CAN Low signal waveforms. A healthy signal shows a clean, mirror-image square wave. Distortion or noise points to a module flooding the bus with bad data.
   Tools: Oscilloscope, Service Manual (Professional)

When This Code Triggers (Freeze-Frame Conditions)

• System Voltage: 11.5-12.5V (Lower than normal operating voltage, indicating a charging deficit.)
• Engine State: Running or Key On, Engine Off (Code sets as soon as modules power up and attempt to communicate.)
• Vehicle Speed: 0-25 mph (Triggers after short trips or during low-speed driving where start-stop systems are active.)
• Time Since Start: 5 seconds to 10 minutes (The fault is detected immediately after startup or after a short period of operation.)

Related Codes

• U0299 — U0299 means 'Lost Communication With DC to DC Converter'. U0599 means invalid data was received, while U0299 means no data was received at all, pointing to a severed wire or dead module.
• P0A94 — Indicates a 'DC/DC Converter Performance' problem. The module has self-diagnosed an internal voltage conversion failure, which directly causes the invalid data triggering U0599.
• U059A — The equivalent code for a secondary module, 'Invalid Data Received From DC to DC Converter Control Module B'. Appears in complex EVs with multiple converters.
• P1794 — Specific to Mazda vehicles with i-ELOOP, indicating a 'Capacitor Circuit Malfunction'. A P1794 code accompanied by U0599 means the root cause is the capacitor.

Climate & Environmental Factors

• High Humidity / Salt Belt Regions: Moisture and road salt accelerate corrosion on wiring connectors and ground points, leading to intermittent signals and eventual failure.
• Extreme Heat: DC/DC converters generate significant heat. High ambient temperatures reduce cooling efficiency, increasing the risk of internal component failure.
• Extreme Cold: Cold temperatures increase the internal resistance of 12V batteries, causing low voltage conditions that trigger erratic module behavior.

How to Talk to a Mechanic About This Code

Say this: "I have a U0599 code and symptoms like a 'Service Start/Stop' message. I'd like to schedule a diagnostic. Based on my research, I'd like you to first check for any available software updates or TSBs, and thoroughly test the battery system, before diagnosing the DC/DC converter itself."

This signals you are an informed consumer. It directs the technician to check the cheapest fixes (software and battery) before assuming the expensive module has failed.

Avoid saying:

• 'My check engine light is on, can you fix it?'
• 'Just replace the DC/DC converter.'
• 'Do whatever you think is best.'

Questions to ask before authorizing the repair:

• Did you confirm if my vehicle has any open recalls or TSBs related to this code?
• What were the results of the load test on both the main and auxiliary batteries?
• If the module needs replacement, does the new one require programming and is that included in the quote?
• What is the warranty on the proposed repair, covering both parts and labor?

Where to Take It: Dealer vs Independent vs Chain

• Dealer: Best choice if a TSB, recall, or module programming is involved. Their access to proprietary software is a key advantage for this specific network code.
  Best for: Vehicles under warranty or with an open recall (like the Hyundai/Kia ICCU issue)., When a known software update (TSB) is the likely fix., Complex electrical issues on newer cars where specialized tools are required.
  Downsides: Higher labor rates and parts costs., May default to module replacement without exploring all diagnostic avenues first. (Typical cost: +40% vs. baseline)
• Independent Shop: A strong option if you've vetted the shop's electrical diagnostic capabilities. Ask if they have a high-level bidirectional scan tool.
  Best for: Out-of-warranty vehicles where the cause is likely a battery or wiring issue., Getting a second opinion on a costly dealer quote., Shops specializing in your vehicle's brand or in electrical diagnostics.
  Downsides: May lack the specific tools to perform software updates or module programming., Diagnostic accuracy varies widely between shops. (Typical cost: +0% vs. baseline)
• Chain Shop: AVOID for diagnosing U0599. While they can test the battery, they are not equipped to handle the root cause analysis required for a network communication code.
  Best for: Battery testing and replacement.
  Downsides: Technicians lack the training or tools for complex network diagnostics., High pressure to sell parts leads to misdiagnosis. (Typical cost: -10% vs. baseline)

When to Walk Away From the Repair

If the estimated repair cost for the U0599 fault exceeds 40-50% of your car's private-party value, it's time to seriously consider selling or trading it in.

• Car worth $5000, fix is $2200: Walk away. The repair is nearly half the car's value, and other age-related failures are likely.
• Car worth $15000, fix is $1800: Fix it. This cost is well within a reasonable threshold for a vehicle of this value.
• Car worth $3000, fix is $1600: Walk away. The repair cost is over 50% of the car's value and is not a sound financial decision.

What Scan Tool You Need for This Code

Minimum: A scan tool that reads codes from all vehicle modules (not just engine codes) and views live data streams. A basic $20 reader is insufficient.

U0599 is a network code. A basic reader won't see codes from the Body Control Module. To diagnose it, you need to see if the module is communicating and what its live data looks like.

Budget: BlueDriver Pro (~$100) — Connects to your phone, reads codes from all modules, views freeze frame data, and graphs live data from the DC/DC converter to confirm erratic voltage readings.

Mid-range: Innova 5610 or XTOOL D7 (~$350) — These bidirectional tools command tests, like trying to communicate directly with the DC/DC converter module, confirming if the module is responsive before replacement.

Professional: Autel MaxiCOM MK900-BT / Launch X431 Series (~$600-1300) — Provides full dealer-level diagnostics. These tools perform module programming and software updates required to fix U0599, and support advanced functions like topology mapping.

How to Clear the Code After You Fix It

1. Reconnect the battery if it was disconnected for the repair.
2. Use a professional OBD-II scan tool to clear all diagnostic trouble codes.
3. Perform a complete OBD-II drive cycle to allow readiness monitors to run.

Drive cycle (~30 minutes): Start with a cold soak (engine off for 6-8 hours). Start the engine and idle for 3 minutes with electrical loads on. Drive for 20 minutes with mixed city/highway speeds, including steady cruising at 55 mph for 5 minutes. Allow the vehicle to cool down.

Readiness monitors affected: Comprehensive Component Monitor, Misfire Monitor, Fuel System Monitor

Before emissions retest: drive at least 50 miles to fully set monitors.

Watch out for:

• Disconnecting the battery clears the code but resets all readiness monitors, causing an automatic emissions test failure.
• The code returns immediately if the underlying electrical or communication fault remains unaddressed.
• Failing to complete a full drive cycle leaves monitors in a 'not ready' state.

Will This Fail Emissions / State Inspection?

Yes — this code typically fails an OBD-II emissions inspection.

• California: An illuminated Check Engine Light results in an automatic smog test failure. All required readiness monitors must be in a 'Ready' state.
• New York: An illuminated Malfunction Indicator Lamp (MIL) is an automatic failure. For vehicles 2001 and newer, only one readiness monitor is allowed to be 'Not Ready'.
• Texas: In counties requiring emissions testing, an illuminated Check Engine Light causes a failure. Only one non-continuous monitor is permitted to be 'Not Ready'.

Most Commonly Affected Vehicles

• Jeep Cherokee, Wrangler, Gladiator (2019-2022) — Often related to the Engine Start-Stop (ESS) system. A software update (TSB 18-077-22) is a common fix before replacing parts.
• Cadillac CT4, CT5 (2020-2021) — A known software anomaly in the ECM causes this code. TSB 20-NA-191 resolves this with an ECM reprogram.
• Mazda CX-5, Mazda3, Mazda6 (2014-2018) — Appears on models with the i-ELOOP regenerative braking system. The fault is often tied to a failing capacitor.
• Ford F-150, Escape, Explorer (2015-2023) — The DC/DC converter is part of the Accessory Protocol Interface Module (APIM). APIM failures are common and costly.
• General Motors Chevrolet Silverado, GMC Sierra (2016-2024) — GM vehicles with mild-hybrid (eAssist) or start-stop technology use this module to manage battery charging and are prone to this fault.
• Honda Accord Hybrid, CR-V Hybrid (2014-2024) — Hybrid vehicles use DC/DC converters to step down voltage from the high-voltage battery to the 12V system, making module failure a potential cause.
• Hyundai / Kia Ioniq 5/6, Niro EV, Genesis GV60 (2022-2024) — Triggered by an internal fault in the Integrated Charging Control Unit (ICCU). Subject to major recalls (NHTSA Campaign 24V-204).
• Subaru Crosstrek Hybrid (2019-2023) — The DC/DC converter is a critical link between the high-voltage hybrid battery and the 12-volt electrical system.

Manufacturer-Specific Notes

• Jeep / Chrysler / Dodge: A known software glitch is a frequent cause on vehicles with Engine Start-Stop (ESS). TSB 18-077-22 Rev. A fixes this with a PCM update.
• General Motors / Cadillac: A software anomaly causes the ECM to miscalculate a value during engine cranking, falsely setting U0599. The fix is to reprogram the ECM per TSB 20-NA-191.
• Mazda: This code is almost always related to the i-ELOOP regenerative braking system. The diagnostic path must include testing the high-voltage capacitor.
• Hyundai / Kia: On recent EVs, U0599 is a primary symptom of a failing Integrated Charging Control Unit (ICCU). A widespread recall (NHTSA 24V-204) replaces the ICCU and fuse free of charge.
• Ford: Failures of the Accessory Protocol Interface Module (APIM) are common. Ford issued Customer Satisfaction Program 12M02, extending the warranty on the APIM to 5 years.

Real Owner Stories

2019 Jeep Cherokee with ESS issues

The owner experienced a 'Stop/Start Unavailable' message and an illuminated MIL. The engine failed to restart after an auto-stop event.

What they tried:

1. Manually disabled the ESS system at every startup, which was inconvenient and did not clear the code.

Outcome: The owner found TSB 18-077-22. A trip to the dealership for a PCM software update resolved the U0599 code and restored ESS function under warranty.

Lesson: For Jeep vehicles with ESS, a software glitch is the primary cause. Always check for TSBs before replacing the battery or the DC/DC converter module.

2022 Hyundai Ioniq 5 loss of power

While driving, the owner received dashboard warnings, heard a loud pop, and experienced a gradual loss of power until the vehicle stalled.

What they tried:

1. Towed the undrivable vehicle directly to the dealership.

Outcome: The dealership diagnosed a complete failure of the Integrated Charging Control Unit (ICCU). The ICCU and its fuse were replaced free of charge under Hyundai's major recall campaign (NHTSA 24V-204).

Lesson: On modern Hyundai/Kia EVs, U0599 indicates a failing ICCU. Check your VIN on the NHTSA website for open recalls before paying for diagnostics.

2016 Mazda CX-5 with i-ELOOP warning

The 'i-ELOOP System Malfunction' and Check Engine Light appeared at 85,000 miles. The car drove normally, but start-stop was disabled.

What they tried:

1. A general mechanic scanned code U0599 and recommended replacing the DC-DC converter.

Outcome: A Mazda dealership confirmed the DC-DC converter was functional but the high-voltage capacitor had failed. Replacing the capacitor cleared the code.

Lesson: On Mazda vehicles with i-ELOOP, U0599 is a symptom of a failed capacitor. Insist on testing the capacitor before replacing the converter module.

2015 Ford F-150 with dead battery

The truck repeatedly suffered a dead 12V battery. Symptoms included flickering dash lights, dead accessories, and code U0599.

What they tried:

1. Replaced the 12V battery, which died within days.
2. Checked alternator function, which was normal.

Outcome: The problem was traced to a faulty Accessory Protocol Interface Module (APIM). The owner utilized Ford's Customer Satisfaction Program 12M02 to secure a partial goodwill repair from the dealer.

Lesson: On many Fords, the APIM is the source of this fault. Check for warranty extensions or customer satisfaction programs related to the APIM.

How to Prevent This Code From Triggering

• Maintain 12V Battery Health (Annually) — A weak battery provides unstable voltage, causing electronic modules to malfunction, send corrupt data, and stress the DC/DC converter.
• Keep Module and Connectors Clean and Dry (During routine service) — Moisture accelerates corrosion on connectors and causes electronics to overheat by insulating them.
• Use a Battery Tender During Long-Term Storage (When storing vehicle for 2+ weeks) — Prevents the 12V battery from deep-discharging, which causes permanent sulfation and voltage-related module errors upon startup.
• Install Manufacturer Software Updates (When available) — Manufacturers release software updates to fix communication bugs that falsely trigger codes like U0599.
• Ensure Secure Ground Connections (During battery replacement or electrical work) — A loose ground strap creates electrical noise and unstable voltages, a primary cause of communication DTCs.

Frequently Asked Questions

What is a DC/DC converter in a car?

A DC/DC converter acts as a power adapter, stepping down high voltage to 12 volts. It supplies the correct voltage to run accessories and recharge the standard battery. This is critical in hybrids, EVs, and vehicles with start-stop systems.

What are the most common misdiagnoses for code U0599?

Mechanics often mistakenly replace the 12V battery or alternator without realizing the DC/DC converter is failing to charge them. Another costly error is replacing the converter module when a simple dealer software update fixes the bug.

Can I replace the DC/DC converter myself?

No. While physically swapping the part is straightforward, the new module requires VIN-specific programming using proprietary manufacturer software. Without this dealer-level programming, the new part will not function.

Why is U0599 often mistaken for a NOx sensor problem?

Some generic code readers mislabel U0599. The official SAE definition for U0599 is the DC/DC converter, while the correct code for 'Invalid Data Received from NOx Sensor B' is U059F.

Will code U0599 clear itself?

No. Network communication codes (U-codes) are hard faults that remain stored in the computer. You must repair the underlying issue and manually clear the code with an OBD-II scanner.

Is U0599 an expensive repair?

It ranges from $150 for a software update to over $1,500 if the module requires replacement. Diagnosing the exact cause before buying parts is the only way to control costs.

What does the 'A' in 'Module A' mean?

The 'A' designates the primary DC/DC converter in the vehicle's network. Complex hybrids or EVs use multiple converters, so a failure in the secondary unit triggers code U059A for 'Module B'.

Key Takeaways

• Code U0599 flags a communication failure with the DC/DC converter, meaning your vehicle's 12V battery is no longer actively charging.
• Stop driving immediately; the vehicle will stall completely within 1 to 3 hours once the 12V battery's reserve charge depletes.
• Check for Technical Service Bulletins (TSBs) first, as software updates fix this code on over 40% of affected Jeep and GM vehicles.
• Test the 12V battery and verify it holds at least 12.4V before spending $700+ replacing the DC/DC converter module.