U0401 on 2015-2019 GMC Sierra 3500: Causes and Fixes for Invalid ECM Data

What's Unique About the 2015-2019 Gmc SIERRA 3500

For this generation of GMC Sierra HD trucks (K2XX platform), a primary suspect for network communication codes like U0401 is physical wiring damage. GM has issued Technical Service Bulletin #PIT5457D, which specifically points to the wiring harness routed under the driver and passenger sill plates. This harness, particularly the Communication Enable Circuit 5986 (often a pink wire), is known to be susceptible to corrosion and damage from water and road salt that can get trapped in the wiring channel. This single circuit failure can cause a loss of communication or corrupted data between essential modules, leading to a cluster of U-codes.

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 illuminated
• "Power Steering Assist Reduced" or "Service Power Steering" message on the Driver Information Center (DIC)
• Loss of power steering assist
• ABS and/or Stabilitrak warning lights on
• "Service Trailer Brake System" message
• Vehicle may enter a reduced power "limp mode"
• Harsh or delayed transmission shifting
• Engine may stall or have trouble starting
• Backup camera inoperative

Most Likely Causes

1. Damaged Wiring Harness Under Sill Plates 🔴 High Probability As documented in TSB #PIT5457D, the wiring harness routed along the floor under the sill plates is a known weak point. Water intrusion and road salt cause corrosion and opens in the Communication Enable Circuit (Circuit 5986, typically a pink wire), which is needed to 'wake up' various modules.
   How to confirm: Remove the driver and passenger side sill plates and peel back the carpet to physically inspect the wiring harness for any signs of chafing, corrosion (often green crust), or broken wires. Pay close attention to the channel where water can sit. A voltmeter should show ~12V on Circuit 5986 when the ignition is on.
   Typical fix: Repair the damaged section of Circuit 5986. This involves cutting out the corroded/damaged wire, splicing in a new section with sealed, waterproof butt connectors, and protecting the harness with loom or high-quality electrical tape. Ensure the channel is dry before reassembly.
   Est. part cost: $10-$50
2. Low or Unstable Battery Voltage 🟡 Medium Probability → Shop Vehicle Battery Modern control modules are sensitive to voltage drops. A weak battery, failing alternator, or poor cable connections can cause modules to boot improperly or send corrupt data during startup or under load, triggering a U0401 code.
   How to confirm: Test the battery with the engine off; it should read at least 12.4 volts. With the engine running, check the alternator output, which should be between 13.7 and 14.7 volts. Perform a load test on the battery to confirm its health. Check battery cable ends for corrosion or looseness.
   Typical fix: Replace the failing battery and/or alternator. Clean or replace corroded battery terminals and cables.
   Est. part cost: $150-$600
3. Faulty Engine Control Module (ECM) ⚪ Low Probability → Shop Engine Control Module (ECM)
   How to confirm: This is typically a last resort after all wiring and power supply issues have been ruled out. A professional shop can use an advanced scan tool to check for internal ECM faults. If the ECM does not respond to the scan tool at all, and its power and ground circuits are confirmed to be good, the module is likely faulty.
   Typical fix: Replace and reprogram the Engine Control Module. This must be done by a dealer or a qualified shop with programming capabilities like GM's TIS2WEB software.
   Est. part cost: $800-$2000

Rare But Worth Checking

• Corroded or Loose Ground Connections: Control modules require a solid ground to function. A loose or corroded ground strap can introduce electrical noise and communication errors. TSB #PIT5405C points to the BCM ground G218, located under the driver's side dash, as a potential cause for various 🎬 See how to locate and fix the G218 ground issue communication issues on this platform.
• Faulty Receiving Module: Less commonly, the module setting the code (e.g., the Power Steering Control Module) may have an internal fault causing it to misinterpret good data from the ECM. GM has issued TSB #20-NA-100 for a PSCM software update to resolve erroneous codes that stay current instead of moving to history.
• Aftermarket Electronics or Tuners: Poorly installed remote starters, alarms, or performance engine tuners can interfere with the CAN bus network by sending unexpected data, leading to communication codes like U0401.

Diagnosis Steps

1. Scan all vehicle modules for DTCs. Note which module is setting the U0401 code and any other communication (U-prefix) codes that are present. This helps identify the scope of the problem.
2. Check battery voltage and charging system operation to rule out power supply issues. A healthy, fully charged battery is crucial.
🎬 Watch: Troubleshooting flickering displays and electrical shut-off issues
3. Following GM TSB #PIT5457D, remove the driver and passenger sill plates to gain access to the main wiring harness in the floor channel.
4. Carefully inspect the entire length of the exposed harness for signs of damage, chafing, corrosion (especially green powder on copper wires), or previous improper repairs. Pay special attention to the Communication Enable Circuit 5986 (often pink).
5. If no visible damage is found, perform a load test on Circuit 5986. With the ignition on, check for ~12V. Then, connect a small bulb (like a 194) between the circuit and a known good ground. The bulb should light brightly. If it's dim or doesn't light, there is high resistance or an open in the circuit that needs to be traced and repaired.
6. If damage is found, repair the affected wires using solder and heat shrink or high-quality sealed butt connectors. Wrap the repaired section in protective loom.
7. Inspect critical ground connections, particularly G218 under the driver's side dash, for cleanliness and tightness.
8. Clear all DTCs and perform a test drive, cycling the ignition several times, to see if the code returns.
9. If the code persists after confirming the wiring and grounds are intact, the issue may be with the ECM itself, which requires professional diagnosis and likely replacement/reprogramming.

Parts You'll Likely Need

• Wiring Repair Supplies — To fix the most common cause: a damaged wiring harness under the sill plates. Includes wire, sealed butt connectors, and Tesa tape/loom.
  Trusted brands: 3M, Tesa
  OEM price range: $20-$50
  Aftermarket price range: $10-$30
• Engine Control Module (ECM) (OEM #12692201, 12720006 (Examples, verify with VIN)) — Needed only if the ECM is confirmed to be the source of the invalid data after all wiring issues are ruled out.
  Trusted brands: ACDelco
  OEM price range: $800-$1500
  Aftermarket price range: $500-$1000

Related Codes That Often Appear With This One

• U0077 — Indicates a general communication bus failure, often linked to the same wiring harness issue on Circuit 5986 mentioned in TSB #PIT5457D.
• U0121 — Loss of Communication with Anti-Lock Brake System (ABS) Control Module. Often appears with U0401 when the EBCM is affected by the network fault. [Bulletin #PIT5457D]
• U0131 — Loss of Communication with Power Steering Control Module (PSCM). This frequently accompanies U0401 when power steering is affected. [Bulletin #PIT5457D]
• U0415 — Invalid Data Received From Anti-Lock Brake System Control Module. This indicates the problem may be bidirectional or involve multiple modules on the same damaged circuit. [Bulletin #PIT5457D]
• U0428 — Invalid Data Received From Steering Angle Sensor Module. This is another code pointing to a fault in the steering system's communication network. [Bulletin #PIT5457D]

Technical Service Bulletins (TSBs) & Recalls

• PIT5457D: Mentions that codes U0077, U0121, U0126, U0131, U0139, U0401, U0415, and/or U0428 can be caused by an open or damaged wire harness (specifically Circuit 5986) located under the driver and passenger sill plates.
• 17-NA-158: Notes that a U0401 code can be set in the Power Steering Control Module as a secondary result of an engine coolant temperature fault (P0128), which could lead to misdiagnosis.
• PIT5405C: Points to a poor BCM ground at G218 as a potential cause for a multitude of communication DTCs and electrical symptoms.

Platform-Specific Known Issues

• A GM Technical Service Bulletin (#PIT5457D) directly addresses loss of communication codes, including U0401, on these trucks. It specifically calls out checking the Communication Enable Circuit (5986) in the wiring harness under the driver and passenger sill plates for damage as a primary diagnostic step.

Mechanic-Grade Diagnostic Values

• High-Speed GMLAN Bus Resistance — expected: ~60 Ohms (measured between Pin 6 and Pin 14 of the DLC with battery disconnected). Failure: A reading of ~120 Ohms indicates a missing terminating resistor or an open in the bus. A reading near 0 Ohms indicates a short between the CAN High and CAN Low wires.
• High-Speed GMLAN Bus Voltage (Key On, Engine Off) — expected: CAN High (Pin 6 to Ground): ~2.6V. CAN Low (Pin 14 to Ground): ~2.3V.. Failure: Voltages stuck high (5V or 12V) or low (0V), or that do not mirror each other, indicate a short to power/ground or a faulty module transceiver.
• Communication Enable Circuit 5986 Voltage (Key On) — expected: ~12V (Battery Voltage). Failure: Low or no voltage indicates an open or high resistance in the circuit, often due to corrosion under the sill plates. A load test with a small bulb will be dim or off.

Hidden / Shadow Codes Worth Checking

• U0401 5A: The '5A' is a symptom byte indicating 'Invalid/Irrational Data'. This provides more specific information that the module is receiving data that doesn't make logical sense, as opposed to no data at all. (see via A professional GM-specific scan tool like a Tech2 or software like GDS2 is required to view symptom bytes.)

Scan Tool Commands That Help

• GDS2 (GM Global Diagnostic System 2): Module Programming & Setup — This is a required procedure after replacing a major control module like the ECM, BCM, PSCM, or EBCM. It loads the correct VIN-specific software and performs necessary security and operational relearns.
• GDS2 (GM Global Diagnostic System 2): Clear All DTCs — As per TSB #20-NA-100, a software error in the PSCM can cause DTCs to remain 'current' even after the fault is fixed. Clearing all codes after a repair and letting the vehicle 'sleep' is a critical step to verify the fix. If codes do not return, a PSCM reprogram is needed to correct the software bug.

Wiring & Ground Locations

• G218 — Under the driver's side dash, on the left instrument panel support bracket, often near the A-pillar behind the kick panel.. This is a primary ground point for the Body Control Module (BCM) and other interior modules. TSB #PIT5405C notes that sound-deadening material can get trapped under this ground, causing a poor connection and a wide range of communication faults.
• Sill Plate Harness Channel — Under the plastic sill plates on both the driver and passenger sides, running from the kick panel area towards the B-pillar.. This is the most common failure location. Water and road salt collect in this channel, corroding critical communication wires like the Communication Enable Circuit (5986), causing codes U0401, U0121, U0131, etc. This is documented in TSB #PIT5457D.
• Engine Harness Chafe Points — Multiple TSBs (#21-NA-149, N192219370) identify chafe points where the engine harness can rub against the ECM/TCM bracket, the upper control arm, and the shock tower bolt in the driver's side wheel well.. While less common than sill plate corrosion for this specific code, a chafed harness can short CAN bus wires to ground or each other, causing a flood of communication DTCs, including U0401.

Real Owner Repair Stories

• gm-trucks.com forum user (2016 GMC Sierra) — U0401, U0131, "Service Power Steering" message on the DIC, loss of power steering.
  ❌ Tried (didn't work) Replacing the battery.
  ✅ What actually fixed it The final fix was repairing a corroded pink wire (Communication Enable Circuit 5986) found under the driver's side sill plate, exactly as described in TSB #PIT5457D.
• YouTube video by 'G218 Ground Issue Fix' (2014-2018 GMC Sierra/Chevrolet Silverado) — Sporadic electrical issues, warning lights on the dash, radio/infotainment glitches.
  ❌ Tried (didn't work) Initial diagnosis was confusing due to the intermittent nature of the problem.
  ✅ What actually fixed it The problem was the G218 ground under the driver's side dash. Sound insulation material was trapped under the ground nut, preventing a solid connection. The fix was to remove the nut, trim away the insulation, and retighten the ground connection.

Model Year Variations Within This Range

• 2015-2019: While the core architecture and the common sill plate wiring issue are consistent across this range, GM released several software updates for various modules (PSCM, BCM, ECM) over the years. A 2019 model may have a more robust PSCM calibration from the factory than a 2015 model, making it slightly less susceptible to setting erroneous codes due to transient voltage drops or minor network glitches.