Toggle Side Panel

Shopping Cart

No products in the basket.

Shopping Cart

No products in the basket.

Close search
Home / DTC / U0031 — Vehicle Communication Bus A (+) Low

U0031 — Vehicle Communication Bus A (+) Low

Detailed page for trouble code U0031.

34,405codes
59brands
11,914generic
22,491specific
All codes P-codes (Powertrain) B-codes (Body) C-codes (Chassis) U-codes (Network/User) Tesla Alerts
Reset
Code

U0031

Generic U — Network/User

Vehicle Communication Bus A (+) Low

Brand: Generic
Type: U — Network/User
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Damaged or chafed CAN_H wiring (short to ground or open)
  • Corroded/loose connectors or pin pushed out at a module or splice
  • Failed control module actively pulling the bus low
  • Missing or incorrect termination (open or wrong resistance)
  • Aftermarket device or accessory tied into the bus creating a fault
  • Power or ground fault affecting a module driving the bus (module fault)

Symptoms

  • Multiple control modules show communication errors or go offline
  • Instrument cluster, ABS, transmission or engine displays erratic behavior or loss of functions
  • DTCs related to lost communication across several modules (U-codes, U0100, etc.)
  • Vehicle may not start if ECM/PCM loses communication or power
  • Intermittent or permanent warning lights on dash

What to check

  • Retrieve stored U0031 and any other U-codes and freeze frame data with a scan tool
  • Verify battery voltage and charging system (low battery can cause network faults)
  • Visually inspect wire harnesses, connectors, splice points and modules on Bus A for damage, corrosion or water intrusion
  • Check fuses and power/ground circuits for modules on Bus A
  • Measure DC voltage between CAN_H and chassis ground with ignition ON (see signal parameters)
  • Measure DC voltage between CAN_H and CAN_L and measure overall bus resistance with ignition OFF (expected ~60 ohms)

Signal parameters

  • CAN bus recessive state: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (each ± ~0.5 V)
  • CAN dominant state (active): CAN_H ≈ 3.5–4.0 V, CAN_L ≈ 1.0–1.5 V (voltage differential ≈ 2.0 V)
  • U0031 condition: CAN_H voltage lower than expected/recessive (CAN_H significantly below ~2.0–2.2 V) or pulled near ground
  • Bus termination: combined resistance across CAN_H to CAN_L ≈ 60 Ω (two 120 Ω terminators in parallel)

Diagnostic algorithm

  1. Confirm the code: record freeze frame, other U/P codes and reproduce conditions if possible.
  2. Check vehicle battery and main power/grounds to modules; ensure stable supply (12 V) before network diagnosis.
  3. Visually inspect Bus A harness, module connectors, splice/junction boxes and any recent repair or aftermarket device connections for damage or corrosion.
  4. With ignition ON (engine off), measure CAN_H and CAN_L DC voltages at a convenient harness connector near the ECM/ECU and at remote modules. Compare to expected values in signal_params.
  5. Measure resistance across CAN_H to CAN_L with ignition OFF; expect ~60 Ω. If open or very low, note results.
  6. Use an oscilloscope to observe CAN_H/CAN_L waveforms while modules are communicating. Look for flatline, clipped, excessive noise, or dominant-state stuck.

Likely causes

  • Short to ground on CAN_H (most likely)
  • Corroded connector or poor pin contact at a junction or module
  • Failed module on Bus A pulling CAN_H low
  • Incorrect or missing CAN termination resistor(s)

Fault status

⚠️ Status
U0031 — Communication Bus A positive line voltage low (CAN_H below expected range); network communication degraded or lost.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5 - 3.0 hours

Similar codes

U0030 U0073 U0100 U0104 U0077
8,297

The library contains 8,297 repair and diagnostic manuals. Choose a brand to open the full manual tree by year, model and trim.

ALFA ROMEO
89
AUDI
1,160
BMW
479
CHEVROLET
456
DAEWOO
75
FIAT
26
GENESIS
207
HUMMER
138
HYUNDAI
371
ISUZU
86
LAND ROVER
320
LEXUS
280
LINCOLN
166
MERCEDES-BENZ
856
MERCURY
296
MITSUBISHI
406
PORSCHE
136
TESLA
58
TOYOTA
2,042
TRIUMPH
131
VOLKSWAGEN
298
VOLVO
221
Your experience will help others
+100 karma for a short comment :)
Keep it short and practical. Up to 500 characters.
Send to email
Code

U0031

HYUNDAI U — Network/User

Vehicle Communication Bus A (+) Low

Brand: HYUNDAI
Type: U — Network/User
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Damaged or chafed CAN_H wiring (short to ground or open)
  • Corroded/loose connectors or pin pushed out at a module or splice
  • Failed control module actively pulling the bus low
  • Missing or incorrect termination (open or wrong resistance)
  • Aftermarket device or accessory tied into the bus creating a fault
  • Power or ground fault affecting a module driving the bus (module fault)

Symptoms

  • Multiple control modules show communication errors or go offline
  • Instrument cluster, ABS, transmission or engine displays erratic behavior or loss of functions
  • DTCs related to lost communication across several modules (U-codes, U0100, etc.)
  • Vehicle may not start if ECM/PCM loses communication or power
  • Intermittent or permanent warning lights on dash

What to check

  • Retrieve stored U0031 and any other U-codes and freeze frame data with a scan tool
  • Verify battery voltage and charging system (low battery can cause network faults)
  • Visually inspect wire harnesses, connectors, splice points and modules on Bus A for damage, corrosion or water intrusion
  • Check fuses and power/ground circuits for modules on Bus A
  • Measure DC voltage between CAN_H and chassis ground with ignition ON (see signal parameters)
  • Measure DC voltage between CAN_H and CAN_L and measure overall bus resistance with ignition OFF (expected ~60 ohms)

Signal parameters

  • CAN bus recessive state: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (each ± ~0.5 V)
  • CAN dominant state (active): CAN_H ≈ 3.5–4.0 V, CAN_L ≈ 1.0–1.5 V (voltage differential ≈ 2.0 V)
  • U0031 condition: CAN_H voltage lower than expected/recessive (CAN_H significantly below ~2.0–2.2 V) or pulled near ground
  • Bus termination: combined resistance across CAN_H to CAN_L ≈ 60 Ω (two 120 Ω terminators in parallel)

Diagnostic algorithm

  1. Confirm the code: record freeze frame, other U/P codes and reproduce conditions if possible.
  2. Check vehicle battery and main power/grounds to modules; ensure stable supply (12 V) before network diagnosis.
  3. Visually inspect Bus A harness, module connectors, splice/junction boxes and any recent repair or aftermarket device connections for damage or corrosion.
  4. With ignition ON (engine off), measure CAN_H and CAN_L DC voltages at a convenient harness connector near the ECM/ECU and at remote modules. Compare to expected values in signal_params.
  5. Measure resistance across CAN_H to CAN_L with ignition OFF; expect ~60 Ω. If open or very low, note results.
  6. Use an oscilloscope to observe CAN_H/CAN_L waveforms while modules are communicating. Look for flatline, clipped, excessive noise, or dominant-state stuck.

Likely causes

  • Short to ground on CAN_H (most likely)
  • Corroded connector or poor pin contact at a junction or module
  • Failed module on Bus A pulling CAN_H low
  • Incorrect or missing CAN termination resistor(s)

Fault status

⚠️ Status
U0031 — Communication Bus A positive line voltage low (CAN_H below expected range); network communication degraded or lost.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5 - 3.0 hours

Similar codes

U0030 U0073 U0100 U0104 U0077
371

Browse 371 HYUNDAI manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.

HYUNDAI

Your experience will help others
+100 karma for a short comment :)
Keep it short and practical. Up to 500 characters.
Send to email
Code

U0031

MERCEDES-BENZ U — Network/User

Vehicle Communication Bus A (+) low

Brand: MERCEDES-BENZ
Type: U — Network/User
AI status
Completed
ready
Completed 100%
Page language: EN

Causes

  • Damaged or chafed CAN_H wiring (short to ground or open)
  • Corroded/loose connectors or pin pushed out at a module or splice
  • Failed control module actively pulling the bus low
  • Missing or incorrect termination (open or wrong resistance)
  • Aftermarket device or accessory tied into the bus creating a fault
  • Power or ground fault affecting a module driving the bus (module fault)

Symptoms

  • Multiple control modules show communication errors or go offline
  • Instrument cluster, ABS, transmission or engine displays erratic behavior or loss of functions
  • DTCs related to lost communication across several modules (U-codes, U0100, etc.)
  • Vehicle may not start if ECM/PCM loses communication or power
  • Intermittent or permanent warning lights on dash

What to check

  • Retrieve stored U0031 and any other U-codes and freeze frame data with a scan tool
  • Verify battery voltage and charging system (low battery can cause network faults)
  • Visually inspect wire harnesses, connectors, splice points and modules on Bus A for damage, corrosion or water intrusion
  • Check fuses and power/ground circuits for modules on Bus A
  • Measure DC voltage between CAN_H and chassis ground with ignition ON (see signal parameters)
  • Measure DC voltage between CAN_H and CAN_L and measure overall bus resistance with ignition OFF (expected ~60 ohms)

Signal parameters

  • CAN bus recessive state: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (each ± ~0.5 V)
  • CAN dominant state (active): CAN_H ≈ 3.5–4.0 V, CAN_L ≈ 1.0–1.5 V (voltage differential ≈ 2.0 V)
  • U0031 condition: CAN_H voltage lower than expected/recessive (CAN_H significantly below ~2.0–2.2 V) or pulled near ground
  • Bus termination: combined resistance across CAN_H to CAN_L ≈ 60 Ω (two 120 Ω terminators in parallel)

Diagnostic algorithm

  1. Confirm the code: record freeze frame, other U/P codes and reproduce conditions if possible.
  2. Check vehicle battery and main power/grounds to modules; ensure stable supply (12 V) before network diagnosis.
  3. Visually inspect Bus A harness, module connectors, splice/junction boxes and any recent repair or aftermarket device connections for damage or corrosion.
  4. With ignition ON (engine off), measure CAN_H and CAN_L DC voltages at a convenient harness connector near the ECM/ECU and at remote modules. Compare to expected values in signal_params.
  5. Measure resistance across CAN_H to CAN_L with ignition OFF; expect ~60 Ω. If open or very low, note results.
  6. Use an oscilloscope to observe CAN_H/CAN_L waveforms while modules are communicating. Look for flatline, clipped, excessive noise, or dominant-state stuck.

Likely causes

  • Short to ground on CAN_H (most likely)
  • Corroded connector or poor pin contact at a junction or module
  • Failed module on Bus A pulling CAN_H low
  • Incorrect or missing CAN termination resistor(s)

Fault status

⚠️ Status
U0031 — Communication Bus A positive line voltage low (CAN_H below expected range); network communication degraded or lost.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5 - 3.0 hours

Similar codes

U0030 U0073 U0100 U0104 U0077
856

Browse 856 MERCEDES-BENZ manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.

MERCEDES-BENZ

Your experience will help others
+100 karma for a short comment :)
Keep it short and practical. Up to 500 characters.
Send to email

Report

Harassment or bullying behavior
Contains mature or sensitive content
Contains misleading or false information
Contains abusive or derogatory content
Contains spam, fake content or potential malware

Block Member?

Please confirm you want to block this member.

You will no longer be able to:

  • See blocked member's posts
  • Mention this member in posts
  • Invite this member to groups
  • Message this member
  • Add this member as a connection

Please note: This action will also remove this member from your connections and send a report to the site admin. Please allow a few minutes for this process to complete.

Report

You have already reported this .