Code
U0031
Generic
U — Network/User
Vehicle Communication Bus A (+) Low
Views:
UK: 22
EN: 24
RU: 25
AI status
Completed
Completed
100%
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
- Confirm the code: record freeze frame, other U/P codes and reproduce conditions if possible.
- Check vehicle battery and main power/grounds to modules; ensure stable supply (12 V) before network diagnosis.
- Visually inspect Bus A harness, module connectors, splice/junction boxes and any recent repair or aftermarket device connections for damage or corrosion.
- 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.
- Measure resistance across CAN_H to CAN_L with ignition OFF; expect ~60 Ω. If open or very low, note results.
- 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
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Code
U0031
HYUNDAI
U — Network/User
Vehicle Communication Bus A (+) Low
Views:
UK: 18
EN: 21
RU: 26
AI status
Completed
Completed
100%
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
- Confirm the code: record freeze frame, other U/P codes and reproduce conditions if possible.
- Check vehicle battery and main power/grounds to modules; ensure stable supply (12 V) before network diagnosis.
- Visually inspect Bus A harness, module connectors, splice/junction boxes and any recent repair or aftermarket device connections for damage or corrosion.
- 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.
- Measure resistance across CAN_H to CAN_L with ignition OFF; expect ~60 Ω. If open or very low, note results.
- 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
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Code
U0031
MERCEDES-BENZ
U — Network/User
Vehicle Communication Bus A (+) low
Views:
UK: 20
EN: 21
RU: 24
AI status
Completed
Completed
100%
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
- Confirm the code: record freeze frame, other U/P codes and reproduce conditions if possible.
- Check vehicle battery and main power/grounds to modules; ensure stable supply (12 V) before network diagnosis.
- Visually inspect Bus A harness, module connectors, splice/junction boxes and any recent repair or aftermarket device connections for damage or corrosion.
- 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.
- Measure resistance across CAN_H to CAN_L with ignition OFF; expect ~60 Ω. If open or very low, note results.
- 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
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