What Does U0031 Mean?
The CAN-H (positive) line voltage is reading lower than specification, typically below 2.0V. This can cause intermittent communication failures or complete loss of network function.
Safety Warning
Safety systems including airbags and ABS may be compromised or non-functional. Do not drive until issue is resolved.
Common Causes
35%
Short to ground on CAN-H wire from damaged insulation or pinched wire
30%
Faulty control module pulling CAN-H line voltage low
20%
Corroded or moisture-contaminated connector causing resistance to ground
10%
Incorrect or damaged terminating resistor creating voltage drop
5%
Poor ground connection at a control module affecting CAN transceiver
Diagnostic Steps
1
Step 1: Measure CAN-H voltage at the diagnostic connector with ignition on (should be approximately 2.5V at rest)
2
Step 2: Check for short to ground by measuring resistance from CAN-H to chassis ground with all modules disconnected (should be >10k ohms)
3
Step 3: Disconnect modules one at a time while monitoring CAN-H voltage to identify module pulling voltage low
4
Step 4: Inspect wiring harness for damage, particularly where it contacts metal surfaces or passes through grommets
5
Step 5: Check connector terminals for corrosion or moisture intrusion and verify proper sealing
Estimated Repair Cost
$150 - $1,000
Parts + labor, varies by vehicle and location
The CAN-H (positive) line voltage is reading lower than specification, typically below 2.0V. This can cause intermittent communication failures or complete loss of network function.
The most common cause of U0031 (Vehicle Communication Bus A (+) Low) is: Short to ground on CAN-H wire from damaged insulation or pinched wire
Typical repair costs for U0031 range from $150 to $1,000, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Safety systems including airbags and ABS may be compromised or non-functional. Do not drive until issue is resolved.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to U0031 to identify the root cause.
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Quick Info
Category
Network
System
Vehicle Communication Network
Difficulty
Type
Generic (SAE)
Recommended Tools
OBD2 Scanner
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