Vehicle Communication Bus A (+) Low
What it means
U0031 means the CAN-High (+) line of communication bus A is reading abnormally low voltage — typically from a short to ground or across to CAN-Low, or a module dragging the line down. Bus "A" is defined by the manufacturer: on many vehicles it's a body/comfort network, but on some it carries powertrain data, so the impact ranges from lost features to reduced power or a no-start. Diagnosis identifies which bus is affected before you decide it's safe to drive.
Can I keep driving? Depends on which bus 'A' is on your vehicle — often drivable, but if bus A carries powertrain data you may see reduced power or a no-start, so diagnose before relying on the car.
Common symptoms
What you'll notice
- Multiple warning lights and modules reporting lost communication
- Features tied to bus A stop working
- In some vehicles, reduced power, limp mode, or no-start if bus A is the powertrain bus
- Intermittent faults that track with wiring movement or moisture
Common causes
Most likely first
- CAN-High (+) wire shorted to ground or to CAN-Low, pulling voltage lowLikely
- Corroded or damaged connector on the bus A circuitCommon
- Chafed or pinched CAN wiringCommon
- A failed module holding the bus voltage downLess often
How to diagnose & fix
Work through it in order
- Scan all modules and record the codes — the affected modules identify which segment of bus A is faulty.
- Back-probe CAN-High and CAN-Low and compare voltages to spec to confirm the low-side fault.
- Inspect the bus A wiring and connectors for shorts to ground, chafing, and corrosion.
- Disconnect suspect modules one at a time to see if bus voltage recovers, isolating a shorted module.
- Repair the wiring short or replace the faulty module, then clear and re-test.
FAQ
Quick answers
Is it safe to drive with U0031?+
Can I just clear U0031?+
Related codes
Often seen with U0031
Read & clear U0031 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.