U0031 code meaning
Vehicle Communication Bus A (+) Low
What U0031 means
U0031 indicates a ground short on the (+) conductor of Vehicle Communication Bus A. Where Bus A is a high-speed-style two-wire CAN, such as the sensor/hybrid CAN Audi assigns to this slot in its 48 V mild hybrids, the (+) wire is CAN-High, which has to rise by about a volt to form a dominant bit. Grounded, it cannot: onsemi's measurements list CAN-High to ground among the shorts under which transmission becomes impossible, so the controllers keep retrying in vain. Every node on Bus A then falls silent together, and modules on other networks record lost-communication codes for them. No manufacturer documents U0031 publicly. With the battery disconnected, a few ohms or less between CAN-High and ground locate the short; chafed harness sections are the usual cause.
U0031 at a glance
- Check engine lightUsually not
- Emissions testNot an emissions code
- Limp modePossible
- Repair difficultyHard
- Typical labor1 to 5 hours
- Clears after repairWhen cleared with a scanner
Symptoms of U0031
- All functions served by Bus A fail at once
- Many lost-communication codes in other modules
- Warning lamps or fallback modes depending on the modules involved
Common causes of U0031
- Most likely(+) wire chafed through against body ground
- PossibleHarness trapped under a panel or bracket
- Less likelyTransceiver of one module shorting the (+) line internally
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Bus a wiringBus a transceiver
What each part of U0031 means
| U | Network system: communication between the control modules (CAN bus). |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 31 | The exact fault in that area. |
What to do next
- Note U0031 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about U0031
What does U0031 mean?
U0031 means "Vehicle Communication Bus A (+) Low". U0031 indicates a ground short on the (+) conductor of Vehicle Communication Bus A. It is a generic code, so it means the same on every make.
Is it safe to drive with U0031?
Drive gently and only to a workshop. With U0031 the car can go into limp mode (reduced power) to protect itself.
Will U0031 fail an emissions test?
U0031 is not an emissions code, so on its own it usually does not fail an emissions test. Other stored codes or a lit check engine light still can.
How hard is U0031 to fix?
Difficulty: hard. A typical repair takes 1 to 5 hours of work. This is a job for a workshop.
Will U0031 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.