U0031: Vehicle Communication Bus C (+) Open
The plus line of vehicle bus C is open. Modules sharing this segment drop offline. A communication loss with this module breaks any function relying on its data — diagnostic tools may show 'no response' and dependent modules can fall into limp mode.
What U0031 means
U0031 is a generic OBD-II network code in the bus & wiring group. In plain English: the computer found an electrical fault in that circuit: an open, short or bad connection, or a failed component.
- Check engine light
- Usually on
- Emissions-related
- No
- Limp mode possible
- Not typical
- Drive cycle to clear
- Not required
Get it fixed
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Symptoms
- Multiple warning lights may illuminate
- Affected modules may go into limp mode or shut down
- Specific functions inoperative depending on module
- Diagnostic tools may show communication faults
Common causes
- 01Bus-C high wire brokenMost likely
- 02Gateway module routing CAN messages incorrectlyLess likely
- 03CAN terminating resistor (120 Ω) drifted out of specLess likely
Can I drive with U0031?
Usually yes, for a short time.
The car will often drive normally, but get it diagnosed soon. If you notice rough running, warning lights flashing, overheating or a burning smell, stop driving.
Why we rate it moderate
- Turns on the check engine light
Severity is our rule-based rating from the code’s flags in OBDex and the system it affects, not an inspection of your car. If the check engine light is flashing, or you notice overheating, low oil pressure, or brake or steering problems, stop driving and get help.
How to diagnose U0031
- 1
Confirm the code and save freeze-frame data
Read the codes with an OBD-II scan tool and note every code, not just U0031. Save the freeze-frame data (speed, load, temperature when the fault set) before clearing anything; it tells you when the fault happens.
- 2
Check for related codes first
Look for U0039, U0042, U0048, U0050. Related codes often share a cause; a circuit or sensor code usually needs fixing before a performance code like this one.
- 3
Check module power, ground and the data bus
Confirm the module named in the code has battery voltage, ignition feed and a good ground. With the battery disconnected, a healthy CAN bus usually measures about 60 ohms between CAN-High and CAN-Low at the OBD port. Inspect connectors for corrosion and pushed-out pins.
- 4
Test the likely causes in order
Start with the most likely cause and work down: Bus-C high wire broken; gateway module routing CAN messages incorrectly; CAN terminating resistor (120 Ω) drifted out of spec. Test before replacing parts.
- 5
Check for recalls and service bulletins
Look up recalls for your exact vehicle with its VIN, and ask a dealer about technical service bulletins (TSBs). Manufacturers sometimes issue software updates or extended warranties for known faults.
- 6
Clear the code and verify the repair
After the repair, clear the code and drive the car under the conditions in the freeze-frame data to confirm it does not return.
U0031 repair cost estimate
Estimate, not a quote. Labor times are typical ranges for this code from OBDex, and $125/hr is an assumed rate. Shop rates and labor times vary by vehicle and region. Diagnosis time may be billed separately.
- DIY difficulty
- Hard
- Good DIY job?
- Usually a shop job
Involved testing or major parts. Most owners use a shop.
Parts and systems involved
- CAN high wire
U0031 FAQ
What does U0031 mean?
U0031 means “Vehicle Communication Bus C (+) Open”. The plus line of vehicle bus C is open. Modules sharing this segment drop offline. A communication loss with this module breaks any function relying on its data — diagnostic tools may show 'no response' and dependent modules can fall into limp mode.
Is U0031 serious? Can I keep driving?
We rate U0031 as moderate severity. Usually yes, for a short time. The car will often drive normally, but get it diagnosed soon. If you notice rough running, warning lights flashing, overheating or a burning smell, stop driving.
What causes U0031?
The most common causes, from most to least likely: Bus-C high wire broken; Gateway module routing CAN messages incorrectly; CAN terminating resistor (120 Ω) drifted out of spec. Test before replacing parts, because several causes can set the same code.
How much does it cost to fix U0031?
Typical labor is about $125–$625 at an assumed $125/hour shop rate, plus parts. That is an estimate, not a quote: labor times and rates vary by vehicle and region, and diagnosis may be billed separately.
Can I fix U0031 myself?
DIY difficulty is hard. Most owners leave this to a shop because it needs involved testing or specialist tools. Always confirm the cause before buying parts.
Will U0031 clear itself?
If the fault was intermittent, the warning may go off once the computer stops seeing it, but the code stays in memory. Fix the cause; clearing the code alone doesn’t repair anything.