U0056
Vehicle Communication Bus D Performance
Communication network (CAN bus)
Generic OBD-II
Medium
What it means
U0056 means communication Bus D is working but not reliably — messages are delayed, corrupted, or dropping out, usually from high resistance, a marginal connection, or a module sending bad data. Because it's an intermittent "performance" fault rather than a dead bus, symptoms tend to come and go. Which features are affected depends on what "Bus D" carries on your specific vehicle.
Medium
Can I keep driving? Usually drivable, but degraded bus performance can make modules drop in and out — expect intermittent warning lights and features until it's fixed.
Common symptoms
What you'll notice
- Intermittent warning lights across different systems
- Modules on Bus D dropping out and reconnecting
- Features that work sometimes and fail other times
- Slow or unreliable scan-tool communication with some modules
Common causes
Most likely first
- High resistance or a loose connection in the Bus D wiring or connectorsLikely
- Corroded, chafed, or partially broken bus wiringCommon
- A failing module sending bad data and disrupting bus trafficCommon
- Marginal or damaged termination resistor affecting signal qualityLess often
- Water intrusion or a weak battery causing erratic communicationLess often
How to diagnose & fix
Work through it in order
- Scan all modules and watch for units that intermittently drop off Bus D.
- Check battery voltage and charging; correct low voltage, then clear and re-test.
- Wiggle-test and inspect Bus D connectors and wiring for corrosion, looseness, or damage.
- Measure bus resistance (should be about 60 ohms across the pair) to check termination.
- Isolate modules one at a time to find a unit flooding the bus with bad data; repair wiring or replace the confirmed module.
FAQ
Quick answers
Is it safe to drive with U0056?+
Usually yes, but intermittent bus faults can disable systems unpredictably, so get it diagnosed rather than ignoring it.
Can I just clear U0056?+
You can, but a performance fault reflects a marginal connection or failing module and will return until that's corrected.
Related codes
Often seen with U0056
Read & clear U0056 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.
✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX