OBD-II trouble code
U0015: Medium Speed CAN Communication Bus (-) Open
A break in the CAN-Low conductor of the medium-speed network. Because this wire sits nearest ground potential, poor module grounding imitates it convincingly, and grounds are the cheapest thing to rule out first.
Quick facts
- System
- Network
- Category
- Network Communication
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $90 – $700
- DIY difficulty
- Shop recommended
What does U0015 mean?
Data on a CAN network exists as the difference between two conductors. At rest both sit near 2.5 volts; during transmission one is driven up and the other driven down, and receivers read only the separation between them. U0015 concerns the wire that gets driven down — CAN-Low, the conductor that swings toward ground — and reports that it has gone open somewhere on the medium-speed network.
That this wire operates close to ground potential is not a trivia point, it is the thing that should shape the diagnosis. Every module on the bus measures the pair against its own local ground reference. If a module's ground strap or eyelet is corroded, its reference sits a little above true chassis ground, and the low side of the pair no longer looks where the module expects it to look. The symptom that produces is indistinguishable from a broken conductor without testing, and the fix is a fraction of the price. Voltage-drop testing the grounds of the modules that have gone quiet is therefore the first thing to do on this code and the last thing anyone thinks of. A ground repair that takes twenty minutes is a much better outcome than a harness search that finds nothing.
The second thing worth understanding is what happens to noise rejection. A CAN pair rejects electrical interference because the two wires are twisted together and any noise that couples into one couples equally into the other, cancelling in the difference. Break one of them and that cancellation is gone. What is left is a long single conductor running through an engine bay and door frames alongside blower motors, wiper motors, defroster grids and ignition circuits. The practical consequence is a distinctive complaint: the fault comes and goes in step with a specific electrical load. A customer who says the display drops out only with the rear defroster on, or only at high blower speed, is describing an intact-but-unpaired bus, not a coincidence, and that observation is worth more than a scope trace.
Because of where it sits electrically, this code also tends to keep company with low-voltage and grounding faults, and often shares a root cause with them — water in a door harness, a corroded body ground behind a kick panel, a battery ground that was never properly retorqued after service. If other modules are reporting supply or reference problems, look for the shared point before assuming two separate faults.
Common causes
- Corroded or loose body ground for one of the modules on the bus, imitating an open
- CAN-Low conductor broken, chafed through, or separated at a splice
- Water intrusion into a door, tailgate or under-carpet connector
- Backed-out or spread CAN-Low terminal in a shared bus connector
- Battery ground or engine-to-body strap left loose after service
- Harness pinched during interior trim or seat work
- Module failing internally and opening its portion of the low-side circuit
Symptoms
- Feature loss that appears and clears in step with a specific electrical load
- Several modules dropping off the scan tool together
- Warning lamps for systems whose status messages have stopped arriving
- Low-voltage or ground-related codes stored in other modules at the same time
- Symptoms worsening in wet weather or after a car wash
- Vehicle usually still starts and drives normally
Diagnostic steps
- 1.Voltage-drop test the grounds of the modules that have gone quiet before touching the bus wiring. A raised local ground reference reproduces this code at a fraction of the repair cost.
- 2.Ask whether the fault correlates with any electrical accessory. A complaint tied to the blower, defroster or wipers points at a bus that has lost its noise cancellation because one conductor is open.
- 3.Scan all modules and note which respond, then check whether the non-responders share a ground point rather than only a bus segment.
- 4.Measure resistance across the bus pair with the network asleep and compare against the expected terminated value for the vehicle.
- 5.Inspect the wettest parts of the route first — door boots, tailgate hinges, under-carpet runs and lower kick panels — since water reaches the low side of a pair as readily as the high side but is found more often here in practice.
- 6.Check for backed-out or corroded terminals at any connector shared by the quiet modules.
- 7.After the repair, road test with the accessories that previously triggered the fault switched on, since a marginal ground can pass a static test and fail under load.
Repair cost
$90 – $700
A corroded ground point cleaned and retorqued can close this out for under $150 including diagnosis. A genuine harness repair typically lands at $150 to $500 once the location is known, with most of the cost in access rather than materials. A module replacement with programming runs $400 to $700 and beyond on premium platforms.
Estimate your repair
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DIY vs shop
Leave this one to a qualified shop. It typically involves emissions-critical components, refrigerant handling, or other work that requires manufacturer-grade tooling, training, or certification. DIY attempts often produce a more expensive problem than the original code.