OBD-II trouble code
U0013: Medium Speed CAN Communication Bus (+) Low
The CAN-High wire is being held down when it needs to be free to swing up. Because almost every metal surface a harness can rub against is already at ground potential, this is the physical-damage code of the medium-speed bus family.
Quick facts
- System
- Network
- Category
- Network Communication
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $100 – $800
- DIY difficulty
- Shop recommended
What does U0013 mean?
CAN is a differential bus, and knowing which direction each wire moves is what makes this code readable. At rest both conductors idle near 2.5 volts. To send a dominant bit, a transmitting module drives CAN-High *upward* toward roughly 3.5 volts while simultaneously pulling CAN-Low down. Receivers never read either wire against ground — they read the gap between the two.
U0013 reports that the CAN-High conductor on the medium-speed network is sitting below where it belongs. The consequence follows directly from the direction of travel: if CAN-High is clamped down, the upward swing that forms a dominant bit cannot happen at all. The wire is present, the modules are powered, the connectors are seated, and yet not a single message can be assembled. That is why a network with this fault can pass every visual inspection while carrying no traffic whatsoever.
Why this specific fault is so common has nothing to do with electronics and everything to do with geometry. A short to ground requires only that the conductor touch something grounded — and in a vehicle, virtually every bracket, panel, seat rail, door frame, floor pan and body seam already is. Medium-speed harnesses are more exposed to this than powertrain harnesses, because the body network runs through doors, under seats, across the floor and up the pillars, spending most of its length in places that flex, get walked on, or get taken apart during trim, audio, seat and carpet work.
The practical lesson is to ask what was recently disturbed inside the vehicle before unplugging a single module. A CAN-High short to ground very often traces to a screw driven back through a harness, a seat bolt torqued down onto a wire, or a door harness grommet that has finally worn through after a decade of opening and closing.
Common causes
- CAN-High conductor chafed through against a grounded bracket, panel edge or seat rail
- Harness pinched or screw-punctured during trim, seat, carpet, audio or body repair work
- Door or liftgate harness worn through inside its flexing grommet
- Water pooling in a footwell or spare-wheel well grounding a damaged splice
- Module with an internally shorted transceiver holding the line down
- Terminal pushed out of its cavity and contacting the connector shell or a grounded pin
- Crushed harness section from accident damage or a previous repair
Symptoms
- Body and comfort features stop working together — climate, audio, lighting, power accessories
- Scan tool cannot establish communication with any module on the medium-speed network
- A long list of lost-communication codes stored in whichever modules can still be reached
- Fault appeared immediately after interior, seat, audio or body work
- Behaviour changes when a door is opened and closed, or when the body flexes
- Engine starts and drives normally
Diagnostic steps
- 1.Ask what was recently worked on inside the vehicle. A short to ground on a body-network conductor is far more often a mechanical injury than a component failure, and that question narrows the search area faster than any meter will.
- 2.Key on, measure CAN-High to ground. It should idle near 2.5 volts with brief upward activity. A reading pinned near zero confirms the clamp.
- 3.Disconnect the battery and check CAN-High to ground for continuity. Direct continuity with the network unpowered proves a hard short and eliminates every module in a single step.
- 4.If the short is hard, split the network. Disconnect it at a mid-point connector or a gateway, measure each half, and halve the search area again from there.
- 5.If there is no hard short, reconnect and unplug modules one at a time while watching the line. A conductor that lifts back to 2.5 volts when one module is removed identifies that module's transceiver.
- 6.Inspect the physical routing along whichever section the measurements identified, concentrating on door grommets, seat mounting points, floor pan seams and any bracket the harness is clipped against.
- 7.Repair the conductor properly — solder and heat-shrink, with the harness re-routed away from the offending edge. Taping over a chafe point without moving the harness simply postpones the fault.
Repair cost
$100 – $800
Diagnosis dominates this repair, because the fault is a location problem rather than a parts problem. Expect $100 to $300 to isolate the short, more where trim has to come out to reach the harness. The wiring repair itself is often $100 to $400 including trim removal and refit. Where a module's transceiver has failed internally, replacement with programming runs $400 to $800 and higher on European platforms.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with module communication / can bus diagnosis preselected. Adjust labor rate and vehicle category to fit your situation.
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.