OBD-II trouble code
U0004: High Speed CAN Communication Bus (+) Low
The CAN-High (positive) line of the high-speed bus is reading too low — usually a short to ground or a bias problem dragging the positive signal down. Communication across the network degrades or fails.
Quick facts
- System
- Network
- Category
- Network Communication
- Severity
- High severity
- Drivable
- No — stop driving until repaired
- Repair cost range
- $100 – $900
- DIY difficulty
- Shop recommended
What does U0004 mean?
U0004 means the high-speed CAN bus's positive line — CAN-High or CAN(+) — is sitting at a voltage that's too low for valid communication. On a healthy bus, CAN-High rests near 2.5 volts and rises toward roughly 3.5 volts when a module is signaling. When something pulls that line down — typically a short to ground or a partial short between CAN-High and CAN-Low — the differential between the two wires collapses and the modules can no longer read messages reliably.
This is the 'low' counterpart to U0005 (CAN-High too high) and U0003 (CAN-High open). Together they describe the three ways the positive bus wire can fail: broken, dragged down, or pushed up. A low condition is most often a wiring or connector fault that creates an unintended path to ground, though a failing module that loads the bus down can produce the same reading. The modules that detect corrupted or missing traffic store U0004, and you'll commonly see module-specific lost-communication codes alongside it.
Because CAN ties together the engine, transmission, ABS, cluster, and body modules, a low CAN-High line can knock out several systems simultaneously. Expect warning lights, possible no-start or limp mode, and a scan tool that struggles to reach the affected modules. U0004 is traced electrically — measuring bus voltages and resistance and isolating the short — rather than by replacing parts on a hunch.
Common causes
- CAN-High wire shorted to ground
- CAN-High partially shorted to CAN-Low, collapsing the differential
- Chafed or pinched CAN-High wire contacting a grounded surface
- Water intrusion or corrosion in a connector creating a low-resistance path to ground
- A failed module loading the CAN-High line down
- Damaged terminating resistor branch biasing the bus low
- Backed-out terminal allowing the wire to touch chassis metal
Symptoms
- Multiple warning lights on simultaneously (check engine, ABS, traction, airbag)
- Engine may not start, or runs in limp mode
- Erratic, frozen, or dead gauges
- Scan tool cannot communicate with several modules
- Module-specific lost-communication U-codes stored along with U0004
- Intermittent loss of features that tracks with vibration or moisture
Diagnostic steps
- 1.Record all stored codes in every module to see which modules are affected and help localize the short.
- 2.Key on, measure CAN-High voltage. It should rest near 2.5 volts; a reading dragged well below that points to a short to ground or to CAN-Low.
- 3.With key off and battery disconnected, measure bus resistance at the OBD port — about 60 ohms is healthy; a near-zero reading indicates a short.
- 4.Inspect the CAN-High wire for chafing, pinch points, and contact with grounded metal or other wires.
- 5.Check connectors for water intrusion and corrosion that could create a path to ground.
- 6.Disconnect modules one at a time while watching the CAN-High voltage; if it recovers when a specific module is unplugged, that module is loading the bus down.
Repair cost
$100 – $900
Diagnosis commonly runs $150-$300 to isolate where the line is being pulled low. A wiring or connector repair to clear a short to ground is often $150-$600. If a failed module is loading the bus, replacement with programming can run $400-$900 or more, higher on European and luxury 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.