High Speed CAN Communication Bus (+) Low
What it means
U0004 means the voltage on the high-speed CAN bus (+) line is lower than it should be — the main network that lets the car's control modules talk to each other. A low bus can garble or cut that communication, so you may see multiple warning lights, erratic module behavior, and in bad cases limp mode or a no-start. It usually points to a wiring short, a bad connector, or a module dragging the bus down.
Can I keep driving? Risky — a high-speed CAN fault can knock out communication between modules, causing warning lights, limp mode, or a no-start/stall. Diagnose before relying on the vehicle.
Common symptoms
What you'll notice
- Multiple warning lights and unrelated communication (U-series) codes set together
- Gauges, modules, or accessories acting erratically or going dead
- Engine limp mode, no-start, or stalling in severe cases
- Check-engine light on
Common causes
Most likely first
- CAN bus high (+) wire shorted to ground or to the low wireLikely
- Damaged, chafed, or corroded CAN wiring or connectorCommon
- A failed module dragging down the bus voltageCommon
- Water intrusion in a connector or moduleLess often
- Faulty terminating resistor or bad ground/power to a moduleLess often
How to diagnose & fix
Work through it in order
- Scan all modules and note which report communication loss to narrow the affected bus segment.
- Inspect the CAN high wiring and connectors for shorts, chafing, corrosion, or water.
- With the key off, measure CAN bus resistance and check the high line for a short to ground.
- Disconnect modules one at a time to find one pulling the bus down; repair wiring or replace the faulty module.
- Repair the fault, clear codes, and verify all modules communicate before driving.
FAQ
Quick answers
Is it safe to drive with U0004?+
Can I just clear U0004?+
Related codes
Often seen with U0004
Read & clear U0004 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.