U0004 High Speed CAN Communication Bus (+) Low

U0004 reports a fault on the CAN-high wire itself rather than in any one module. The network that lets the controllers talk to each other is disturbed, so several unrelated modules can set codes at the same time.

Code details

Generic OBD-II · U0

System: Computer
Code type: Generic (SAE / ISO controlled)
Standard: SAE J2012 / ISO 15031-6

Diagnostic boundary

A diagnostic trouble code reports that a monitored test failed. It does not prove one specific part is bad, and manufacturer logic can add vehicle-specific conditions.

Do not replace a control module for a bus fault. The code says the network is disturbed, not that any particular controller has failed, and a module fitted to a faulty bus will report the same thing.

Symptoms of U0004

Check-engine light, sometimes with several unrelated codes at once.
Warning lights, a no-start, or intermittent electrical faults.
Communication or module-reset problems on a scan tool.

Common causes of U0004

Low battery voltage, poor grounds, corroded connectors, or blown fuses.
Wiring or network (CAN bus) faults between modules.
A module software/programming issue, or rarely a failed controller.

Useful checks before replacing parts

Read codes from every module, not just the engine, because a bus fault usually shows up as a cluster of unrelated communication codes across the vehicle.
Measure resistance across the bus with the ignition off — a healthy high-speed CAN pair reads about 60 ohms end to end, which is the two 120-ohm terminating resistors in parallel.
Inspect connectors, splices and any recently fitted accessory wiring before suspecting a module, since aftermarket installs and rodent damage are common causes of a shorted or open bus.

Related OBD-II codes

These nearby computer entries can help compare wording and pin down the fault area.