U0004 code meaning
High Speed CAN Communication Bus (+) Low
What U0004 means
U0004 means the (+) wire of the high-speed CAN, CAN-High, is dragged below its valid range, usually by a short to ground. This is one of the faults the physical layer cannot absorb: a sending node is unable to produce a dominant bit, every frame attempt fails, the error counters rise and the controllers end up bus-off, as onsemi's bench measurements show. Transceivers with bus diagnosis, NXP's TJA1043 for example, recognise this short up to roughly 50 ohms. Typically all powertrain and chassis modules on the network fall silent at the same moment, accompanied by U0001 and a string of lost-communication codes. No manufacturer documents U0004 publicly; an FCA bulletin describes the same picture under U0001, caused by a harness pinched beneath a console bolt.
U0004 at a glance
- Check engine lightUsually not
- Emissions testNot an emissions code
- Limp modePossible
- Repair difficultyHard
- Typical labor1 to 6 hours
- Clears after repairWhen cleared with a scanner
Symptoms of U0004
- Network-wide communication loss, scan tool cannot connect
- Engine, gearbox, ABS and steering warnings together
- Vehicle may not start or run in limp mode
Common causes of U0004
- Most likelyCAN-High insulation damaged and touching body ground
- PossibleHarness clamped by a screw, bracket or seat rail
- Less likelyDefective transceiver in one module pulling CAN-High down
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
CAN high wireCAN transceiver
What each part of U0004 means
| U | Network system: communication between the control modules (CAN bus). |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 04 | The exact fault in that area. |
What to do next
- Note U0004 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about U0004
What does U0004 mean?
U0004 means "High Speed CAN Communication Bus (+) Low". U0004 means the (+) wire of the high-speed CAN, CAN-High, is dragged below its valid range, usually by a short to ground. It is a generic code, so it means the same on every make.
Is it safe to drive with U0004?
Drive gently and only to a workshop. With U0004 the car can go into limp mode (reduced power) to protect itself.
Will U0004 fail an emissions test?
U0004 is not an emissions code, so on its own it usually does not fail an emissions test. Other stored codes or a lit check engine light still can.
How hard is U0004 to fix?
Difficulty: hard. A typical repair takes 1 to 6 hours of work. This is a job for a workshop.
Will U0004 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.