U0040 code meaning
Vehicle Communication Bus B (+) Low
What U0040 means
U0040 indicates the (+) line of Vehicle Communication Bus B pulled down to ground potential. On a high-speed-type CAN that is CAN-High, and onsemi's bench tests list it among the shorts that stop all transmission: each failed start of frame raises the transmit error counter by eight, the controller turns error passive after a handful of attempts and goes bus-off once the count passes 255. Meanwhile the CANH driver runs into its current limit. The same short is one of the failures that NXP's fault-tolerant TJA1055 rides out without corrupting frames. So the effect ranges from a completely silent Bus B to a degraded but working one, depending on the bus type. No manufacturer documents U0040 publicly. Measure CAN-High against ground with the battery off; chafed insulation is the usual culprit.
U0040 at a glance
- Check engine lightUsually not
- Emissions testNot an emissions code
- Limp modePossible
- Repair difficultyHard
- Typical labor1 to 5 hours
- Clears after repairWhen cleared with a scanner
Symptoms of U0040
- All functions on Bus B fail at once (high-speed type)
- Many lost-communication codes in modules on other networks
- Fallback modes or warning lamps depending on the modules involved
Common causes of U0040
- Most likely(+) wire chafed through against body ground
- PossibleHarness crushed by a seat rail, bracket or panel
- Less likelyTransceiver of one module pulling the (+) line to ground
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Bus b wiringBus b transceiver
What each part of U0040 means
| U | Network system: communication between the control modules (CAN bus). |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 40 | The exact fault in that area. |
What to do next
- Note U0040 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 U0040
What does U0040 mean?
U0040 means "Vehicle Communication Bus B (+) Low". U0040 indicates the (+) line of Vehicle Communication Bus B pulled down to ground potential. It is a generic code, so it means the same on every make.
Is it safe to drive with U0040?
Drive gently and only to a workshop. With U0040 the car can go into limp mode (reduced power) to protect itself.
Will U0040 fail an emissions test?
U0040 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 U0040 to fix?
Difficulty: hard. A typical repair takes 1 to 5 hours of work. This is a job for a workshop.
Will U0040 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.