Vehicle Communication Bus B (+) Low
What it means
U0040 means the voltage on the positive wire of vehicle communication bus B is lower than expected — typically a short to ground or a bad connection on the data network. Modules on bus B struggle to communicate, causing warning lights and lost features. How serious it is depends on which systems use bus B: a body network just disables features, while a powertrain network can cause stalling or a no-start. It is normally a wiring or connector problem.
Can I keep driving? Depends on the network — a low signal on bus B can mean anything from a few dead features to a no-start if it carries the powertrain, so diagnose before relying on the car.
Common symptoms
What you'll notice
- Several warning lights on together
- Modules on bus B missing or intermittently responding
- Features served by bus B stop working
- Possible hard start or stalling if bus B serves the engine or transmission
Common causes
Most likely first
- Bus B positive circuit shorted to ground, pulling the voltage lowLikely
- Corroded or water-damaged connector dragging the signal downCommon
- Chafed wiring touching chassis groundCommon
- A failing module loading the busLess often
- Damaged splice or terminal from a prior repairLess often
How to diagnose & fix
Work through it in order
- Check which modules on bus B have dropped off to narrow down the affected segment.
- Inspect the bus B positive wiring for chafing or a short to ground, especially where the harness passes metal edges.
- Measure the (+) circuit voltage and resistance to ground against spec to find the low point.
- Disconnect suspect modules one at a time to see if the bus voltage recovers.
- Repair the shorted wiring or replace the faulty module, then clear and re-test.
FAQ
Quick answers
Is it safe to drive with U0040?+
Can I just clear U0040?+
Related codes
Often seen with U0040
Read & clear U0040 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.