CAN bus off — module went into protection
Where to start
The unit disconnected itself from the bus, going into protection because the line was noisy. A common cause is someone else's splice into the wiring: an alarm, a stereo, a tracking device. If the code appeared soon after something was fitted, start there. The difference from U0001: there the bus fails as a whole, here a SINGLE unit went into protection, and which unit it was usually leads straight to the splice — it is next to it. ⚠️ The code often returns after a clear: the protection trips again as soon as the noise on the line comes back.
«No communication» is about the bus, not the unit
⛔ «No communication» almost never means «the unit is broken». It means one unit stopped hearing another, and the usual causes are power, ground and the bus — not the unit itself. Replacing a control unit on this code is the most expensive mistake in the whole reference.
Read the number of them. Several «no communication» codes at once point to something shared: power, ground, or a break in the bus. One code alone points to that unit's own connector and its power. Five missing units almost never mean five broken units.
⚠️ These codes also appear on their own after the battery has been disconnected, and clear themselves after a few trips. Clearing them without finding the cause is fine only in that case — otherwise they come back before you leave the yard.
How to put together a report a mechanic will actually read
How the code is built
All codes in «Communication between modules»
How the app helps
CarScan reads this code from your car through an OBD-II adapter, shows whether it is active now or only stored, names the module that reported it, and puts it into a plain-text report for your mechanic.