Communication / CAN Bus (U codes) — Remapping Solutions

The U codes point to communication faults between the vehicle's control units. The engine ECU, the automatic gearbox, the ABS, the instrument cluster, the immobilizer and smart sensors such as NOx sensors exchange messages over the CAN network. When a module stops receiving the messages it expects, or receives them with invalid data, it stores a U code.

In a remapping workshop they mostly appear in three situations: when removing a component (NOx sensors or the AdBlue module after an SCR OFF, the automatic gearbox in a manual conversion, the ABS in motorsport), on engine swaps and engines running outside their vehicle, which lose the BCM, the cluster or the immobilizer, and after working on the ECU on the bench or in boot mode, or after loading software that does not match the other modules.

Not all of them are solved the same way. When the module has been removed on purpose, the solution is a calibration that stops expecting it, plus DTC OFF of the associated codes. When the problem is the network itself (short circuits, termination, wiring) or incompatible software, remapping is not the answer, and each code page says so. These are the U codes we work on: