What does code U0029 mean?
Vehicle communication bus A performance. The module detects errors on the CAN network (lost, corrupted or late messages) without it going down completely. In tuning it appears on swaps with an incomplete CAN network (missing modules or terminating resistor) and with damaged or badly spliced wiring.
Symptoms seen in the workshop
- Several intermittent warning lights
- U codes from several modules at once
- Faults that come and go with temperature or vibration
Common causes of code U0029
- Swap with removed modules the ECU still expects on the network
- Missing bus terminating resistor (it was in a removed module)
- Poor CAN wiring splices or CAN-H/CAN-L wires not twisted
- Damaged wiring, corroded connector
- A faulty module adding noise to the network
Fix by ECU remapping
It depends on the cause:
If removed modules on a swap cause it (DTC OFF): calibrating the ECU to stop expecting messages from those modules and deactivating U0029 and the U codes of each missing module.
If it is the network: measure the resistance between CAN-H and CAN-L (about 60 Ω with both terminations), check splices and connectors and find the module adding noise. That is a repair, not software.
⚠️ Important: Disabling the DTC without the correct calibration of the related system (DTC OFF) is not a valid fix. Both the calibration maps and the diagnostic mask have to be modified.
Frequently asked questions about U0029
What does U0029 mean?
The module detects performance errors on CAN bus A: lost, corrupted or late messages.
Why does U0029 appear on a swap?
Because the network is incomplete: modules are missing, or the terminating resistor that was in one of them.
How is the CAN network checked?
With the ignition off, measure the resistance between CAN-H and CAN-L: about 60 Ω with both terminations, about 120 Ω with only one.
