A dead battery, A faulty ABS module, A problem with ABS module circuit, A problem with the CAN bus
On Audi vehicles, U0121 is logged when the ECU detects a fault in the communication / CAN bus: “A dead battery, A faulty ABS module, A problem with ABS module circuit, A problem with the CAN bus”. U0121 is a generic SAE code — see the universal U0121 page too.
Typical triggers for a communication / CAN bus code like U0121 include faulty or ageing sensors, wiring/connector faults, a clogged or removed component, or a value drifting outside the ECU's expected window. Symptoms range from a check-engine light only, to limp mode, rough running, higher consumption or failed emissions tests.
When a Audi has had hardware changed for off-road/motorsport use (for example a removed DPF or EGR), the related monitor must be disabled in the ECU map or the code re-appears. Softechpro Solutions performs DPF/EGR/AdBlue/DTC off and recalculates the checksum, so codes like U0121 stay cleared after the flash. Genuine faults should always be repaired, not just hidden.
Softechpro Solutions does DPF/EGR/AdBlue/DTC off + checksum for 260+ ECU families — from €98.99/mo.
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