U0121: Kommunikationsverlust mit dem Steuergerät A des Antiblockiersystems (ABS)
Was zu tun ist
Your brakes still work. The braking system is hydraulic underneath the electronics, and the anti-lock system only ever releases pressure you have already applied — so losing it does not stop the car stopping.
What you have lost is protection in an emergency stop. Without anti-lock braking the wheels can lock if you brake hard on a wet, icy or loose surface, and a locked wheel gives you almost no steering — the car carries on in the direction it was already going however much you turn the wheel.
Until it is fixed: leave more distance, and brake earlier and more gently, especially in the wet. If you ever do have to stop hard and feel the wheels lock, ease off the pedal slightly and reapply rather than pushing harder.
Expect stability control, traction control and hill-hold to be off too — they are built on the same unit. Note also that an anti-lock warning lamp will fail a roadworthiness test, so it needs dealing with regardless.
As with all network codes, this often is not the module itself. Its power supply, earth, connector and the bus wiring come first — which matters here because the module is usually part of the hydraulic unit and expensive to replace.
Was dieser Code bedeutet
The anti-lock braking module has stopped being heard from on the data network. The general reasoning about what a lost communication code does and does not mean is at U0100.
The first thing to establish, because it is what people actually want to know: the brakes still work.
The braking system on every road car is hydraulic and mechanical underneath the electronics. Pressing the pedal moves fluid, and the fluid clamps the pads. No computer is involved in that, and none is needed — the anti-lock system sits on top as a set of valves that can release pressure the driver has already applied. Lose the module and you lose the releasing, not the braking. The car stops.
What is genuinely lost is what happens at the limit. Anti-lock braking exists so that a wheel about to stop turning is briefly released, because a locked wheel provides almost no steering — it slides in whatever direction it is already travelling regardless of where the front wheels are pointed. Without it, an emergency stop on a wet, icy or loose surface can lock the wheels, and the car goes straight on while the driver steers.
So the change is invisible until the one moment it matters, which is why this deserves more than the shrug it usually gets.
It rarely goes alone. Stability control, traction control, hill-hold and often the electric park brake are built on the same hydraulic unit and read the same wheel speed sensors — so one lost module takes several capabilities with it, and the dashboard usually says so with several lamps at once.
Two practical notes. An illuminated anti-lock braking warning is a roadworthiness test failure in most jurisdictions, so this is not something to leave. And the module here is usually part of the hydraulic unit rather than a separate box, which makes replacement expensive and makes checking its power, earth, connector and the bus first — as U0100 argues — worth more than usual.
Wahrscheinliche Ursachen
Hier anfangen
- Versorgung oder Masse des Steuergeräts ausgefallen · mittel
- Korrodierter oder loser Stecker · einfach
- Datenbusleitungen beschädigt · mittel
- Sicherung durchgebrannt oder Versorgung ausgefallen · einfach
Seltener
- Ein anderes Steuergerät zieht den Bus auf Masse · Fachwerkstatt
- ABS-Steuergerät defekt · Fachwerkstatt
Symptome
Beteiligte Bauteile
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