U0129: Lost Communication With Brake System Control Module A
What to do
Your brakes still work, but on many modern cars the pedal will have become very hard and you will need to push much harder to stop. Stopping distances get considerably longer, so leave far more room and slow down earlier. Push firmly — much harder than feels normal — and the car will stop.
This is worth getting recovered rather than driven any distance, particularly in traffic.
On a hybrid or electric car you may also notice the car no longer slows when you lift off the accelerator, and the pedal feels different. That is the regenerative braking being switched off, so the ordinary brakes are doing all the work — which also means the pads are wearing far faster than usual while this is unfixed.
As with all network codes, the module itself is often not the fault. Its power, earth, connector and the bus wiring come first.
What this code means
The brake system control module has stopped being heard from. The general reasoning is at U0100.
On an older vehicle this would be a convenience. On a vehicle with electrically assisted braking it is not, and knowing which one you have is the whole of the difference.
A conventional brake servo multiplies pedal effort using engine vacuum. Turbocharged engines, hybrids and electric vehicles cannot rely on manifold vacuum — the argument at P2525 — so many use an electrically driven booster instead, and this module is what runs it. On such a vehicle, losing it means losing power assistance.
The brakes still work. The pedal becomes very hard and stopping distances get considerably longer, because the driver is now supplying all the force that reaches the pads. That is a serious change in an emergency and a surprising one at low speed, but it is not a loss of braking.
On hybrids and electric vehicles there is a second effect worth understanding: these vehicles normally slow using the motor as a generator and blend friction braking in underneath it. With this module quiet, the blending stops — regeneration is disabled and the friction brakes do everything. The pedal feel changes noticeably, the car coasts rather than slowing when the driver lifts off, and brake pad wear rises sharply, because pads on such a vehicle are usually doing a small fraction of the work.
Marked drivable on the same basis as P2525 and P0556: assistance is lost, braking is not.
Likely causes
Start here
- Failed power or earth supply to the module · moderate
- Damaged data bus wiring · moderate
- Corroded or loose connector · easy
- Blown fuse or failed supply · easy
Less common
- Low system voltage · easy
- Faulty brake system control module · specialist
Symptoms
Components involved
Information, not instruction
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Community experience
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