C0004: TCS Control Channel B Valve 2
What to do
Your ordinary brakes still work — this is the anti-lock assistance switching off, not the brakes failing. Stopping distances get longer on slippery surfaces and the wheels can lock under hard braking, which also takes away your steering while you brake. Leave more room, brake earlier, and take extra care in rain, snow or on gravel. Have it looked at soon; in most countries the car will fail its roadworthiness test with this light on.
What this code means
Traction control channel B valve 2 has a circuit fault.
Traction control is ABS run backwards. ABS stops a wheel locking under braking; traction control stops a wheel spinning under power — and it does it with the same hydraulics, by applying the brake to the spinning wheel so the differential sends torque to the one that grips.
That needs valves ABS does not. ABS can only ever reduce the pressure a driver has already applied. Traction control has to create pressure at a wheel with the driver's foot nowhere near the brake, so the modulator carries extra valves to admit pump pressure into a channel on its own. These codes name those valves.
A channel is a hydraulic circuit, not a wheel. Channels are usually paired diagonally or by axle, so one channel's fault can affect two corners — which is why the code names a channel and a valve number rather than a corner.
The most important thing first: the car still stops.
ABS, traction control and stability control are all additions to a braking system that works without them. When the module finds a fault it switches itself off and hands back a conventional, fully hydraulic brake system — the pedal, the master cylinder, the lines and the calipers are untouched. You have not lost your brakes.
What you have lost is the help. Without ABS the wheels can lock under hard braking, which lengthens the stopping distance on a slippery surface and — the part people forget — takes away your ability to steer while braking hard, because a locked wheel cannot generate cornering force. Without stability control the car will not catch a slide for you.
So drive it, but drive it as though it were forty years old: brake earlier, leave more room, and be especially careful in rain, snow and on gravel.
Likely causes
Start here
- Faulty ABS hydraulic modulator · specialist
- Corroded or loose connector · specialist
- Damaged wiring harness · specialist
Less common
- Faulty stability control module · specialist
- Poor module ground connection · specialist
Symptoms
Components involved
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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