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DTC codes / C05xx · Brakes, steering and suspension / C0566

C0566Brake Booster Solenoid Circuit/Open

C0566 indicates that the electronic brake control module has detected an open circuit or excessive resistance in the brake booster solenoid circuit, which is used on some vehicles to control vacuum or hydraulic assist to the brake booster. The module monitors the solenoid's electrical path and sets this code when the expected current flow is not present, often disabling the associated function and illuminating the brake warning light.

SystemChassis
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaBrakes, steering and suspension

Symptoms you may notice

  • Brake warning light or red brake system indicator illuminated on the dashboard
  • Often no symptoms besides the warning light, with normal brake pedal feel and stopping performance
  • A slightly harder or less responsive brake pedal if the solenoid fails to provide assist
  • Audible click or lack of click from the brake booster area when the ignition is turned on or the brake pedal is pressed
  • Reduced or disabled electronic brake assist features such as hill-start assist or emergency brake assist
  • Intermittent warning light that comes and goes with vibration or temperature changes

Common causes, most likely first

  1. Broken, corroded, or disconnected wire in the brake booster solenoid circuit
  2. Failed brake booster solenoid coil (open winding or internal short)
  3. Corroded or loose electrical connector at the solenoid or at the brake control module
  4. Blown fuse or faulty relay that supplies power to the brake booster solenoid
  5. High resistance in the ground circuit for the solenoid or control module
  6. Faulty brake control module driver circuit, though this is less common than wiring or solenoid failure
  7. Aftermarket accessories or poor previous repairs that disturbed the solenoid wiring harness

How to diagnose and fix it

  1. Verify the code with a scan tool and check for any related brake system codes before clearing anything
  2. Inspect the brake booster solenoid connector and wiring for corrosion, chafing, or loose pins, and repair as needed
  3. Check the fuse and relay that feed the brake booster solenoid circuit, replacing any failed component
  4. Measure resistance across the solenoid coil with a multimeter and compare to the manufacturer's specification
  5. Perform a voltage drop test on the power and ground sides of the solenoid circuit while commanding the solenoid on with a scan tool
  6. If wiring and power are good but the solenoid does not actuate, replace the brake booster solenoid or the booster assembly as required
  7. Clear the code and perform a road test to confirm the repair, ensuring the brake warning light stays off and assist functions work

What a brake booster solenoid code hides

  • A brake booster solenoid circuit code can be set by a simple blown fuse that also powers other brake components, so always check fuses before condemning the solenoid.
  • The solenoid is often part of the brake booster assembly on many vehicles, meaning a failed solenoid may require replacing the entire booster rather than a separate serviceable part.
  • Intermittent open circuits frequently occur at the connector where the harness flexes near the booster, and a wiggle test while watching live data can reveal the fault.
  • This code does not always mean the solenoid itself has failed; a poor ground connection at the chassis can create the same open-circuit condition.
  • Some vehicles use the brake booster solenoid for emergency brake assist, so a fault may disable that feature without affecting normal braking, leading drivers to ignore the warning light.

Can I keep driving with C0566?

The vehicle remains safe to drive in most cases because the base hydraulic brakes still function, but you should avoid towing or heavy loads and schedule diagnosis soon. If the brake pedal feels unusually hard or stopping distance increases, stop driving and have the vehicle towed.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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