What it means
The vacuum pump control circuit B is below the expected electrical threshold. This indicates a low circuit condition, not necessarily a short to ground or failed winding.
At a glance
- Usually lights the check-engine light
How it works
Circuit B is the identified control circuit for the vacuum pump system; the code does not establish the pump design or its vehicle function. The module detects circuit B below its expected electrical threshold, a low-circuit condition that can arise from wiring or pump faults and does not by itself prove a short to ground.
Common causes
Most likely first.
- Control wire shorted to ground
- Internal short in pump motor winding
Symptoms
- Brake pedal may feel harder than usual
- HVAC airflow or operation may be reduced
How to diagnose it
- Review freeze-frame data and circuit B voltage or command readings to confirm when the low condition occurred.
- Inspect the circuit B wiring and pump connector for damaged insulation, corrosion, loose terminals, or contact with ground.
- Check the pump winding for an internal short, if the vehicle uses an electric pump, and compare results with the manufacturer's specification.
- Test circuit B for shorts to ground and excessive voltage drop using the manufacturer's procedure.
- If the wiring and pump pass, test the module's circuit B output for a low-voltage fault.
Common fixes
- Repair a damaged circuit B wire or connection causing the low reading.
- Clean or replace corroded pump connector terminals.
- Replace a pump with a confirmed shorted winding.
- Repair the module control output if testing isolates the low condition there.
Can I keep driving?
If the brake pedal feels harder than usual, leave extra stopping distance and do not continue driving until brake assist is checked. If braking feels normal, arrange diagnosis promptly and watch for changes in pedal effort or HVAC operation.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.