Chassis, generic

C0561 Vacuum Sensor A/B Correlation

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

Where two vacuum signals describe the brake booster supply, the module expects them to track each other; C0561 means sensor A and sensor B disagree beyond the allowed band. No maker documents this code number publicly. The principle is patented: Ford compares the booster vacuum sensor with a second reservoir sensor after the vacuum pump stops and flags a sensor mismatch; GM checks the booster sensor against manifold vacuum at closed throttle and marks it biased high or low after set times. Sensors such as HELLA's run on 5 V and output 0.5–4.5 V. GM uses C0561 differently: in its brake modules it means stability control was disabled because another module reported a fault. Brake assist and stop/start may be limited.

Likely causes

One vacuum sensor drifting or reading offset
Leaking or kinked vacuum hose or check valve between the two measuring points
Corroded connector or damaged wiring on one sensor
Unstable 5 V reference or ground shared by the sensors

Symptoms

  • Brake or stability warning lamp
  • Electric vacuum pump runs too often or not at all
  • Stop/start or brake assist unavailable on some cars

Repair estimate

DifficultyMediumDIY possible
Parts and labour€20–300
Time0.3–1.5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

vacuum sensor avacuum sensor b

Related codes

C0547C0549C054AC054DC054EP0556

References: patents.google.com, patents.google.com, hella.com, bosch-mobility.com.