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C0561 — Vacuum Sensor Ab Correlation

Generic OBD-II · all makesChassis

Code C0561 indicates that the vehicle's control module has detected a correlation error between two vacuum sensor signals or between a vacuum sensor and an expected value. This often relates to the brake booster vacuum sensor circuit and can affect braking assist and engine performance.

Symptoms

What causes it — most likely first

1. Faulty vacuum sensor

The vacuum sensor itself may be defective, providing incorrect voltage readings to the control module.

2. Vacuum leak in brake booster or hoses

A leak in the brake booster, vacuum supply hose, or check valve can cause actual vacuum to differ from expected values.

3. Damaged or corroded wiring/connectors

Wiring issues such as shorts, opens, or high resistance in the sensor circuit can distort the signal.

4. Faulty brake booster

An internal leak in the brake booster diaphragm can lead to insufficient vacuum and trigger the code.

5. Clogged or restricted vacuum lines

Blockages in the vacuum lines can prevent proper vacuum from reaching the sensor or booster.

6. Control module failure (rare)

The engine control module (ECM) or ABS module may have an internal fault, but this is the least likely cause and should only be considered after all other possibilities are ruled out.

How it's diagnosed

  1. Use a scan tool to verify the C0561 code and check for any related codes. Record freeze frame data to understand the conditions when the code was set.
  2. Visually inspect all vacuum hoses, connections, and the brake booster for cracks, disconnections, or signs of leakage. Listen for hissing sounds with the engine running.
  3. Test the vacuum sensor by comparing its voltage output to a known good vacuum gauge while applying vacuum. Verify the sensor's signal on the scan tool matches actual vacuum.
  4. Check the sensor wiring and connector for damage, corrosion, or loose terminals. Test for proper reference voltage, ground, and signal continuity back to the control module.
  5. If the sensor and wiring are good, perform a brake booster leak test by pumping the brake pedal with the engine off, then starting the engine to see if the pedal drops. Inspect the booster check valve.
  6. As a last resort, verify the control module's inputs and outputs, but replacement should only be considered after all other components and circuits have been thoroughly tested and confirmed functional.

Code facts & related codes

SystemChassis
StandardizationSAE generic — identical on every make
Related codesC0559 — Power Steering Control Module RangeperC0560 — Brake Pressure Sensor Missing CalibratC0562 — Brake Pedal Position Sensor Supply VolC0563 — Abs Control Module PerformanceC0564 — Abs Control Module System Voltage LowC0565 — Abs Control Module System Voltage High

Frequently asked

What does the C0561 code mean?

It means the vehicle's computer has detected a mismatch between the vacuum sensor signal and another reference, often indicating a problem with the brake booster vacuum system.

Can I drive with a C0561 code?

It may be possible to drive, but the vehicle could experience reduced brake assist, making it harder to stop. It is safer to diagnose and repair the issue promptly.

Is the PCM/ECM a common cause of C0561?

No, control module failure is very rare for this code. Many drivers are incorrectly told to replace the PCM, but the problem is almost always a faulty sensor, vacuum leak, or wiring issue.

How do I fix a C0561 code?

Start by inspecting vacuum hoses and the brake booster for leaks. If none are found, test the vacuum sensor and its circuit. Replacing the sensor is often the fix, but only after confirming it is defective.