P0559 indicates an intermittent signal from the brake booster pressure sensor circuit to the powertrain control module. The sensor measures vacuum pressure in the brake booster to help the PCM determine brake‑assist needs; an unstable or missing signal triggers the code.
The sensor itself may develop an internal intermittent open or short, causing erratic voltage output to the PCM.
Wiring harness chafing, pin corrosion, or a loose connector can produce an intermittent circuit interruption.
A wire that intermittently contacts chassis ground or breaks internally will cause the sensor signal to drop out.
Leaks can cause rapid pressure fluctuations that the sensor interprets as an intermittent signal, even if the sensor is functional.
A failing control module could misinterpret the sensor signal, but this is rare and should be considered only after sensor and wiring are verified good.
No. PCM failure is uncommon for this code; the sensor, its wiring, or a vacuum leak are far more likely causes.
Driving is possible but brake assist may be reduced or inconsistent, increasing stopping distance; caution is advised, especially in emergency braking situations.
With the key on and engine off, measure the sensor’s signal voltage at the connector and compare it to the specified range; a fluctuating or out‑of‑range reading indicates a problem.
A leaking booster can create erratic pressure readings that the sensor interprets as an intermittent circuit fault, so the booster should be inspected for leaks.