P0559 Brake Booster Pressure Sensor Circuit Intermittent

The P0559 trouble code indicates that the vehicle’s control module has detected an intermittent electrical signal from the brake booster pressure sensor circuit.

The brake booster helps reduce the amount of physical effort required to press the brake pedal. Depending on the vehicle, the booster may use engine vacuum, an electric vacuum pump, hydraulic pressure, or another assist system.

The pressure sensor allows the vehicle to monitor the pressure or vacuum associated with the brake booster system. The control module uses this information to determine whether the brake-assist system is operating as expected.

With P0559, the important word is "Intermittent." The control module is not necessarily seeing a permanently high or permanently low signal. Instead, the sensor signal is dropping out, changing unexpectedly, becoming implausible, or intermittently losing electrical continuity.

What Does "Circuit Intermittent" Mean?

P0559 is an electrical circuit diagnosis rather than a direct statement that the brake booster has mechanically failed.

An intermittent sensor circuit can behave normally for several minutes and then suddenly produce an incorrect signal.

For example, the control module may see a normal brake-booster pressure signal, then suddenly see the signal disappear or jump to an implausible value before returning to normal.

This can be caused by:

• A failing pressure sensor

• A loose connector

• Corroded terminals

• A damaged wire

• An intermittent ground

• An unstable reference voltage

• A poor sensor power supply

• Wiring that changes position with engine movement

• A problem inside the brake booster pressure-sensing system

The code therefore needs to be diagnosed as a signal problem first, rather than immediately assuming that the brake booster itself needs replacement.

What Does the Brake Booster Pressure Sensor Do?

The exact design varies between vehicles.

In a vacuum-assisted braking system, the sensor may monitor the vacuum or pressure difference associated with brake-booster operation.

In other systems, the sensor may be integrated into or associated with an electronically controlled brake-assist or electric vacuum system.

The control module can use the sensor information to determine whether sufficient brake assistance is available.

If the expected pressure or vacuum signal is missing or inconsistent, the vehicle may recognize that something is wrong with the brake-assist system.

This is why P0559 should not be dismissed simply because the brake pedal still feels normal.

Common Causes of P0559

Faulty brake booster pressure sensor

The sensor itself can develop an intermittent internal fault.

It may produce a correct signal when cold and become unstable as it warms up, or it may fail when subjected to vibration.

Loose or damaged connector

A connector that is not fully locked can intermittently interrupt the sensor circuit.

Corrosion or contamination can also increase resistance and cause the signal to become unstable.

Damaged wiring

A wire can be internally broken while its insulation still looks intact.

Engine vibration can move the harness enough to make the electrical connection disappear temporarily.

Poor ground

Many pressure sensors depend on a stable ground reference. A poor ground can cause the sensor output to become erratic.

Unstable sensor supply or reference voltage

Depending on the system, the sensor may receive a regulated supply or reference voltage.

If that voltage becomes unstable, the sensor output can appear incorrect even when the sensor itself is healthy.

Mechanical vibration

A loose harness, poorly secured connector, or damaged wiring near the brake booster can produce an intermittent electrical problem when the engine moves or when the vehicle travels over bumps.

Brake booster or vacuum-system problem

A genuine pressure or vacuum problem can sometimes result in an implausible sensor signal.

However, P0559 specifically identifies an intermittent circuit condition, so the electrical side should be investigated carefully rather than automatically blaming the mechanical brake booster.

Control-module problem

A fault in the module's sensor input circuit is possible but should generally be considered only after the sensor, wiring, power, ground, and connector have been verified.

Symptoms You May Notice

The symptoms depend on the vehicle's brake-assist design and how frequently the signal becomes invalid.

Possible symptoms include:

• Brake warning light

• Check Engine Light

• ABS or brake-system warning

• Cruise control becoming unavailable

• Intermittent change in brake-pedal feel

• Harder brake pedal in some situations

• Reduced brake assistance

• Longer or more difficult braking effort

• Brake booster or vacuum-pump operating abnormally

• P0559 returning intermittently

It is also possible for the vehicle to store P0559 with no obvious change in braking performance, particularly when the electrical signal only drops out for a short period.

Why an Intermittent Fault Can Be Difficult to Find

A permanent electrical fault is usually relatively easy to reproduce.

An intermittent fault is different.

The sensor may work perfectly when the vehicle is sitting in the workshop.

The problem might only appear:

• When the engine is hot

• During vibration

• Over rough roads

• During braking

• During acceleration

• When the harness moves

• After the vehicle has been driven for a certain amount of time

• When a connector reaches a particular position

This is why simply scanning the vehicle and seeing that the code is stored is not enough.

The technician needs to determine what changed when the fault occurred.

Checking Live Data

A scan tool capable of displaying brake-booster pressure or vacuum information can be extremely useful.

Monitor the sensor value while:

• The engine is idling

• The brake pedal is pressed

• The brake pedal is released

• The engine speed changes

• The vehicle is driven

• The harness and connector are carefully moved where safe and appropriate

The exact expected value depends on the vehicle.

The important thing is whether the sensor signal behaves smoothly and plausibly.

If the reading suddenly drops out, jumps to an impossible value, or freezes while the physical conditions are changing, the sensor circuit deserves close attention.

Inspecting the Connector

The sensor connector should be examined carefully.

Look for:

• Corrosion

• Moisture

• Loose terminals

• Damaged locking mechanisms

• Bent pins

• Terminals pushed backward into the connector

• Contamination

• Signs of previous repairs

A connector can appear connected while one terminal has insufficient contact pressure.

This type of fault can be especially difficult to find because vibration can temporarily restore or interrupt the connection.

Checking the Wiring

The wiring should be inspected from the brake booster pressure sensor toward the relevant control module.

Pay particular attention to areas where the harness:

• Runs close to the engine

• Passes near moving components

• Is exposed to heat

• Can rub against brackets

• Moves with the engine

• Is close to the brake booster or vacuum equipment

If the wire is internally damaged, gently moving the harness while monitoring the sensor signal may cause the fault to appear.

This type of wiggle test should be performed carefully and according to safe diagnostic procedures.

Checking Sensor Power and Ground

If the sensor uses a dedicated supply, reference, signal, and ground circuit, each should be tested according to the manufacturer's wiring diagram.

Do not assume that every brake-booster pressure sensor uses the same wiring configuration.

A typical electronic pressure sensor may have:

• Power/reference

• Ground

• Signal

But the actual arrangement varies between manufacturers and systems.

The important thing is to verify that the sensor receives a stable supply and ground and produces the expected signal.

Voltage-Drop Testing Can Reveal Hidden Problems

A continuity test may show that the wiring is technically connected, yet the circuit can still have excessive resistance.

This is why voltage-drop testing can be valuable.

A poor ground connection, corroded terminal, or damaged wire may behave normally with almost no current flowing through it but become problematic during actual operation.

If the sensor supply or ground changes when the circuit is loaded or the harness is moved, the wiring or connection should be investigated.

Could P0559 Mean the Brake Booster Is Bad?

Not necessarily.

This is one of the most important points when diagnosing this code.

P0559 identifies an intermittent problem in the brake booster pressure sensor circuit.

That does not automatically mean the mechanical brake booster has failed.

If the sensor is reporting an unstable electrical signal, replacing the entire brake booster without testing the sensor circuit could be an unnecessary and expensive repair.

The sensor, connector, wiring, power, ground, and actual pressure/vacuum conditions should be evaluated before condemning the booster.

What About a Vacuum Leak?

A vacuum leak can certainly affect a vacuum-assisted brake booster system, but a vacuum leak and an intermittent electrical circuit are not the same diagnosis.

If the vehicle uses engine vacuum for brake assistance, the technician should determine whether:

1. The actual vacuum/pressure is incorrect, or

2. The actual pressure/vacuum is normal but the sensor is reporting it incorrectly or intermittently.

That distinction can prevent unnecessary parts replacement.

If the vehicle uses an electric vacuum pump, the pump and its control system should also be evaluated according to the manufacturer's diagnostic procedure.

P0559 and Cruise Control

Although P0559 is not itself a cruise-control code, brake-booster information can be important to other vehicle systems.

Cruise control is designed to disengage when the vehicle detects certain braking-system problems.

Therefore, a brake-assist or brake-pressure sensor fault can sometimes result in cruise control being disabled.

This does not mean that cruise control is the original cause of P0559. The brake-assist system should be diagnosed independently.

Can You Drive With P0559?

This code deserves more caution than a typical convenience-system DTC because it involves the brake-assist system.

If the brake pedal feels normal and the code appears to be an intermittent electrical fault, the vehicle may still have normal braking performance.

However, if you notice:

• A hard brake pedal

• Reduced brake assistance

• Increased pedal effort

• Brake warning lights

• ABS/brake-system warnings

• Unusual brake behavior

the vehicle should be inspected promptly.

If braking assistance is actually reduced, continuing to drive should not be treated as a minor issue.

Remember that the basic hydraulic brakes may still function even if the booster assist is reduced, but the driver may need substantially more pedal force.

The Most Important Point About P0559

P0559 means the control module has detected an intermittent problem with the brake booster pressure sensor circuit.

The code does not automatically mean that the brake booster itself has failed.

The fault may be caused by the pressure sensor, connector, wiring, power supply, ground, reference circuit, or an actual pressure/vacuum problem.

The best diagnostic approach is to monitor the sensor's live data, inspect the connector and wiring, verify power and ground, perform appropriate voltage-drop tests, and compare the sensor's behavior with the manufacturer's specifications.

Because this system is directly related to brake assistance, any change in brake pedal feel or braking effort should be taken seriously rather than treating P0559 as merely another electrical warning.