OBD-II trouble code
P0559: Brake Booster Pressure Sensor Circuit Intermittent
The booster vacuum signal drops out and returns. Unlike most intermittent faults, this one usually tracks the weather rather than the road — and on a stop-start vehicle it produces a complaint that sounds nothing like a brake problem.
Quick facts
- System
- Powertrain
- Category
- Brakes / Vacuum Supply
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $0 – $480
- DIY difficulty
- Intermediate DIY
What does P0559 mean?
An intermittent code is stored when the module sees the signal briefly leave its valid range and return, without the fault lasting long enough to be classed as a hard low or hard high failure. The circuit is dropping out rather than sitting wrong, and by the time anyone connects a scan tool the reading is usually perfect.
What gives this particular intermittent its own character is the environment the connector lives in. The brake booster is bolted to the bulkhead behind the engine, underneath the cowl — the tray at the base of the windscreen that collects everything running off the glass and channels it to drains at either end. When those drains block, which happens on any vehicle that spends time under trees, the tray holds standing water instead of shedding it, and the connector sits in it. Water bridging the terminals is a fault; the water evaporating is the recovery.
That gives this code an unusual signature. Most intermittent faults are hunted by driving over rough ground or by wiggling harnesses, because most of them are mechanical. This one correlates with weather. Owners describe it in terms of days rather than roads: the light came on the morning after a storm, or after a car wash, and cleared by the middle of the week. If someone can tell you that, they have narrowed the diagnosis further than a scan tool will.
The reproduction test follows directly. Rather than a wiggle test, run water over the base of the windscreen with the key on and live data displayed and watch for the drop-out. It is the rare case where a garden hose is the right diagnostic tool.
The consequence side is where this code earns real attention, and it produces a complaint that sounds like something else entirely. On a vehicle with automatic stop-start, the module needs a valid vacuum reading to keep the engine switched off at a light. When the signal drops out mid-stop, the safe response is to restart immediately. The driver experiences the engine shutting off at a red light and then firing back up on its own a few seconds later, for no apparent reason, and reports it as a battery fault or a hybrid system fault. Vehicles have had charging systems and auxiliary batteries replaced over this. Connecting the restart complaint to a stored P0559 is the observation that avoids that.
One closing note on urgency. An intermittent on a steering pressure sensor is an inconvenience. An intermittent on the circuit that verifies brake assist reserve deserves more weight, not because the brakes will fail — the hydraulic system is mechanical and unaffected — but because a connection that is already opening and closing does not stay intermittent forever, and the fallback behaviours it triggers work the auxiliary vacuum pump harder than it was built for.
Common causes
- Water in the cowl tray intermittently bridging the connector terminals
- Blocked cowl drains, the underlying condition that keeps putting water where the connector is
- Corroded terminals that conduct when dry and open when damp
- Washer fluid residue in the connector shell that becomes conductive when it absorbs moisture
- Loose or corroded shared ground point serving several bulkhead-mounted sensors
- Chafed insulation on the harness section that passes through the firewall grommet
- Terminal pushed partly out of its lock during earlier cowl or wiper service, making marginal contact
- Sensor with an internal connection that opens as it heats and closes as it cools
Symptoms
- Check engine light that comes and goes, often following rain, snow melt or a car wash
- Stop-start shutting the engine off at a light and then restarting it unprompted seconds later
- Owner reporting a suspected battery or hybrid system fault that tests perfectly good
- Auxiliary vacuum pump running at unpredictable times
- Booster vacuum reading that flickers or momentarily disappears on live data
- Stored code with no fault present at the time of inspection
- Standing water or slow drainage visible in the wiper area during rain
- Brief brake pedal firmness during a stop, resolving on its own
Diagnostic steps
- 1.Ask when it happens before asking what it reads. An intermittent on this circuit usually correlates with wet weather rather than with rough roads, because the connector lives in the cowl tray where water off the windscreen is collected. If the owner can tell you the light comes on after rain or a wash and goes out a day or two later, you have most of the diagnosis already.
- 2.Reproduce it with water rather than with a wiggle test. Run a hose over the base of the windscreen for a few minutes with the key on and live data displayed, and watch for the reading to drop out. This is the reproduction test that works on this code and it is not one you would use anywhere else in the engine bay.
- 3.Check the cowl drains as part of the same inspection. If they are packed with leaf litter, water sits in the tray instead of draining through, and any repair that does not clear them is temporary. On many vehicles the drains are reachable from below with a length of trimmer line.
- 4.Separate the connector and look for water staining, silt lines and green corrosion on the terminals. Unlike the fretting wear you would find on a vibration-driven intermittent, this environment leaves visible deposits, and a tide mark inside the shell is direct evidence of the mechanism.
- 5.Consider the intermittent restart complaint if the vehicle has stop-start. A driver reporting that the engine starts itself again a few seconds after stopping at a light is describing this fault, not a battery or hybrid problem, and connecting those two observations early avoids an expensive detour into the charging system.
- 6.If the fault will not reproduce with water, log the circuit's reference voltage alongside the signal. A reference that dips at the same moment the signal drops out moves the search to a shared connection point rather than to this sensor's own wiring, and shared points in this area serve several devices at once.
Repair cost
$0 – $480
The cheapest genuine repair on this code is clearing blocked cowl drains and drying and cleaning the connector, which costs nothing but time and resolves a meaningful share of these. A shop diagnosis, including a water reproduction test, is $100 to $190 because it takes time to provoke a fault that is not present on arrival. Cleaning and re-terminating a corroded connector is $130 to $300. Repairing the harness where it passes the bulkhead grommet is $150 to $350. Sensor replacement at $150 to $400 is the least likely fix, since the sealed sensor is the part in this circuit with no exposed contact surfaces. Whatever else is done, clear the drains — a repaired connector under a tray that still fills with water is a repair with an expiry date.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with brake booster pressure sensor replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.