OBD-II trouble code
P0558: Brake Booster Pressure Sensor Circuit High Input
Signal voltage pinned at the ceiling on the booster vacuum sensor. The most valuable thing on this page is the code arithmetic: P0558 arriving alongside an unrelated cowl-area sensor code means one bad ground, not two bad sensors.
Quick facts
- System
- Powertrain
- Category
- Brakes / Vacuum Supply
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $0 – $800
- DIY difficulty
- Intermediate DIY
What does P0558 mean?
P0558 is stored when the booster pressure sensor's signal line sits at or above the top of its legitimate voltage window. As with most three-wire sensors, two quite different faults produce that reading: a short to a voltage source, or an open somewhere in the sensor's ground or signal path. The second is much more common, and the reason is that the module feeds the circuit through an internal pull-up, so anything that breaks the connection allows the line to drift to reference voltage and stay there.
The most useful thing to know about this particular code is what it means when it does not arrive alone. This sensor's ground and its reference feed are frequently shared with other devices mounted in the same area — the bulkhead and cowl are dense with connections, and manufacturers economise on ground studs. So a fault at one shared point takes several sensors offline at once. When P0558 appears in a list next to a code from a sensor that has nothing to do with brakes or vacuum, that pairing is the diagnosis: one connection, not several parts. Reading the code list as a set instead of as individual entries is worth a few hundred dollars here, and it is the single mistake most likely to turn this repair into a parts-replacement exercise.
Timing is the second lead. Opens are caused by disturbance, and this sensor lives in one of the most frequently disturbed spaces on the vehicle. The cowl comes off for wiper motor work. The cabin filter is often behind it. Windscreen replacement involves lifting the trim directly above it. The master cylinder is right there. Any of those jobs can leave a connector half-seated or a terminal pushed back out of its lock, and the code that results is this one. "What was done recently" is a better first question than "what should I measure."
The consequence side is where P0558 diverges from its low-side twin, and it carries a cost that is easy to miss. A full-scale reading on many calibrations is interpreted as no vacuum available. The module's response is to run the auxiliary vacuum pump — continuously, since the reading never improves no matter how long it runs. These pumps are designed for intermittent duty measured in seconds. Left running for weeks on end, they overheat, wear their vanes or diaphragms and fail. It is entirely possible to arrive with a $200 wiring fault and leave with a $600 pump as well, purely because the code was left stored. That, rather than any braking risk, is the reason to deal with this one promptly.
Common causes
- Open or high-resistance ground shared with other cowl and bulkhead sensors
- Terminal backed out of its lock in a connector disturbed during nearby service work
- Open in the signal wire between the sensor and the engine controller
- Sensor connector left unseated after wiper motor, cabin filter or master cylinder work
- Failed sensing element that has gone open internally
- Signal wire shorted to a switched 12-volt source through a pinched or chafed section
- Corrosion at a shared ground stud on the bulkhead raising its resistance under load
- Damaged harness from windscreen replacement, where trim above the sensor is lifted
Symptoms
- Check engine light with a second, apparently unrelated sensor code stored alongside it
- Auxiliary vacuum pump running constantly or cycling far more than normal
- Vacuum pump becoming audible from inside the cabin when it never used to be
- Booster vacuum value pinned at the top of its scale and unresponsive to the brake pedal
- Stop-start disabled and the engine refusing to shut off at lights
- Code appearing shortly after wiper, cowl, cabin filter or windscreen work
- Brake warning lamp illuminating alongside the check engine light on some vehicles
- Vacuum pump failing outright some weeks after the original code was first noticed
Diagnostic steps
- 1.Read the whole code list before you touch this circuit. A high-side fault here plus a fault from another sensor in the cowl or bulkhead area is a pattern, not a coincidence — those devices commonly share a ground stud or a reference feed, and one bad shared connection produces both codes. Fixing the shared point clears both; replacing two sensors clears neither.
- 2.Ask what was worked on recently. This sensor sits behind the master cylinder under the cowl, which is disturbed by wiper motor replacement, cowl panel removal, cabin filter changes and windscreen work. Open circuits do not appear spontaneously, so recent service in that space is a stronger lead than any measurement.
- 3.Measure the sensor ground with the circuit loaded rather than checking it for continuity. A ground that reads continuous with an ohmmeter can still drop several hundred millivolts under load, which is enough to push the output to full scale. Voltage drop testing finds this; a continuity beep does not.
- 4.Back-probe the signal pin and jumper it to a known good ground with the key on. If the value drops to the bottom of its range, the wire and the module are both healthy and the fault is upstream in the sensor or its ground. If it stays high, the open is between that point and the module.
- 5.Check the auxiliary vacuum pump duty on vehicles that have one. If the module has read a full-scale value as no vacuum available and is running the pump continuously, the pump is on a clock — note how long the fault has been stored, because a pump that has run flat out for months is likely to need replacing along with the circuit fault.
- 6.Inspect the connector for a backed-out or spread terminal rather than for corrosion. High-side faults on this sensor are dominated by lost connections, and a terminal that has been pushed out of its retainer during a nearby repair looks perfectly normal until the connector is separated and the pins are examined individually.
Repair cost
$0 – $800
A connector pushed home or a ground stud cleaned and retightened costs nothing in parts, and that is a genuine outcome here often enough to set the floor at zero. Diagnosis to get there is $90 to $170. Repairing an open in the signal wire or replacing a terminal runs $130 to $320, with access behind the master cylinder driving the labour. Sensor replacement is $150 to $400. The figure that turns this into an expensive repair is the auxiliary vacuum pump: a module that has read full scale as no vacuum and run the pump continuously for weeks can destroy it, adding $250 to $700. That cost is avoidable and it is the reason not to leave this code stored.
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.