Does this sensor measure brake fluid pressure?

No, and this is the single most common misunderstanding about the code. It measures air pressure — specifically the vacuum — inside the brake booster, the round assembly between the pedal and the master cylinder that multiplies the force your foot applies. The hydraulic side of the brakes, the fluid, the lines and the calipers are all completely separate and are not monitored by this circuit. That is also why the code carries a P prefix: the engine controller owns the vacuum supply, not the ABS module.

Are my brakes safe with this code stored?

The brakes still work. Everything from the master cylinder outward is hydraulic and mechanical and does not depend on this sensor. What you lose is the module's ability to confirm there is enough vacuum reserve to give you power assist, so most vehicles fall back to conservative behaviour: run the auxiliary vacuum pump more, refuse to shut the engine off at stops, and warn you. Treat it as something to fix soon rather than something to ignore, because if the booster itself is losing vacuum you will find out with a hard pedal at an inconvenient moment.

Why did my stop-start feature quit working?

Because the module will not switch the engine off unless it can prove there is enough vacuum stored to give you at least one more assisted stop. This sensor is how it proves that. With no valid signal, the safe answer is to keep the engine running, so auto-stop is disabled without any message explaining why. It is a deliberate design choice rather than a second fault, and it usually returns on its own once the circuit is repaired.

How is this different from the other four codes in this family?

P0555 is the general one — the module has no usable signal at all and has not narrowed it down further. The other four are more specific: P0556 means the signal exists but does not make sense against what the engine is doing, P0557 means it is below the electrical floor, P0558 means it is above the ceiling, and P0559 means it comes and goes. If you have P0555 on its own, expect a dead sensor, a lost reference or an open circuit rather than a mechanical vacuum problem.

OBD-II trouble code

P0555: Brake Booster Pressure Sensor Circuit

A general fault on the sensor that measures vacuum inside the brake booster. It carries an engine code prefix and a brake system consequence, which is an unusual and important combination.

Medium severityPowertrainBrakes / Vacuum SupplyDrivable short-term

Quick facts

System: Powertrain
Category: Brakes / Vacuum Supply
Severity: Medium severity
Drivable: Usually safe to drive short-term
Repair cost range: $110 – $900
DIY difficulty: Intermediate DIY

What does P0555 mean?

The first thing to be clear about is what this sensor measures, because the name misleads almost everyone who reads it. It does not measure brake fluid pressure and it has nothing to do with the hydraulic side of the braking system. It measures the air pressure — in practice, the vacuum — inside the front chamber of the vacuum brake booster, the drum-shaped assembly bolted between the pedal and the master cylinder. P0555 is the general circuit code for that sensor: the module is not getting a usable output from it at all.

Older vehicles had no such sensor because they did not need one. A naturally aspirated engine with a throttle plate produces abundant manifold vacuum at idle, a check valve traps it in the booster, and the booster quietly multiplies pedal effort without anyone monitoring it. Two changes broke that arrangement. Turbocharged and direct-injection engines run much smaller throttle restrictions and spend time at positive manifold pressure, so vacuum becomes scarce and intermittent. And stop-start systems switch the engine off at every red light, which means the vacuum source disappears exactly when a driver may need to brake again.

Both problems are solved the same way: an auxiliary vacuum pump, either mechanical off the camshaft or electric, supplements what the engine makes. And a pump that runs on demand needs something to tell it when demand exists. That is this sensor. It is the module's only measure of how much assist reserve is left in the booster, which is why it also feeds the decision about whether an automatic engine stop is safe.

So the consequence of losing the signal is not electrical inconvenience. With no reading, most calibrations fall back to a conservative default: run the auxiliary pump more than necessary, refuse to auto-stop the engine, and in some cases light the brake warning lamp alongside the check engine light. Braking itself is not disabled — the booster still holds whatever vacuum reaches it, and the hydraulic system behind it is entirely mechanical — but the safety net that guarantees assist is available has been removed, and the fallback strategy quietly runs the auxiliary pump toward an early death.

One practical note that shapes the repair. On a good number of applications this sensor is not a separate part at all. It is integrated into the booster housing, or into the check valve that screws into it, and it is supplied only as an assembly. Confirm how the part is sold for the specific vehicle before quoting the job, because the difference between a standalone sensor and a complete booster is several hundred dollars.

Common causes

Failed brake booster pressure sensor with no output on the signal line
Sensor connector unlatched or damaged during cowl, wiper or master cylinder work
Open or shorted signal wire in the harness section that crosses the bulkhead
Loss of the 5-volt reference or the sensor ground at a shared connection point
Water intrusion into the connector from blocked cowl drains above the booster
Corroded terminals in a connector mounted low in the water-management area behind the engine
Engine controller output driver fault on the reference supply feeding this circuit
Sensor integrated into a check valve or booster housing that has failed as an assembly

Symptoms

Check engine light, sometimes accompanied by a brake system warning lamp
Automatic engine stop-start refusing to operate, with no explanation given to the driver
Auxiliary vacuum pump running frequently, for longer, or at times it normally would not
Firmer brake pedal than usual, particularly on the second or third application after shutdown
Live data showing no plausible booster vacuum value at any engine speed
Hill hold or brake hold features unavailable on vehicles that use them
No change in the reading when the brake pedal is applied and released
Cruise control or adaptive cruise dropping out on vehicles that require a valid assist reserve

Diagnostic steps

1.Establish what the vehicle actually has before diagnosing anything. Find out whether the sensor is a standalone part, part of the check valve, or built into the booster, and whether the vacuum source is the manifold alone, a camshaft-driven pump or an electric pump. Those three answers change the entire test plan and the cost of the repair.
2.Look at the live vacuum value with the engine idling and then press the brake pedal firmly two or three times. A healthy system shows a steady vacuum that dips with each application and recovers between them. A value that does not exist, reads as a flat zero, or never responds to the pedal confirms the module has nothing usable to work with.
3.Back-probe the connector with the key on and check for reference voltage and a good ground before condemning the sensor. A missing reference explains a dead signal completely, and it usually points at a shared connection rather than at this circuit alone.
4.Check whether the connector is wet or corroded. This sensor sits under the cowl behind the engine, which is where water shed off the windscreen is supposed to drain away. Blocked drains put standing water exactly where the connector lives, and the shell often tells the story before any meter does.
5.Scan for other codes from sensors in the same area. If several devices sharing a reference or ground point are reporting at once, fix the shared connection rather than treating each code as a separate fault.
6.Verify the vacuum supply mechanically with a hand pump or gauge at the booster check valve. This does not diagnose the electrical fault, but it tells you whether the booster is holding vacuum at all — which is the difference between a sensor that cannot report and a system that has nothing to report.

Repair cost

$110 – $900

Where this lands depends almost entirely on how the manufacturer sells the part. A standalone sensor is $25 to $230 and half an hour to two hours of labour, putting a typical job at $150 to $400 including diagnosis. Harness or connector repair behind the master cylinder is $120 to $300, and access is the reason it is not cheaper. The expensive outcome is an application where the sensor is integrated into the check valve or the booster housing: a complete booster is $200 to $900 in parts with two to four hours of labour, because the master cylinder has to come off and the brakes have to be bled afterwards. Establish which case you are in before authorising work.

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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.