OBD-II Code P0555 — What It Means & How to Fix It

What Does This Error Mean?

P0555 means the PCM detected a fault in the brake booster pressure sensor circuit. The brake booster uses vacuum or hydraulic pressure to multiply your braking force — making it easier to stop the vehicle. A faulty pressure sensor means the PCM cannot monitor booster function. This code is considered safety-critical — braking performance may be reduced. Do not ignore this code.

Affected Models

Vehicles with vacuum-assisted brake boosters and electronic pressure monitoring — many 2005 and newer models
Hybrid vehicles with electric vacuum pumps for brake boost — Toyota Prius, Honda Insight, Ford Escape Hybrid
Turbocharged vehicles that use intake vacuum for brake boost
Trucks and SUVs with large brake boosters and dedicated pressure sensors
European vehicles with electrohydraulic brake boost systems

Common Causes

Brake booster pressure sensor has failed — internal fault in the sensor sending an out-of-range signal
Vacuum leak in the brake booster hose or at the booster check valve — reducing vacuum pressure below sensor threshold
Wiring fault in the sensor circuit — broken wire, corroded connector, or short to ground or voltage
Brake booster check valve has failed — not holding vacuum, causing pressure to drop rapidly
On hybrid vehicles, the electric vacuum pump has failed and is not maintaining adequate booster pressure

How to Fix It

Check your brake pedal feel immediately. If the pedal feels hard, requires more force than normal, or if stopping distance has increased, treat this as an emergency. Do not drive the vehicle until it is inspected.

A hard brake pedal is a sign of reduced or lost brake boost. In normal traffic this may still allow stopping, but emergency braking performance is compromised.
With the engine off, pump the brake pedal 5 to 6 times to exhaust any stored vacuum. Then press the pedal firmly and start the engine. The pedal should drop slightly as engine vacuum builds. If it does not move, the booster or vacuum supply has failed.

This test only works on vacuum-boosted systems. Hybrid vehicles with electric pumps may behave differently.
Inspect the vacuum hose running from the engine intake to the brake booster. Check for cracks, kinks, loose connections, or collapsed sections. Replace any damaged hose with the correct diameter vacuum hose.

The booster vacuum hose is typically a large-diameter reinforced hose. A cracked hose will cause both a vacuum leak and trigger the pressure sensor fault.
Test the booster check valve. Remove it from the booster and blow through it in both directions — it should allow airflow in only one direction. A valve that passes air both ways or is blocked completely has failed and needs replacement.

A failed check valve is a common cause of P0555. The valve is inexpensive and straightforward to replace.
If the vacuum supply and check valve are good, inspect the brake booster pressure sensor connector and wiring. A scan tool can show the sensor's voltage reading — an out-of-range reading with good vacuum supply points to a faulty sensor requiring replacement.

On hybrid vehicles, also check the electric vacuum pump operation using a scan tool — the pump should cycle to maintain booster vacuum.

When to Call a Professional

Brake system faults are safety-critical — if your brake pedal feels hard or your stopping distance has increased, stop driving and call for service immediately. Brake booster and vacuum system diagnosis is best performed by a qualified technician. Expect $150 to $600 for diagnosis and repair depending on the root cause.

Frequently Asked Questions

Is it safe to drive with P0555?

Only if your brakes feel completely normal. If braking feels harder than usual or your stopping distance has increased, do not drive the vehicle. Even if braking feels fine, get this code diagnosed promptly — a brake system fault is not something to delay.

What does the brake booster pressure sensor actually do?

It monitors the vacuum or hydraulic pressure inside the brake booster. The PCM uses this data to verify the booster has enough pressure to multiply braking force. On hybrid vehicles, the PCM also uses this reading to control the electric vacuum pump — switching it on when pressure drops too low.

Can P0555 turn on my ABS or traction control warning light?

Yes — on many vehicles, brake system faults trigger multiple warning lights including ABS and stability control warnings. Those systems rely on consistent braking performance, so a booster fault may disable them as a precaution.