What does it mean?
P1578 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Brake Booster Vacuum Sensor Circuit Low Vacuum .
Symptoms
- Brake pedal becomes hard and requires much more effort, especially after repeated braking
- Stopping distance increases, which is a genuine safety concern and needs prompt attention
- Hissing noise from behind the pedal or from under the bonnet when braking
- Engine idle drops or stumbles when the brake is applied, if the booster is leaking
- Check engine light on and automatic stop-start disabled
- Warning message about brake assistance on vehicles that display it
Likely causes
- Leaking booster diaphragm, which lets vacuum bleed away as the pedal is used
- Failed one way check valve at the booster inlet
- Cracked, softened or collapsed vacuum hose between the engine or vacuum pump and the booster
- Worn or failing mechanical vacuum pump on turbocharged and direct injection engines
- Vacuum leak elsewhere in the intake system reducing available manifold vacuum
- Restricted vacuum line or a blocked fitting at the manifold
- Sensor reading correctly but the engine simply not producing vacuum at high altitude or under heavy load
How to fix it
- Treat this as a braking safety issue and verify pedal assist before returning the vehicle to use
- Connect a vacuum gauge at the booster line and compare the actual reading with the sensor value in live data
- Inspect the vacuum hose for cracks, softening and collapse, and confirm the check valve holds in one direction only
- Test the booster by pumping the pedal with the engine off, then starting the engine and feeling for the pedal to drop
- Measure vacuum pump output where one is fitted, and inspect its drive and seals
- Check the intake system for leaks that would reduce available vacuum
- Replace the failed hose, check valve, pump or booster, then clear the code and confirm strong pedal assist