What does it mean?
P1577 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Brake Booster Vacuum Sensor Circuit Low Input .
Symptoms
- Check engine light on with the code stored
- Automatic stop-start disabled and a warning message displayed
- Other pressure sensors sharing the same reference may set codes at the same time
- Idle speed may be raised by the module as a precaution
- Brake performance is normally unaffected, since the booster itself is not the problem
- Fault may appear immediately after work has been carried out in the engine bay
Likely causes
- Signal wire shorted to ground where the harness has rubbed through
- Loss of the 5 V reference voltage feeding the sensor, which often affects other sensors at the same time
- Open circuit in the signal wire or a terminal pushed out of the connector
- Internally failed sensor producing no output
- Corrosion or moisture bridging the pins in the sensor connector
- Connector left unplugged after brake or engine work
- Faulty reference supply inside the control module
How to fix it
- Check whether other sensors sharing the 5 V reference have also set codes, which points to a shared reference fault
- Confirm the sensor connector is fully seated and undamaged
- With the ignition on, measure the reference voltage and ground at the harness side of the connector
- Backprobe the signal wire and check whether it is sitting at zero volts, indicating a short to ground or an open
- Test the signal wire for continuity to the module and for a short to ground with both ends disconnected
- Apply vacuum with a hand pump and confirm the live data value responds
- Replace the sensor once the wiring and reference supply are proven good, then clear the code and retest