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P1578

BBV Sensor Circuit Low Vacuum.

  • GM
  • Powertrain
  • Manufacturer code
  • High severity

How this code is built

P
SystemPowertrain
1
Defined byThe manufacturer
5
AreaVehicle speed, idle control and auxiliary inputs
78
FaultThe specific fault in that area

What it means

On GM vehicles, P1578 means the control module has detected a low vacuum condition on the brake booster vacuum (BBV) sensor circuit. The module compares the sensor signal with expected engine vacuum and found that the booster is not getting or holding enough vacuum. The cause may be a vacuum supply fault or a sensor or wiring fault, and the details depend on the model and engine.

Symptoms

  • Hard or stiff brake pedal, needing more effort than normal
  • Brake pedal feels high or firm after the engine has been running
  • Hissing noise from under the dash or at the booster when braking
  • Rough or unstable idle when the brake pedal is pressed
  • Brake warning or service message and a stored fault code
  • Check engine light on

Possible causes

  • Leaking or split vacuum hose between the intake or vacuum pump and the brake boosterlikely
  • Faulty or cracked check valve at the brake boosterlikely
  • Faulty brake booster vacuum sensorpossible
  • Wiring or connector fault in the sensor circuit (open, short, corrosion or chafing)possible
  • Leaking brake booster diaphragm or master cylinder sealpossible
  • Weak vacuum supply from the engine or vacuum pump, where fitted, because of an engine fault or a failing pumprare

How to diagnose

  1. Read all stored codes and freeze frame data, noting engine speed, load and whether other vacuum or brake codes are present
  2. Visually inspect the booster vacuum hose, fittings and check valve for splits, kinks, loose clips and oil contamination
  3. Inspect the sensor connector and wiring for damage, corrosion, water ingress and poor pin tension
  4. With the engine off, pump the brake pedal to clear the vacuum, then hold the pedal down and start the engine. The pedal should sink slightly; if it does not, suspect the booster or its vacuum supply
  5. Check for a hiss at the booster or hose when the pedal is pressed, and listen for leaks at idle
  6. Test the check valve for one-way flow, following the manufacturer's procedure
  7. Measure actual vacuum at the booster with a vacuum gauge and compare it with live sensor data and the specification for that model
  8. Check the sensor supply, ground and signal with a multimeter against the wiring diagram for the specific vehicle

How to fix

  • Replace any leaking, split or perished vacuum hose and refit clips securely
  • Replace a faulty check valve (or the hose assembly if the valve is integral)
  • Repair damaged wiring or connectors in the sensor circuit
  • Replace the brake booster vacuum sensor if its signal is out of range with good wiring and good vacuum
  • Replace the brake booster or master cylinder if internal leaks are confirmed
  • Repair the underlying engine vacuum or vacuum pump fault if supply is low

Can I keep driving?

High severity

Low vacuum reduces power assistance, so the brakes will still work but need much more pedal force and stopping distances may increase. Avoid driving until the fault is checked, or limit it to a short, careful trip to a workshop.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.

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