P1578 on a GM vehicle means the powertrain control module (PCM) has detected a low vacuum condition at the brake booster vacuum sensor, indicating that the engine is not producing enough vacuum or that vacuum is not reaching the booster as expected. The PCM compares the sensor signal against expected values for the current engine operating conditions and sets this code when the vacuum level falls below the calibrated threshold.
Guidance is general and written for GM vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.
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