OBD-II code C0550 indicates that the powertrain control module (PCM) has detected a voltage signal from the vacuum sensor B circuit that is above the expected range. This typically points to an electrical fault such as a short to voltage, an open ground, or a failed sensor, rather than an actual high vacuum condition.
Internal short to voltage within the sensor causes the signal line to read high, often due to sensor element failure or contamination.
The sensor signal wire may be chafed or pinched, making contact with a 5V reference wire or battery voltage, pulling the signal high.
A broken or corroded ground wire to the sensor prevents proper voltage drop, causing the signal to float near reference voltage and be interpreted as high.
Loose, corroded, or backed-out terminals in the sensor connector can create high resistance or an open circuit, leading to an elevated signal reading.
Chafing, melting, or rodent damage to the harness can cause shorts between wires or open circuits in the sensor wiring.
An internal fault in the PCM’s input circuit for the vacuum sensor can falsely read a high voltage, but this is the least likely cause and should only be considered after exhaustive wiring and sensor checks.
Short-distance driving may be possible, but drivability issues like stalling or poor engine performance can occur. It is safer to diagnose and repair the problem promptly.
Yes, the vacuum sensor is the most common cause of this code. However, a thorough inspection of the wiring and connector should be done before replacing the sensor to avoid misdiagnosis.
PCM failure is extremely rare for this code. The vast majority of cases are resolved by repairing wiring or replacing the sensor. Only consider the PCM after all other possibilities have been ruled out by a professional.
It measures vacuum in a specific part of the engine or brake system, providing data to the PCM for engine management, emissions control, or brake assist monitoring. The exact function depends on the vehicle model.