C0549

Code

Generic C — Chassis

Vacuum Sensor A Circuit/Open

Brand: Generic

Type: C — Chassis

Views: UK: 25 EN: 38 RU: 22

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Malfunction indicator lamp (MIL) or chassis/system warning light illuminated
Loss or reduction of function in vacuum-dependent systems (may be reduced brake assist, EVAP control, or other vacuum-actuated features) depending on vehicle
Diagnostic trouble codes stored; freeze-frame data may show open-circuit condition
Intermittent or no change in vacuum sensor reading on a scan tool
Possible degraded engine idle or drivability if sensor is used by engine control

Read and record freeze-frame and diagnostic data with a scan tool; confirm C0549 is active or history
Visually inspect sensor, connector, and wiring for damage, corrosion, or disconnection
Backprobe sensor connector and verify reference voltage (typically 5 V) with key ON, engine OFF
Check sensor ground continuity to chassis ground (near 0 ohms)
Measure sensor output voltage while applying known vacuum (hand pump) or while engine is running
Perform continuity check between sensor connector and control module pin for open circuit

Typical sensor type: analog vacuum/pressure sensor using a reference (commonly 5 V) and ground with 0–5 V output (vehicle-specific)
Common output range: ~0.5–4.5 V across operating vacuum range (consult vehicle spec for exact values)
Open-circuit symptom: no output change, scanner shows circuit open or voltage at connector abnormally high or low (often 0 V or pulled to reference)
Reference supply: typically 5 V DC from control module (some systems may use 12 V or switched supply—check wiring diagram)
Ground: near 0 ohms between sensor ground pin and chassis ground
Resistance: open circuit will show infinite resistance between sensor signal pin and module input

Confirm code and review freeze-frame/scan-tool data. Note whether the code is active or history.
Inspect the sensor and harness visually for obvious damage, loose pins, corrosion, or water intrusion. Repair obvious issues and retest.
Locate wiring diagram for the vehicle to identify sensor pins: reference voltage, signal, and ground.
With ignition ON (engine OFF), backprobe the connector: verify reference voltage present (commonly 5 V) and proper ground continuity. If reference or ground missing, trace harness to fuse/module.
Measure signal voltage at the sensor connector while applying vacuum with a hand pump or while engine runs. If signal is absent or fixed, test continuity from the sensor signal pin to the module input.
If continuity is OK and reference/ground are present but signal still absent, substitute a known-good sensor (or bench-test sensor if possible).
Repair any wiring faults (replace damaged section, repair corroded terminal, clean connector pins) or replace the sensor if faulty.
Clear codes and perform a test drive or reproduce operating conditions to verify the repair. Re-scan to ensure C0549 does not return.
If all wiring and the sensor are good but fault persists, suspect control module input circuit; perform module-level tests or consult manufacturer procedures before replacing the module.