Chassis, generic

C0548 Vacuum Sensor Supply Voltage Low

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

The supply reaching the vacuum sensor is present but sags below its lower limit. With a ratiometric sensor every output voltage scales with the feed, so an under-supplied sensor reports a wrong vacuum even if it is healthy. HELLA rates its brake booster vacuum sensor for 4.5 to 5.5 V and maps 0 to −1000 hPa onto 0.5 to 4.5 V; a feed near the bottom of that window already shifts the reading. On a Saab 9-5 the brake module reacts to a sensor supply under 4.4 V within 0.06 s. Typical causes are a weak module output, an extra load on a shared 5-V rail or high resistance in the feed wire. No maker documents C0548 publicly. C0547 covers an open or shorted supply, C054B and C054F low sensor signals.

Likely causes

Another sensor on the same 5-V rail drawing it down
Corroded contact or damaged feed wire adding resistance
Weak sensor supply output in the control module

Symptoms

  • Brake or ESC warning lamp on
  • Vacuum reading implausible in live data
  • Further codes for sensors on the same supply

Repair estimate

DifficultyMediumDIY possible
Parts and labour€30–300
Time0.3–2 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

vacuum sensorsensor supply

Related codes

C0547C054BC054F

References: hella.com, saabwisonline.com, bosch-mobility.com.