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DTC codes / C05xx · Brakes, steering and suspension / C0548

C0548Vacuum Sensor Supply Voltage Low

The C0548 code indicates that the vehicle's computer has detected an abnormally low voltage on the supply circuit for a vacuum sensor, which is used to monitor vacuum pressure in systems such as the brake booster or EVAP. The computer expects a certain voltage range on the sensor's power feed, and a reading below the threshold triggers this code.

SystemChassis
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaBrakes, steering and suspension

Symptoms you may notice

  • Illumination of the check engine light or service vehicle soon light
  • Often no symptoms besides the warning light
  • Reduced engine performance or poor fuel economy if the vacuum sensor affects engine load calculations
  • Erratic operation of vacuum-assisted systems like power brakes, leading to a hard brake pedal
  • Possible stalling or rough idle if the sensor influences air/fuel mixture
  • Transmission shift quality issues if vacuum is used for shift control

Common causes, most likely first

  1. A faulty vacuum sensor with an internal short or degraded circuitry
  2. Damaged or corroded wiring in the sensor's supply circuit, including open or high-resistance connections
  3. Poor electrical connection at the sensor or computer connector, such as loose pins or corrosion
  4. Low system voltage due to a weak battery or failing alternator, affecting the reference voltage
  5. A short to ground in the supply wire, pulling the voltage below the expected level
  6. A malfunctioning powertrain control module (PCM) providing an incorrect reference voltage

How to diagnose and fix it

  1. Use a scan tool to verify the code and check for any related codes or freeze-frame data
  2. Perform a visual inspection of the vacuum sensor and its wiring harness for obvious damage or corrosion
  3. Measure the reference voltage at the sensor connector with a multimeter and compare to specifications
  4. Check the sensor's ground circuit for integrity and proper continuity to the chassis
  5. Test the sensor's signal output with a vacuum gauge and scan tool to confirm sensor operation
  6. Inspect and test the supply wire for shorts to ground or high resistance using a wiring diagram
  7. If all circuits test good, replace the vacuum sensor and clear the code to see if it returns

Don't overlook the simple electrical checks

  • A low voltage code for a vacuum sensor is more often caused by a poor connection or corroded wire than by a failed sensor itself.
  • The vacuum sensor may share a reference voltage circuit with other sensors, so a fault in one can affect multiple readings and set related codes.
  • A weak battery or charging system can cause a low reference voltage that triggers this code without any sensor fault.
  • After replacing a sensor, always clear the code and perform a road test to ensure the fix is complete and the code does not reset.
  • Some vehicles use the vacuum sensor for brake assist, so a low voltage can affect braking performance and should not be ignored.

Can I keep driving with C0548?

It is generally safe to drive with this code if the vehicle runs normally, but avoid heavy braking or towing until the fault is repaired because vacuum-assisted brakes may be compromised. If you notice a hard brake pedal or engine performance issues, have the vehicle inspected promptly.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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