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OBD-II · Engine and powertrain (P)

P0651

Sensor reference voltage B circuit/open

What it means

The 5 V reference supply B that the ECU sends to several sensors is out of range or open. Several sensor codes often appear at the same time.

Symptoms

  • several sensor fault codes at once
  • limp mode or reduced power
  • possible no start
  • engine warning light

Likely causes, most likely first

Likely causes, most likely first
No.Cause
01shorted sensor on the shared reference circuit (for example pressure, pedal or camshaft sensors)
02chafed or damaged wiring
03water in a sensor connector
04internal ECU fault

What to check, step by step

  1. 01Check which sensors share this reference circuit using the wiring diagram
  2. 02Measure the reference voltage (normally 5 V) at the sensors
  3. 03Unplug the sensors on that circuit one at a time: if the voltage recovers, the last sensor unplugged is shorted
  4. 04Inspect the wiring harness for chafing or water damage
  5. 05Suspect the ECU only if the reference stays wrong with all sensors unplugged and the wiring good