Computer & output circuits · Powertrain · generic (SAE-defined)
P0651: Sensor Reference Voltage "B" Circuit/Open
P0651 means the engine computer found a problem in sensor reference voltage circuit "B", a second regulated supply that powers a separate group of sensors.
Standard title per SAE J2012 / ISO 15031-6 · Updated 2026-11-07 · By CarWhere Research
How P0651 sets
Many powertrain control modules (PCMs) use more than one regulated reference supply, typically 5 volts, so a single short does not disable every sensor. The PCM monitors reference "B" and sets P0651 when the voltage is too low, too high or appears open. The sensors assigned to circuit "B" vary by manufacturer and engine.
Common causes of P0651
Roughly most likely first; the actual cause depends on the vehicle and needs testing to confirm.
- A shorted sensor powered by reference "B"
- Reference "B" wiring shorted to ground or chafed through
- Corroded or damaged sensor connectors
- Harness damage from heat or rodents
- Internal PCM fault
P0651 symptoms
- Check engine light on
- Reduced engine power
- Several sensor codes appearing together
- Rough running, stalling or hard starting
- Electronic throttle warning light on some vehicles
How P0651 is diagnosed
- Record freeze frame data and every code present, especially sensor circuit codes
- Identify the sensors on reference "B" from factory wiring information
- Check reference voltage at those sensor connectors with the key on
- Disconnect each sensor in turn to see whether the reference voltage recovers
- Inspect the harness for chafing, heat damage or rodent damage
- Test the PCM output only after the sensors and wiring are cleared
Typical fixes for P0651
- Replace the shorted sensor
- Repair damaged reference wiring
- Repair or replace corroded connectors
- Replace and program the PCM if the internal supply has failed
Replace parts only after testing confirms the cause — a code names the circuit or condition that failed a check, not necessarily the faulty part. We don’t publish per-repair prices; for what households actually spend on upkeep, see car repair costs.
Related codes & warning lights
P0651 FAQ
What does code P0651 mean?
P0651 means the engine computer found a problem in sensor reference voltage circuit "B", a second regulated supply that powers a separate group of sensors. The standard definition is "Sensor Reference Voltage "B" Circuit/Open."
Can I drive with a P0651 code?
Drive only as far as needed to reach a repair shop. Depending on which sensors share reference "B", the vehicle may go into reduced power, run poorly or stall.
What is the difference between P0641 and P0651?
They point to different reference supplies, "A" and "B". Each powers its own group of sensors, so the affected sensors and symptoms differ between the two codes.
Can a bad accelerator pedal sensor cause P0651?
On some vehicles one track of the pedal sensor is powered by reference "B", so a shorted pedal sensor can set this code. Check the wiring diagram for your vehicle.
Keep researching
More computer & output circuits codes
Sources
- SAE J2012 — Diagnostic Trouble Code Definitions (standard code titles)
- ISO 15031-6 — Road vehicles: communication between vehicle and external equipment for emissions-related diagnostics, Part 6: Diagnostic trouble code definitions
- U.S. EPA — Vehicle Emissions Inspection and Maintenance (I/M) programs
- U.S. EPA, 40 CFR 86.1806-17 — Federal onboard diagnostics requirements for light-duty vehicles
General information written by CarWhere Research, not a diagnosis of your vehicle. Manufacturers can add their own sub-definitions and test conditions; confirm with the factory service information for your year, make and model. Before paying for a repair, check NHTSA for open recalls and your warranty booklet: federal law requires an emissions warranty on major emission-control parts such as the catalytic converter and engine computer.