P0651: Sensor Reference Voltage "B" Circuit/Open
P0651 means Sensor Reference Voltage "B" Circuit/Open. Several sensors share a 5 V reference supply from the engine computer. This code means the 5 V reference circuit "B" is shorted or open, so the sensors connected to it cannot report correctly.
Can I keep driving? Limit driving and diagnose promptly. Continued driving can cause damage, stalling or limp mode.
Common symptoms
- Check engine light on
- Several sensor codes at the same time
- Limp mode
- Stalling or no-start
Possible causes
- A shorted sensor pulling the reference down
- Chafed wiring shorted to ground or voltage
- Water in a connector
- Engine computer fault (rare)
How to diagnose P0651
- Note which other sensor codes are present: they share the same reference.
- Measure the reference voltage at a sensor connector.
- Unplug the sensors on that circuit one at a time; if the voltage recovers, the last unplugged sensor is shorted.
- Inspect the harness for chafing.
What the code means, digit by digit
Generic codes have the same definition on every OBD-II vehicle (petrol cars from model year 1996 in the US, 2001 in the EU; diesels later). Manufacturers can add their own enhanced codes; those are not listed here.
Related codes
General guidance only. Fault codes describe what the engine computer saw, not which part failed: confirm with testing before replacing parts, and follow the vehicle's service information.