P0651: Sensor Reference Voltage B Circuit Open
What to do
Expect reduced power. As with P0641, do not authorise replacing several sensors that failed together — they share a supply, and one shorted sensor or damaged wire explains all of them. Ask whether the reference voltage was measured.
What this code means
The second sensor reference voltage supply has failed. Everything said about P0641 applies here — one regulated supply feeding several sensors, all of which report nonsense together when it collapses — but the practical detail differs in a useful way.
Modules provide more than one reference deliberately, partly for capacity and partly so that a fault on one does not disable every sensor at once. Which sensors sit on reference B rather than A is specific to the engine, so the set of codes appearing alongside this one identifies the affected circuit far faster than tracing wiring does.
The procedure is the same: measure the reference at an affected sensor, then unplug sensors on that circuit one at a time until it recovers.
Likely causes
Start here
- Shorted sensor dragging the reference down · moderate
- Failed sensor reference voltage supply · moderate
- Damaged wiring harness near exhaust heat · moderate
Less common
- Water ingress into connector · easy
- Failed control module · specialist
Symptoms
Components involved
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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Nothing posted about this yet. Whatever you found is worth writing down — even a dead end saves someone the same afternoon.
What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
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