P0651Generic code

P0651 Code: Sensor Reference Voltage "B" Circuit/Open

The P0651 code means there is a fault in the 5 volt reference B voltage the computer sends to a group of sensors. It often sets several sensor codes at once and puts the vehicle in limp mode. The typical cause is a short to 12 volts or an open circuit.

Severity
High
Can I keep driving?
Yes, but with caution
Estimated cost
$20 to $400
Difficulty
Medium
System
Computer and Outputs
Applies to
All OBD2 vehicles

Updated Reviewed by Renzo Chacca

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What the P0651 code means

The computer powers many sensors (MAP, throttle position, fuel pressure, A/C pressure, camshaft and others) with a 5 volt reference voltage. So it does not depend on a single supply, it splits the sensors into several groups: reference A, B, C and so on. The P0651 code means reference B is out of range. Reference B is a separate group from reference A, so the sensors that set codes together are not necessarily the same ones.

When one sensor in the group shorts, it drags down the whole reference. Every sensor in that group sets a code at once, and the vehicle can go into limp mode (limited throttle) or even fail to start. That is why, when several unrelated sensor codes show up together, the reference is the first suspect.

The most effective method is to unplug the sensors in the group one at a time while you measure the reference (or watch the code on the scan tool). When you unplug the guilty one, the voltage returns to about 5 volts.

To confirm the fault, measure the continuity of each wire between the component connector and the computer connector with the key off. It should be under 1 ohm, and an infinite reading means the wire is open. If the wiring is good, measure the component itself: infinite resistance confirms it is open inside.

P0651 code symptoms

  • Several unrelated sensor codes at the same time.
  • Limp mode with limited power.
  • Erratic dash gauges (on some vehicles).
  • The engine may not start.

Likely causes of the P0651 code

  1. Reference wire shorted to 12 volts by rubbing against another wire.
  2. Open reference circuit. A damaged connector or splice.
  3. Sensor with an internal fault that disturbs the shared reference.
  4. Computer. Uncommon.

How to diagnose the P0651 code step by step

  1. Read all the codes and identify which sensors are affected. They share the reference.
  2. Measure reference B at a sensor in the group with the key on. It should read 4.8 to 5.1 volts.
  3. Unplug the sensors in the group one at a time until the reference returns to 5 volts. The last one you unplugged is the culprit.
  4. If it does not return, check the wiring and the splices in the reference circuit.
  5. Look at the wiring diagram to see which sensors share the reference on your vehicle.

How to fix the P0651 code

  • Replace the shorted sensor.
  • Repair the damaged wire or splice.
  • Clear the codes of all the affected sensors and verify the repair.

Technical reference data

  • Reference voltage: 4.8 to 5.1 volts.
  • Each reference group powers several sensors.
  • Reference values: check the manufacturer data for your vehicle.

Brand-specific notes

A shared 5 volt reference is a classic cause of limp mode, and a few makes are known for it.

P0651 on GM

On GM, the 5 volt reference shared by the A/C pressure sensor and the throttle sensor is a classic cause of limp mode.

P0651 on Ford

On Ford, the fuel pressure sensor or the MAP sensor often pulls the reference down.

P0651 on Dodge

On Dodge, the fuel pressure sensor or the MAP sensor often pulls the reference down.

Frequently asked questions about P0651

Can I drive with a P0651 code?
With caution. The vehicle can go into limp mode or stall, so avoid long trips and busy roads until it is repaired.
Why do so many codes show up at once?
Because all the sensors in the group share the same 5 volt supply. When it fails, every one of them sets a code.
Is it the computer?
Rarely. Almost always it is a sensor or a wire in the group.
How much does it cost to fix P0651?
Between $20 and $400. It costs more if the computer turns out to be damaged.
What does the P0651 code mean?
It is a generic OBD2 code that means sensor reference voltage B is out of range. In this case it points to an open circuit or a short.
What is the most common cause of P0651?
A reference wire shorted to 12 volts, or an open circuit at a connector or splice. With a P0651 code, always unplug the sensors one by one to find the culprit.

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