P0651

What it means

P0651 means the engine control module has detected a fault in the 'B' sensor reference voltage circuit, usually a 5 volt supply shared by several sensors. The module saw the reference voltage fall outside its expected range, typically from an open circuit, a short to ground or a short to voltage. Which sensors share the 'B' supply depends on the vehicle and engine.

Symptoms

• Engine warning light on
• Poor or erratic throttle response
• Rough running, hesitation or stalling
• Reduced power or limp mode
• Hard starting or no start
• Other sensor fault codes stored at the same time

Possible causes

• Damaged, chafed or corroded wiring in the 5 volt reference supply or its shared sensor feedslikely
• A faulty sensor on the shared 5 volt supply pulling the voltage down by an internal short (for example a pedal, throttle, pressure or position sensor, depending on the model)likely
• Poor connections, corroded or loose terminals at sensor plugs or the control module connectorpossible
• Water ingress into a connector or harness jointpossible
• Poor power or earth supply to the engine control modulepossible
• Internal fault in the engine control module's reference voltage outputrare

How to diagnose

1. Read all stored codes and freeze frame data, noting any other sensor circuit codes that point to a shared supply
2. Check the manufacturer wiring diagram to identify which sensors share the B reference supply on this model and engine
3. Visually inspect the harness and connectors for chafing, heat damage, corrosion, water and pushed-out pins
4. With the ignition on, measure the reference voltage at the sensors and compare with the specification, normally around 5 volts
5. Unplug the sensors on the shared supply one at a time and watch whether the reference voltage recovers, which points to the faulty sensor
6. Test the reference wire for continuity, and for shorts to ground and to voltage, with the module and sensors disconnected
7. Check the control module's power and earth supplies and the battery condition
8. If wiring and sensors all test fine, have the module checked before replacing it

How to fix

• Repair or replace damaged wiring and fix any short or open circuit found
• Replace the sensor that is pulling the reference supply down
• Clean or repair corroded connectors and terminals, and dry out any water ingress
• Restore faulty power or earth connections to the control module
• Repair or replace the engine control module only once it has been confirmed faulty, following the manufacturer's procedure

Can I keep driving?

Nearby codes

• P0650GenericMIL Control Circuit/Open
• P0652GenericSensor Reference Voltage B Circuit Low