P0651

Sensor Reference Voltage "B" Circuit/Open

Car Fault Codes Engine computer & electrical

Can you still drive? With care, short trips only

Who can fix it? Needs a workshop

Typical repair time 30 minutes – 4 hours

Typical cost SAR 100 to 5,000

What it means

The engine computer (ECU) distributes reference voltage to the sensors through more than one independent circuit, and circuit B is one of them. The computer detected an open or fault in this circuit, so the sensors connected to it lose their correct signal; which sensors are affected varies between vehicles.

Symptoms4

Check engine light on with codes for sensors fed by circuit B.
Limp mode with poor accelerator response.
Hesitation, stalling or hard starting.
Illogical readings from some sensors on the scan tool.

Possible causes5

Broken or corroded reference voltage B wire or splice point in the harness.
Loose or corroded engine computer connector.
Damaged connector at one of the sensors on the circuit.
Wiring damaged by heat or rodents.
Faulty reference voltage supply inside the engine computer.

How to diagnose5

Identify the sensors fed by circuit B from the manufacturer wiring diagram, since they vary between vehicles.
Compare the stored sensor codes with the list of sensors on the circuit.
Measure the reference voltage at each of those sensor connectors with the key on.
Check wire continuity from the computer to each sensor, focusing on splice points.
Inspect the computer connector for corrosion or loose pins.

How to fix5

Repair the broken wire or damaged splice.
Clean and tighten corroded connectors.
Replace the damaged sensor connector.
Replace and program the engine computer only if no reference voltage leaves it despite good power supply.
Clear the codes and confirm sensor readings are normal and the code does not return.

Repair cost

0–500

500–2,000

2,000–5,000

5,000+

In Saudi riyals; the highlighted part is the expected range for this fault.

The fix is usually a wire or connector repair at low cost; the upper end applies only if the voltage supply inside the computer has failed.

Prices are rough estimates for common cars at independent workshops and vary with the car and the source of the part.
Prices last updated: September 2026

Frequently asked questions

What is the difference between circuit A and circuit B?

The computer splits the sensors across more than one reference circuit so a single fault does not take them all out. Which sensors sit on each circuit varies by manufacturer.

Should I replace the sensors that set codes?

Not before the reference circuit is repaired, because the sensors are usually fine and have simply lost their supply.

This guide is for information only and does not replace an inspection by a qualified technician. If the fault involves brakes, steering, engine temperature, electrics or refrigerant, do not attempt the repair yourself.

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