P0641

What it means

P0641 means the engine control module has detected a fault in sensor reference voltage circuit A, which is the regulated 5 volt supply (typically) feeding a group of engine sensors. The module saw the voltage out of range or the circuit open, so the sensors it feeds may give unreliable readings. Which sensors share supply A depends on the vehicle and engine.

Symptoms

• Check engine light on
• Engine runs poorly, hesitates or lacks power
• Rough idle or stalling
• Hard starting or no start
• Car goes into limp mode with reduced power
• Other sensor codes stored at the same time

Possible causes

• Short to earth or to voltage on the shared 5 volt reference wire, often from chafed or damaged wiringlikely
• A faulty sensor on the shared supply pulling the reference voltage down (for example a pressure, position or throttle sensor)likely
• Corroded, loose or water-damaged connector or terminal on the reference circuitlikely
• Open circuit in the reference supply wiring or a broken wire inside the loompossible
• Poor earth or damaged engine wiring harness connection affecting the sensor circuitspossible
• Internal fault in the engine control module reference voltage supplyrare

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. Look up the wiring diagram for the specific vehicle to see which sensors share reference supply A
3. Visually inspect the harness and connectors for chafing, corrosion, water ingress, heat damage or previous repairs
4. Check battery condition and the engine control module power and earth connections
5. With the ignition on, measure the reference voltage at the sensors and compare it with the manufacturer specification
6. Unplug the sensors on the shared supply one at a time and see whether the voltage returns to normal, which identifies a pulling-down sensor
7. Test the reference wire for continuity, short to earth and short to voltage with the module disconnected
8. Only once wiring and sensors are cleared, check for a module fault following the manufacturer procedure

How to fix

• Repair or replace damaged wiring, and secure the harness away from sharp edges and heat
• Replace the sensor found to be overloading the reference supply
• Clean or replace corroded connectors and terminals, and restore proper terminal tension
• Repair the open circuit in the reference supply wire
• Restore faulty earth or power connections to the control module
• Repair or replace the engine control module only if testing confirms an internal fault, and apply any required programming

Can I keep driving?

Nearby codes

• P0640GenericIntake Air Heater Control Circuit
• P0642GenericSensor Reference Voltage A Circuit Low