P0641: Sensor reference voltage "A" circuit/open
The 5 V supply that the engine computer feeds to a group of sensors is missing, too low or too high. Every sensor on that supply then reads wrong, so the engine may go into limp mode.
Updated
What P0641 means
Many sensors, such as pressure sensors and the accelerator pedal, get a steady 5 V supply from the engine computer: the reference voltage. Each sensor returns part of it as its signal.
Most engine computers have more than one such supply, named A, B and so on, each shared by a group of sensors. Which sensors are on which supply varies from car to car; the wiring diagram for your car shows it.
P0641 means reference A is open. When a supply fails, every sensor on it reads wrong at once, which is why this code often comes with several sensor codes.
P0642, P0643 and P0651 to P0653
P0642 and P0643 mean reference A is too low or too high. P0651 is the same fault as P0641 on reference B, and P0652 and P0653 are reference B too low or too high. A low reference often means a short to earth in one of the sensors or wires on it.
P0642: Sensor Reference Voltage A Circuit Low
P0643: Sensor Reference Voltage A Circuit High
P0651: Sensor Reference Voltage B Circuit/Open
P0652: Sensor Reference Voltage B Circuit Low
P0653: Sensor Reference Voltage B Circuit High
Can I keep driving?
Drive gently
Drive gently and have it checked soon. Several sensors share this supply, such as the accelerator pedal, throttle or pressure sensors on many cars, so the engine can suddenly lose power (limp mode), and on some cars stall. Avoid motorways and overtaking until it’s fixed.
General guidance based on the code alone, not on the car’s actual condition. If in doubt, stop and ask a mechanic.
Look at the other codes stored at the same time: the sensors they name are probably on the faulty supply, and show where to look.
Symptoms
- Reduced power or limp mode, sometimes with a throttle or engine warning.
- Several sensor codes at the same time, for sensors that seem unrelated.
- On some cars, rough running, stalling or no start, depending on which sensors share the supply.
Common causes
- A short in one of the sensors on that supply, or in its wiring. One shorted sensor pulls the whole supply down. Unplugging the sensors on it one by one often finds it.
- Wiring or connector fault in the supply circuit. Look for chafed wiring near hot or moving engine parts, and for water in connectors.
- Internal fault in the engine computer. Possible, but suspected only after the sensors and the wiring check out.
Typical fixes
- Read all codes, and note the sensors they name.
- Check the connectors and wiring of those sensors for damage, chafing and water.
- Have the reference voltage measured, then the sensors on it unplugged one at a time: if the voltage comes back, the last one unplugged is shorted.
- Repair the wiring or replace the sensor. The engine computer comes last.
Questions
Can I drive with P0641?
Gently, and have it checked soon. Several sensors share this supply, so the engine can suddenly lose power (limp mode), and on some cars stall. Avoid motorways and overtaking until it’s fixed.
Why are there so many codes with P0641?
Every sensor on the faulty supply reads wrong at once, so each may store its own code. Fix the supply first, then clear the codes and read them again.