P0642: Sensor reference voltage A: circuit low
The 5 volt supply the ECU feeds to several sensors is too low. Usually a sensor or a wire shorting to earth.
| Code | P0642 |
|---|---|
| System | Engine and transmission |
| Group | P06xx, Control unit and auxiliary outputs |
| Type | Generic: it means the same on every make |
| Can you keep driving? | With care |
What P0642 means
The ECU powers several sensors with a fixed voltage, usually 5 volts, called reference A. This code means that voltage has dropped below normal. Typically a faulty sensor or a chafed wire is pulling it down to earth, and several sensors then fail at the same time.
Most common causes
- A sensor on the line with an internal short
- Reference wire chafed and touching earth
- Sensor plug with water or corrosion
- Wiring loom pinched or burnt near the engine
- Internal ECU fault
Symptoms
- Engine warning light on
- Several sensor codes at once
- Limp mode or unresponsive accelerator
- Engine may not start
Can you drive with P0642?
With care The car may have very little power or refuse to start. Only drive when essential until it is repaired.
How to check it
On the scan tool, look at the sensors that use this supply, such as the accelerator pedal or pressure sensors: if several show nonsense values at once, the problem is shared. Pass that on to the garage with all the codes.
How to read and clear P0642 with Final OBD
Plug an OBD2 scanner in under the steering wheel, switch the ignition on and open Final OBD. Under Faults you’ll see P0642 with this same explanation and whether it’s confirmed, pending or permanent.
With Free you read this code with its short description for nothing, and clear it if the engine stores it over standard OBD. With Pro you get the full explanation, how the car was running when it was logged (revs, temperature, load) and clearing in every control unit.
Clearing the code only turns the light off: if the cause is still there, it comes back. Clear it once it’s fixed, and not right before the ITV. It’s all in how to read and clear fault codes and in the ITV and OBD.
Similar codes
- P0622 Generator field F control: circuit fault
- P0627 Fuel pump A control: circuit open or faulty
- P0638 Throttle actuator control: range or performance (bank 1)
- P0641 Sensor reference voltage A: circuit open or faulty
- P0643 Sensor reference voltage A: circuit high
- P0645 A/C clutch relay control: circuit fault
- P0650 Malfunction indicator lamp (MIL) control: circuit fault
- P0651 Sensor reference voltage B: circuit open or faulty
- P0600 Serial communication link fault
- P0601 Internal control module memory checksum error
- P0602 Control module programming error
- P0603 Internal control module keep alive memory (KAM) error
P0642 is a generic code, so it means the same on any make, for example on Alfa Romeo, Dacia, Jeep, Kia, Nissan, Škoda. See what Final OBD does on your car in makes and models.
What does code P0642 mean?
The ECU powers several sensors with a fixed voltage, usually 5 volts, called reference A. This code means that voltage has dropped below normal. Typically a faulty sensor or a chafed wire is pulling it down to earth, and several sensors then fail at the same time.
Can you drive with P0642?
The car may have very little power or refuse to start. Only drive when essential until it is repaired.
How do you clear P0642?
With an OBD2 scanner and an app like Final OBD, which clears it for free if it’s stored by the engine control unit. But clearing it only turns the light off: if the cause isn’t fixed, P0642 comes back.
Read it on your car
Final OBD reads the faults of every control unit for free, including each make’s own codes, and with Pro explains each one in plain English. Free clears engine faults at no cost.
Coming soon
Mid to late October 2026
If you want to try it in your car before anyone else, send us your email and we will add you as a tester.
info@finalobd.com