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.
Open the bonnetthe in-depth explanation
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 its description and whether it’s confirmed, pending or permanent.
If the engine stores it over standard OBD, with FREE you read this code with its explanation for nothing and clear it. With PRO you also read the faults of every control unit and clear them in all of them.
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 an inspection. It’s all in how to read and clear fault codes and in inspections and OBD.
Similar codes
- P063E The input needed to configure the throttle control unit automatically is missing
- P063F The automatic configuration input for the engine coolant temperature sensor is
- P0640 The intake air heater control has an electrical fault
- P0641 Sensor reference voltage A: circuit open or faulty
- P0643 Sensor reference voltage A: circuit high
- P0644 The driver display series communication has an electrical fault
- P0645 A/C clutch relay control: circuit fault
- P0646 The air conditioning clutch relay control gives a lower signal than normal
- 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
To understand it properly
We explain how that system works, with drawings and no waffle, in our mechanics guide: Accelerator.
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 and clears engine faults through standard OBD for free, each one explained in plain English. With PRO it also reads the faults of every control unit, each make’s own codes included, and clears them in all of them.
