P0642 means Sensor Reference Voltage A Circuit Low. The 5-volt reference circuit A is being pulled low, usually because a sensor or wire on that circuit is shorted to ground. It is a generic OBD-II code, so the definition is the same on any make, but the most likely cause varies by engine; confirm with a scan tool and the service information for your vehicle before buying parts.
What P0642 means
Most engine sensors receive a 5-volt reference and return a signal between roughly 0 and 5 volts. Codes ending in circuit high or circuit low usually point to an electrical fault: an open circuit, a short or an unplugged connector. Range/performance codes mean a signal is present but disagrees with what the computer expects from the other sensors, which more often points to a dirty sensor, an air leak or a mechanical problem.
P0642 symptoms
- Several sensor codes at once
- Check engine light, reduced-power mode or stalling
- Erratic gauges or throttle response on some vehicles
Common causes of P0642
- A shorted sensor on the shared reference circuit
- Wiring chafed against the engine or frame
- Water in a connector
- Internal computer fault (less common)
How to diagnose P0642
- Read all stored codes; the sensors that share the circuit point to it
- Unplug the sensors on that circuit one at a time while watching the reference voltage
- Inspect harnesses for chafing
- Check connectors for water and corrosion
Can you drive with P0642?
Use caution: sensors such as the throttle and pedal position may be affected, which can limit power.
P0642 tips
Unplug sensors on the circuit one by one while watching reference voltage; when it jumps back to about 5 volts, the last sensor unplugged is the short.
Will P0642 clear itself?
If the fault stops, the computer turns off the check engine light by itself after several drive cycles without seeing it again, and the code stays in memory as history for a while. Clearing P0642 with a scan tool after a repair is fine, but if the cause is still there the code returns, sometimes within one drive. Disconnecting the battery also clears codes on most vehicles, but it erases learned fuel trims and idle settings as well.
Related codes
| Code | Meaning |
|---|---|
| P0122 | Throttle/Pedal Position Sensor/Switch A Circuit Low Input |
| P0107 | Manifold Absolute Pressure/Barometric Pressure Circuit Low Input |
| P0335 | Crankshaft Position Sensor A Circuit |
Guides that help
When it is more than a sensor
Sensor codes are usually fixed with a sensor, a connector, wiring or a leak repair rather than major parts. If testing turns up internal engine trouble instead, see how to tell if an engine is blown and compare engines by code on our engines page.
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