What P0642 means
The engine module feeds several sensors (pedal, throttle, pressure and position sensors) from shared, regulated 5 V reference circuits; 'A' is circuit 1.
How the engine computer detects it
A short on one sensor or its wire pulls down every sensor on that circuit and triggers many follow-up codes – so look for the one faulty sensor or chafed wire, for example by unplugging the sensors one at a time. P0642 means the voltage of reference circuit A is too low – typically the 5 V wire or a connected sensor is shorted to ground. Ford checks continuously from ignition on and sets it below 4.75 V after 5 s; GM Europe's 2.0 L gasoline after 1 s or 10 s cumulative, lamp in the first trip. GM North America reports it as P0641. All sensors on the circuit then read near 0 V, with matching 'low' codes. High is P0643, range/performance P06A6.
Written from public technical sources and not yet checked by a mechanic. How we write these pages
Symptoms
- Check engine light
- Limp mode with reduced power
- Many codes from several sensors at once
- Possibly starting or boost problems
Common causes
- 5 V wire or sensor shorted to ground Most likely
- Internal short in a supplied sensor (e.g. pressure sensor) Possible
- Chafed harness or water ingress Possible
- Module reference regulator faulty Less likely
How to diagnose P0642
- Read every stored code with an OBD-II scanner. If misfire or fuel trim codes are there too, fix those first: they often cause this one.
- Inspect the parts involved: sensor reference voltage circuit, sensor wiring, engine control module.
- Work through the causes above from most to least likely, cheapest checks first.
- Clear the code and drive a full drive cycle to confirm the repair held.
Reading the code
- P
- Powertrain
- 0
- Generic (SAE) code
- 6
- Computer and output circuits
- 42
- Specific fault
Questions drivers ask
Can I drive with code P0642?
Drive gently and only as far as you need to. The engine computer may limit power to protect the engine, so avoid highways and heavy traffic until it's fixed. If the check engine light starts flashing, pull over.
Will my car pass an emissions inspection with P0642?
Probably not in states that test vehicles with an OBD-II scan: an emissions-related code that keeps the check engine light on is usually an automatic fail. Rules differ by state and county, so check yours.
This page is general information, not a substitute for a professional diagnosis. Repair costs are rough estimates for parts and labor and vary by vehicle and region.