OBD-II code · Powertrain (generic)
P0642 Sensor Reference Voltage "A" Circuit Low
The 5 V reference voltage on the A circuit (used by multiple sensors) is below threshold — short to ground or excessive current draw by a single sensor pulling the rail down.
Is it safe to drive with P0642?
The car may go into limp mode (reduced power). Get it diagnosed before a long trip.
Common causes
- Most likelySensor or harness shorted to ground
- Less likelyECU reference output damaged
- Less likelyConnector pins corroded - high contact resistance
Symptoms
- Check engine light on
- Possible limp mode or no-start
- Multiple follow-up faults across modules
- Reset/relearn may be needed after repair
Repair at a glance
- Difficulty
- Hard
- DIY friendly
- Best left to a mechanic
- Typical labour
- 0.5–4 h
- Emissions test
- Likely to fail
- Parts involved
- Sensor Reference Circuit, Ecu
Related codes
Code data: OBDex open dataset (CC0). Labour times are typical ranges, not a quote.