P0642

Sensor Reference Voltage A Circuit Low

Sensor reference voltage Generic OBD-II High

What it means

P0642 indicates the 5-volt reference voltage the ECM supplies to many sensors has dropped too low on the 'A' circuit, usually from a short to ground or a sensor drawing too much current. Because several sensors share this feed, you may see multiple sensor codes, erratic readings, poor running, or limp mode.

High

Can I keep driving? Often limited — because many sensors share this 5-volt feed, the engine may run poorly or drop into limp mode, so have it diagnosed soon.

Real-world scan data

Seen in

494

scans in the past year

Found in

0.1%

of scans

0.1%

of scans that found faults

Appears alongside · share of P0642 scans

P0340in 28.5% of scans

P0193in 27.5% of scans

P0122in 21.7% of scans

Figures reflect only diagnostic scans recorded through our own apps and tools. Cars are typically scanned because a symptom or warning light prompted it, so these counts describe our scan records — not the incidence of faults in any model, and not the quality, reliability, or safety of any vehicle. No comparison between manufacturers or models is implied. All trademarks belong to their respective owners; no affiliation or endorsement is implied.

Common symptoms

What you'll notice

Check-engine light on, often with several sensor codes at once
Erratic sensor readings or sensor 'failures'
Rough idle or poor acceleration
Reduced engine performance or limp/failsafe mode

Common causes

Most likely first

Short to ground in the 5-volt reference 'A' circuit wiringLikely
A shorted sensor pulling the reference voltage downCommon
Damaged or corroded wiring or connectors on the reference circuitCommon
Faulty powertrain control module (PCM)Less often

How to diagnose & fix

Work through it in order

Scan for all stored codes; several sensors sharing the same reference often point to the common feed.
Inspect the reference circuit wiring for a short to ground or chafed insulation.
Disconnect sensors on the circuit one at a time to find one dragging the voltage low.
Check connectors for corrosion; if wiring and sensors are good, the PCM's reference supply may be at fault. Clear and re-test.

FAQ

Quick answers

Is it safe to drive with P0642?+

Not ideal — a low sensor reference can upset fueling and put the car in limp mode. It's drivable to a shop, but don't leave it long.

Can I just clear P0642?+

It returns until the short or supply fault is fixed. Because it can knock out several sensors at once, it's worth diagnosing rather than clearing.

Read & clear P0642 yourself

Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.

▶ Get it on Google Play

✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX