P0642🔧 Powertrain computer sensor supply (5-volt reference)Drive with caution, fix soon

P0642: Sensor Reference Voltage A Circuit Low (Meaning, Causes, Fixes)

Reviewed 2026-10-25 · Based on 2 cited source(s)

What P0642 means

The PCM sends a steady reference voltage, typically 5 volts, to many sensors over a switched circuit that is powered with the key on, and each sensor returns a changing signal the PCM reads. These supplies are grouped into circuits labeled A, B and so on. RepairPal says the PCM sends a 5-volt reference signal to all applicable engine sensors and sets P0642 when it detects an incorrect output signal. OBD-Codes.com describes P0642 as the PCM detecting a low reference voltage signal for the sensor designated "A", with the code storing when the input voltage is below a programmed limit. It is a generic code for OBD-II vehicles, and the repair steps vary by make and model. The sources differ on which sensors are involved: RepairPal talks about engine sensors and lists 'engine may not start' as the symptom, while OBD-Codes.com says the sensor is usually tied to the automatic transmission, transfer case or one of the differentials, so its symptoms are mostly shifting and 4WD problems. Which sensors share reference circuit A depends on the vehicle, so check a service manual for yours. OBD-Codes.com adds that P0642 will almost always come with a more specific sensor code, that those codes should be diagnosed first (keeping the low reference voltage in mind), and that a P0642 stored by itself suggests a PCM programming error. P0641 is the open-circuit code for the same circuit, P0643 is the circuit A high code, and P0652 is the low code for reference circuit "B".

Severity: can I keep driving?

Drive with caution, fix soon

Get it diagnosed soon. RepairPal rates P0642 Moderate, meaning it may not threaten safety right away but should be checked within a reasonable time. OBD-Codes.com says how serious it is depends on which sensor circuit has the low voltage and what other codes are stored with it. RepairPal notes the engine may not start, and if the car shifts badly, won't engage a gear or 4WD, or the speedometer stops working, drive gently and get it checked before relying on it.

Common symptoms

Likely causes

Roughly most to least common, based on the sources below. Your vehicle may differ.

  1. Failed sensor on the reference circuit (OBD-Codes.com); engine sensor failure (RepairPal)
  2. Defective or blown fuse or fusible link (OBD-Codes.com)
  3. Faulty system power relay (OBD-Codes.com)
  4. Open circuits or bad connectors (OBD-Codes.com); wiring issue (RepairPal)
  5. Faulty engine computer (PCM) (RepairPal), or a PCM programming error when P0642 is stored alone (OBD-Codes.com)

Diagnostic steps you can do

⚠ Before you start

Work on a cool engine, parked on level ground with the parking brake set. Never work under a car supported only by a jack.

  1. Read all stored codes and write them down in the order they were stored, with any freeze frame data. Fix any more specific sensor codes first, as OBD-Codes.com recommends.
  2. Look over the sensor wiring and connectors for damage, burned spots or loose pins, and repair what you find. Then clear the code and test drive to see if it comes right back.
  3. If it returns, use a multimeter at the sensor connector. OBD-Codes.com says you should normally find about 5 volts and a good ground there.
  4. If the voltage and ground are present, test the sensor's resistance against the specs for your vehicle. If the reference voltage is low at the sensor, check resistance and continuity of the wiring between the sensor and the PCM. Disconnect related control modules before any resistance checks, since OBD-Codes.com warns skipping that can damage the PCM.
  5. After a repair, clear the code and drive normally. OBD-Codes.com says if the PCM reaches readiness mode without the code returning, the repair worked.

Typical fixes

Repair costs vary widely by vehicle, region, parts brand and shop, so we don't quote prices. Get a written estimate after diagnosis.

🛠 When to call a pro

  • The engine won't start
  • The transmission won't engage or the 4WD system won't switch
  • The reference voltage at the sensor is low and you can't find the wiring fault
  • P0642 is the only code stored, which may point to a PCM programming issue

Where to buy

Plain search links. Always match parts to your exact year, make, model and engine.

OBD2 scanners on Amazon Parts at Advance Auto Parts catalog at RockAuto

FAQs

Can I drive with P0642?

Often for a short time, but get it checked soon. RepairPal rates it Moderate, and OBD-Codes.com says the severity depends on which sensor circuit has low voltage and what other codes are stored. If the engine won't start, the transmission won't engage or 4WD won't work, don't rely on the car until it's fixed.

What's the difference between P0642 and P0641?

Both are about the same sensor reference voltage circuit "A". P0641 means the PCM sees an open circuit, while P0642 means it sees the voltage on that circuit is lower than it should be.

What's the difference between P0642 and P0652?

They describe the same low-voltage problem on two different reference supplies: P0642 is circuit "A" and P0652 is circuit "B". Which sensors each one feeds depends on the vehicle.

Why is P0642 stored with other codes?

OBD-Codes.com says this code is generally a support code for a more specific sensor code, so fix those first. If P0642 is stored by itself, it suggests a PCM programming error.

How much does it cost to fix P0642?

Neither source gives a repair cost for P0642. A fuse or connector repair is much smaller than a sensor or PCM replacement, so find the cause before buying parts.

Sources

  1. OBD-Codes.com: P0642 Sensor Reference Voltage A Circuit Low (Stephen Darby, ASE Certified Technician)
  2. RepairPal: P0642 Sensor Reference Voltage "A" Circuit Low (rated Moderate)

Last reviewed 2026-10-25. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.