P0642

Sensor Reference Voltage "A" Circuit Low

Car Fault Codes Engine computer & electrical

Can you still drive? With care, short trips only

Who can fix it? Needs a workshop

Typical repair time 30 minutes – 4 hours

Typical cost SAR 100 to 5,000

What it means

The engine computer (ECU) found that the reference voltage it sends to the group of sensors on circuit A is lower than normal. The most common cause is one of the connected sensors failing internally and pulling the voltage down, or a wire shorted to ground.

Symptoms4

Check engine light on with low-voltage codes for several sensors.
Limp mode and reduced power.
Hard starting or intermittent stalling.
A/C cutting out or wrong gauge readings on certain vehicles.

Possible causes5

Internal failure in one of the sensors on the circuit, shorting it to ground.
Reference wire shorted to the body or a ground wire through chafed insulation.
Moisture or corrosion in a sensor connector.
Wiring damaged by heat near the engine or exhaust.
Faulty reference voltage supply inside the engine computer.

How to diagnose5

Identify the sensors fed by circuit A from the manufacturer wiring diagram.
Watch or measure the reference voltage, then unplug the sensors one at a time; the sensor whose removal restores the voltage is the faulty one.
If the voltage stays low with all sensors unplugged, look for the wire shorted to ground.
Inspect connectors for moisture or corrosion.
Disconnect the computer connector and test the wire's isolation from ground before condemning the computer.

How to fix5

Replace the sensor that pulls the voltage down.
Repair the wire shorted to ground, re-insulate it and route it away from heat.
Clean and tighten corroded connectors.
Replace and program the engine computer only if the voltage stays low with all load removed from the circuit.
Clear the codes and confirm the reference voltage is normal and the code does not return.

Repair cost

0–500

500–2,000

2,000–5,000

5,000+

In Saudi riyals; the highlighted part is the expected range for this fault.

The cost is low when a wire or an inexpensive sensor is at fault, and rises if the sensor is costly or the computer is proven faulty.

Prices are rough estimates for common cars at independent workshops and vary with the car and the source of the part.
Prices last updated: September 2026

Frequently asked questions

How do I find which sensor is the cause?

Watch the reference voltage and unplug the shared sensors one at a time; the sensor whose removal brings the voltage back up is the one pulling it down.

Does this code mean the computer is bad?

Rarely. Most cases are a failed sensor or a wire shorted to ground, and the computer should only be condemned after all load is removed from the circuit.

This guide is for information only and does not replace an inspection by a qualified technician. If the fault involves brakes, steering, engine temperature, electrics or refrigerant, do not attempt the repair yourself.

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