P0641 - OBD Trouble Code

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Diagnostic trouble code P0641 is technically described as Sensor Reference Voltage “A” Circuit Open. This code indicates that a fault has been detected in the sensor reference voltage “A” circuit. The letter “A” isolates a particular area of the circuit more than a particular component or wire. This code is usually detected when PCM/ECM monitors the sensor 5 volt reference circuit voltage is higher or lower than expected.

Code P0641 stands for Sensor Reference Voltage 'A' Circuit Open. In the event of a sensor reference “A” malfunction, a trouble code will be stored and a service engine soon lamp may be illuminated. PCM supplies a reference voltage to many of the onboard sensors. Typically, this reference signal is 5-Volts. The sensors then verify this voltage signal in accordance with whatever they're measuring and send it back to the PCM. Code P0641 indicates the PCM has detected voltage is out of specification on the 5-Volt reference circuit. The 'A' part of the code indicates the problem is with the number one sensor, if more than one sensor is in question.

What causes this problem with the Sensor Reference Voltage "A" Circuit Open?

• Faulty Sensor
• Damaged, corroded, broken or loose wiring and connectors
• Poor electrical connection
• Inoperative or blown fuses
• Defective system power
• Failed PCM

1. Sensor - A sensor is a device that measures a physical quantity and converts it into a 'signal' which can be read by an observer or by an instrument and here the sensor in question should be thoroughly inspected and replaced if found faulty.
2. Transmission Parts - Transmission refers simply to the gearbox that uses gears and gear trains to provide speed and torque conversions from a rotating power source to another device. In motor vehicles, the transmission adapts the output of the internal combustion engine to the drive wheels. Each and every part of the transmission should be checked and repaired or replaced as necessary.
3. PCM - The Powertrain Control Module PCM is an on-board electronic control unit that primarily controls the charging, emission and transmission systems of the vehicle. However, PCM fails rarely but if you find any of its components faulty, replace or repair it quickly.
4. Connectors - Connectors are used to connect control units, sensors and actuators with the cables of the vehicle electrical system. These should not be loose, corroded or broken else can become one of the causes storing this error code.

To identify an error code, one should be aware of the symptoms of that error code. Below mentioned symptoms will help you to identify the error code P0641:

• Erratic and harsh gear shifting
• Inoperative Speedometer
• Transmission may not shift between pre-programmed modes
• Transmission may not shift between 2WD and AWD modes
• Transfer case may not shift between high and low ranges
• Lack of front differential engagement
• FWD hubs may not engage
• Decreased Fuel efficiency
• Engine misfires

It is always recommended to take your vehicle to the reputed repair facility for diagnosis and repair because a trained eye can catch even the little errors fairly quickly. Following repairs may help you to rectify this error code:

• Using digital multimeter inspect the reference circuits. The meter should show a good reference voltage being supplied to the sensor by the PCM. If not, the wiring between the sensor and PCM should be checked and repairs as needed.
• Using diagnostic scan tool check the sensors carefully. The data produced by the sensor in question can be compared the manufacturer's specifications. If the value is out of specification, the sensor is probably faulty and should be replaced.
• In rare cases, the PCM may be at fault. The PCM should supply a 5-volt reference to the sensors in question. If it does not, it may be faulty or require reprogramming.

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Here are some common steps to diagnose the error code P0641:

• To diagnose this code, you need diagnostic scanner and a digital volt/ohmmeter.
• Determine the location and function of the sensor in question.
• Check system fuses and fusible links with the circuit under a full load and blown fuses should be replaced.
• Perform a visual inspection of sensor system related wiring harnesses and connectors. Repair or replace damaged or burned wiring, connectors, and components as required.
• Use the DVOM to test reference voltage and ground signals at the sensor in question.
• If the voltage and ground signals are present on the sensor connector, continue by testing sensor resistance and continuity levels.
• Sensors that fail to comply with these specs should be replaced.
• Replace open or shorted circuits as required.

Common Mistakes When Diagnosing the P0641 Code

As we always recommend all to seek help of an experienced technician while diagnosing any error code because a trained eye can catch and solve all the issues in less time and without any mistake. The common mistake while diagnosing this error code is unnecessary replacements of sensors where in the actual culprit is something else.