Description of the P0791 fault code.
OBD2 Error Codes

P0791 Intermediate shaft speed sensor "A" circuit malfunction

26.05.2022 /

P0791 – OBD-II Trouble Code Technical Description

Trouble code P0791 indicates a circuit malfunction in the intermediate shaft speed sensor "A".

What does the fault code mean P0791?

Trouble code P0791 indicates a problem with the intermediate shaft speed sensor “A” circuit. This sensor is typically used to communicate shaft speed data to the transmission control module (PCM). Normally, the speed of the intermediate shaft should gradually increase until the gears are changed. If this does not occur, the PCM will not be able to determine the shift strategy, resulting in a P0791 fault code. This error can also occur if the PCM is not receiving a signal from the shaft speed sensor or is receiving an erroneous signal.

Fault code P0791.

Possible reasons

Some possible reasons for the P0791 trouble code:

  • The intermediate shaft speed sensor itself is faulty.
  • Problems with electrical connections or wiring in the sensor circuit.
  • There is an error in the engine control module (PCM), which cannot correctly interpret signals from the sensor.
  • Failure in the intermediate shaft transmission itself, including problems with the gear shift mechanism or drive.
  • Damage or wear to transmission mechanical components that may affect countershaft speed.
  • Electrical noise or interference in the sensor signal.

What are the symptoms of a fault code? P0791?

Symptoms for DTC P0791 may include the following:

  • Shifting Problems: The vehicle may have difficulty shifting gears or there may be a noticeable delay in shifting.
  • Unusual Gear Shifts: Unusual or unexpected gear shifts may occur, including sudden or unusual changes in engine speed.
  • Limp-On Mode: In some cases, the vehicle may enter a limp-on mode in which only one gear or a limited set of gears operates.
  • Check Engine Light: Often the P0791 code will be accompanied by the Check Engine light on the instrument panel.

How to diagnose a fault code P0791?

The following steps are recommended to diagnose DTC P0791:

  1. Scanning Trouble Codes: Use an OBD-II scanner to identify all trouble codes in your vehicle, including code P0791. This will help you narrow your search and see if there are other problems affecting the transmission.
  2. Checking Electrical Connections: Check the electrical connections and wiring associated with the countershaft speed sensor. Make sure the connections are secure and there is no damage to the wiring.
  3. Checking the sensor itself: Check the intermediate shaft speed sensor itself for correct installation and possible damage. Replace the sensor if necessary.
  4. Transmission Diagnostics: Perform a thorough transmission diagnostic to identify other problems that may be affecting transmission performance or countershaft speed.
  5. PCM Check: If necessary, check the engine control module (PCM) for errors or malfunctions that may be causing countershaft speed problems.
  6. Testing Mechanical Components: Check the mechanical components of the transmission, such as the shift mechanisms and drive, for wear or damage that may cause improper operation.

Diagnostic errors

When diagnosing DTC P0791, the following errors may occur:

  • Incorrect interpretation of data: Incorrect understanding of data or incorrect interpretation of error codes can lead to incorrect determination of the cause of the malfunction and, as a result, to diagnostic errors.
  • Insufficient Inspection: Make sure that all components associated with the countershaft speed sensor have been thoroughly inspected, including the electrical connections, the sensor itself, and the mechanical components of the transmission.
  • Ignoring Additional Problems: It is important to remember that the P0791 code can be caused not only by a faulty countershaft speed sensor, but also by other problems in the transmission system or with the engine control module. It is necessary to conduct a comprehensive diagnosis to exclude other potential causes.

How serious is the fault code? P0791?

Trouble code P0791 can be serious because it indicates a problem with the transmission countershaft speed sensor. This sensor is important for the proper operation of the transmission, as it transmits shaft speed data to the automatic transmission control module (PCM), which allows for correct gear shifting.

If the intermediate shaft speed sensor is not working correctly, it may cause the gears to shift incorrectly, which may cause the following problems:

  • Incorrect gear shifting: The vehicle may shift into inappropriate gears, which can result in loss of power, poor performance, and even dangerous driving conditions.
  • Increased Transmission Wear: Improper gear shifting can cause wear and damage to internal transmission components, requiring costly repairs or replacement.
  • Poor Fuel Economy: An improperly functioning transmission can result in increased fuel consumption due to inefficient use of gears.

Therefore, the P0791 code should be taken seriously and it is recommended that diagnosis and repair be started immediately to avoid further problems and ensure the safety and proper operation of the vehicle.

What repair will help eliminate the code? P0791?

Troubleshooting trouble code P0791 may include the following steps:

  1. Replacing the Intermediate Shaft Speed ​​Sensor: If the sensor is faulty or fails, it should be replaced with a new one that meets the original vehicle specifications. After replacing the sensor, it is recommended to check and calibrate the system.
  2. Checking Electrical Connections: Check the electrical connections, including the wires and connectors associated with the countershaft speed sensor. Make sure the connections are intact and securely fastened.
  3. Transmission System Diagnosis: In addition to the sensor, other components of the transmission system, such as the PCM and other sensors, should also be checked to rule out other potential causes of the error.
  4. Programming and Calibration: After replacing a sensor or other components, it may be necessary to program or calibrate the system to ensure proper transmission operation.

It is recommended that repairs be carried out under the guidance of an experienced auto mechanic or an authorized service center to ensure that the problem is properly corrected and to avoid reoccurrence of the error.

How to Diagnose and Fix P0791 Engine Code - OBD II Trouble Code Explain

P0791 – Brand-specific information

Trouble code P0791 can have different meanings depending on the manufacturer and model of the car, several possible meanings for different brands:

These are just a few examples of P0791 codes for different car brands. It is also recommended that you consult the repair and service documentation for your specific brand and model for more accurate information.

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