P0791 code definition
The P0791 trouble code detects an issue with the intermediate shaft speed sensor circuit
What the P0791 code means
The P0791 code is a generic OBD-II trouble code that notes an issue with the transmission intermediate shaft speed sensor circuit, which relays shaft speed data to the powertrain control module (PCM). In general, the intermediate shaft speed should slowly increase until the shift point, but if the PCM notes that the output speed is not slowly increasing, then shift strategy cannot be determined, and the P0791 code will be stored. Additionally, if the PCM receives no signal, or an erratic signal from the shaft speed sensor, the code will also be detected.
What causes the P0791 code?
Potential causes for the P0791 trouble code include:
- Malfunctioning intermediate shaft speed sensor
- Malfunctioning input speed sensor
- Malfunctioning output speed sensor
- Defective shift solenoids
- Dirty transmission fluid
- Malfunctioning engine temperature sensors, or possibly other drivability sensors
- Defective or damaged electrical components, such as wires and connectors
- In very rare cases, a malfunctioning PCM
What are the symptoms of the P0791 code?
The P0791 code will usually be joined by the Check Engine Light illuminating on the instrument panel. The car will likely experience transmission issues, such as an inability to shift, or very harsh shifting. There may also be engine performance issues, including constant stalling or throttle hesitation, frequent engine misfires, and diminished fuel efficiency. It is also common for the speedometer to fail, or act erratically. In rare cases, the vehicle may have no drivability issues.
How does a mechanic diagnose the P0791 code?
The P0791 code will be diagnosed with the assistance of a standard OBD-II trouble code scanner. A reputable technician will use the scanner to assess the freeze frame data and gather information about the P0791 code. The mechanic will also check for the presence of any additional trouble codes, as codes should always be addressed in the order in which they appear on the scanner. The mechanic will then reset the codes and restart the vehicle, to see if the trouble code returns. If it does not, it should be assumed that there is an intermittent issue, or that the code was triggered erroneously.
If the trouble code does return, then the technician should start with a visual inspection of all of the system’s electrical components. Any damaged wires or connectors should be replaced. If it is noticed that the electrical components have been damaged by the failure of another component in the vehicle, then that should be addressed immediately. The transmission fluid should then be checked, to make sure it is clean and at a sufficient level.
The mechanic will then check the intermediate shaft speed sensor’s reference voltage and ground signals, and use a digital volt/ohmmeter to look for continuity in the circuits. Any circuits that do not meet the manufacturer’s specifications will require replacement. Following this, the input and output sensors can be inspected, along with the shift solenoids. If no issues have been found, an oscilloscope can be used to gather information from the transmission intermediate shaft speed sensor.
After any component is replaced, the mechanic should once again reset the codes, restart the vehicle, and check to see if the code returns. By doing this, the technician will know as soon as the problem has been fixed.
Common mistakes when diagnosing the P0791 code
The most common mistake that is made when diagnosing the P0791 code comes from failing to properly follow the OBD-II trouble code diagnosis protocol. It is imperative that the protocol be adhered to at all times, to assure that all inspections, diagnoses, and repairs are successful and efficient. It is very common for the vehicle speed sensor to be erroneously replaced.
How serious is the P0791 code?
In most cases, a vehicle with the P0791 code cannot be driven, or will experience serious performance problems when it is driven. Continuing to drive the vehicle in this condition can be dangerous, and can cause further damage. Because of this, the P0791 code should be addressed as soon as it is detected, even if the car is drivable.
What repairs can fix the P0791 code?
Some repairs for the P0791 trouble code include:
- Replacement of the intermediate shaft speed sensor
- Replacement of the input speed sensor
- Replacement of the output speed sensor
- Replacement of shift solenoids
- Replacement of electrical components
- Replacement or addition of transmission fluid
- Replacement of drivability sensors
- Replacement of the PCM
Additional comments for consideration regarding the P0791 code
It is very rare for the PCM to be defective, and it should not be considered the problem unless all other potential issues have been checked. If the PCM does need to be replaced, it will also need to be reprogrammed.
Need help with a P0791 code?
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