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P0791 is a diagnostic trouble code (DTC) for "Intermediate Shaft Speed Sensor “A” Circuit". This can happen for multiple reasons and a mechanic needs to diagnose the specific cause for this code to be triggered in your situation. Our certified mobile mechanics can come to your home or office to perform the Check Engine Light diagnostic for $70.00. Once we are able to diagnose the problem, you will be provided with an upfront quote for the recommended fix and receive $30.00 off as a credit towards the repair. All our repairs are backed by our 12-month / 12,000-mile warranty.
The P0791 trouble code detects an issue with the intermediate shaft speed sensor circuit
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.
Potential causes for the P0791 trouble code include:
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.
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.
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.
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.
Some repairs for the P0791 trouble code include:
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.
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