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Technical Description of OBD-II Trouble Code
Circuit A of the Intermediate Shaft Speed Sensor
What does that signify?
This is a generic diagnostic trouble code (DTC) that is commonly found in OBD-II vehicles with automatic transmissions. This includes vehicles from various manufacturers such as Mazda, Toyota, Chrysler, Ford, VW, Dodge, Jeep, Mercedes, Lexus, Chevrolet, etc.
While it is a generic code, the specific repair steps may vary depending on the year, make, model, and powertrain configuration of the vehicle.
The intermediate shaft, also known as the layshaft, helps distribute rotational force from the input drive to the output shaft inside the transmission. The speed of the intermediate shaft depends on the gear you are in. In a manual transmission, the gear selector determines this, so there is no need to monitor the speed of the intermediate shaft.
However, in an automatic transmission, the gear you are in (such as “D” drive) is determined by the TCM (Transmission Control Module) using multiple sensor inputs. One of these sensors is the Intermediate Shaft Speed Sensor. The TCM relies on this specific input to determine and adjust hydraulic pressures, shift points, and patterns. Experience in diagnosing other types of speed sensors (such as VSS and ESS) will be helpful, as most speed sensors have similar designs.
The ECM (Engine Control Module) and TCM (Transmission Control Module) may activate the P0791 code and associated codes (P0792, P0793, P0794) when they detect a fault in the intermediate shaft speed sensor or its circuits. Sometimes, when the sensor fails, the TCM uses other speed sensors within the transmission to maintain a “backup” hydraulic pressure and keep the automatic transmission functional. However, this can vary significantly between manufacturers.
The code P0791, which indicates a fault in Circuit A of the Intermediate Shaft Speed Sensor, is set by the ECM and/or TCM when they detect a general malfunction in the A speed sensor or its circuit. Consult a vehicle-specific repair manual to determine which part of the circuit corresponds to “A” in your particular application.
NOTE: If multiple warning lights are illuminated (such as Traction Control, ABS, VSC, etc.), record any active codes in other systems.
Photo of a transmission speed sensor:
What is the severity of this DTC?
I would classify this fault as moderately severe. As mentioned earlier, your automatic transmission may still be functioning properly. However, it could also indicate one or more significant underlying issues. It is best to diagnose any transmission issue as soon as possible.
What are some of the symptoms of the code?
Symptoms of a P0791 trouble code may include:
What are some of the common causes of the code?
Possible causes for this P0791 engine code may include:
What are some P0791 troubleshooting steps?
Basic Step #1
If you are researching this code, I will assume that you have already checked the transmission fluid level. If not, start by doing so. Make sure the fluid is clean and at the appropriate level. Once you have addressed any fluid-related issues, locate the intermediate shaft speed sensor. Often, these sensors are mounted directly to the transmission housing.
You may be able to access the sensor from under the hood, but this may require removing additional components such as the air cleaner and box, miscellaneous brackets, wires, etc. Ensure that the sensor and its corresponding connector are in good condition and securely connected.
TIP: If the ATF (Automatic Transmission Fluid) has a burnt smell, it is a sign that new fluid is needed. Consider performing a full transmission service with a new filter, gasket, and fluid.
Basic Step #2
Remove and clean an easily accessible speed sensor. This is a low-cost step, and if you find that the sensor’s pickup is excessively dirty after removal, cleaning it may resolve the issue. Use brake cleaner and a rag to ensure a clean sensor pickup. Dirt or shavings on the pickup can affect the sensor’s readings, so make sure it is clean!
NOTE: Any signs of rubbing on the pickup may indicate inadequate spacing between the reluctor ring and the pickup. Most likely, the sensor malfunctioned and is now hitting the ring. If replacing the sensor does not resolve the issue, refer to the manufacturer’s procedures for adjusting the pickup/reluctor gap.
Basic Step #3
Test the sensor and its circuit. To test the sensor itself, use a multimeter and refer to the specific manufacturer specifications to measure different electrical values between the sensor’s pins. One useful trick is to perform these tests using the same wires but at the corresponding ECM or TCM connector pins. This will verify the integrity of the harness involved, as well as the sensor.
Professional Service
While we always recommend seeking mechanical solutions for this issue, if that is not possible, we offer the option to permanently delete this specific DTC. All other codes will continue to function. If you are interested in this option, please upload your ECU file to our portal.
