What does it mean?
This is a generic diagnostic trouble code (DTC) that generally applies to OBD-II vehicles with automatic transmissions. This may include, but is not limited to, vehicles from Mazda, Toyota, Chrysler, Ford, VW, Dodge, Jeep, Mercedes, Lexus, Chevrolet, etc.
Although generic, the exact repair steps can vary depending on the year, make, model, and powertrain configuration.
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 dictates this, so monitoring the intermediate shaft speed is not necessary.
On the other hand, in an automatic transmission, if you are in “D” mode, the gear ratio is determined by the TCM (Transmission Control Module) using several sensor inputs that all contribute to smooth and efficient shifting. One of the sensors included here is the intermediate shaft speed sensor. The TCM needs this specific input to help determine and adjust hydraulic pressures, shift points, and patterns. Experience in diagnosing other types of speed sensors (e.g., VSS (Vehicle Speed Sensor), ESS (Engine Speed Sensor), etc.) will help here, as most speed sensors are of similar design.
The ECM (Engine Control Module) together with the TCM (Transmission Control Module) can activate P0791 and related 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 in the transmission and determines a “backup” hydraulic pressure to keep the automatic transmission functional, but this can vary significantly from one manufacturer to another.
The P0791 intermediate shaft speed sensor A circuit is defined by the ECM (Engine Control Module) and/or the TCM (Transmission Control Module) when it detects a general malfunction within the speed sensor A or its circuit. Refer to a vehicle-specific repair manual to determine which part is the “A” circuit for your particular application.
NOTE: Record all active codes in other systems if multiple warning lights are on (e.g., traction control, ABS, VSC, etc.).
How severe is this DTC?
I would say this fault is moderately severe. As mentioned earlier, your automatic transmission may still function properly. That said, it could also indicate one or more larger and more urgent issues. Diagnosing any transmission problem as soon as possible is the best strategy.
What are some of the symptoms of the code?
Symptoms of a P0791 error code may include:
- Harsh shifting
- Many dashboard warning lights illuminated
- Poor driving performance
- Irregular engine RPM
What are some common causes of the code?
Causes of this P0791 engine code may include:
- Faulty or damaged intermediate shaft speed sensor
- Electrical fault in the wiring between the speed sensor and the modules used
- Internal ECM and/or TCM problem
- Other related sensors/solenoids damaged or faulty (e.g., input shaft speed sensor, output shaft sensor, shift solenoid, etc.)
- Dirty or low automatic transmission fluid (ATF)
What are the troubleshooting steps for P0791?
Basic Step #1
If you are looking up this code, I will assume you have already checked the transmission fluid level. If not, start with that. Ensure the fluid is clean and properly filled. Once the fluid is checked, you need to locate the intermediate shaft speed sensor. Often, these sensors are mounted directly on the transmission case.
You may even access the sensor under the hood; this may involve removing one or more components such as the air filter and box, various brackets, wires, etc., to reach it. Make sure the sensor and its corresponding connector are in good condition and fully connected.
TIP: Burnt-smelling ATF (automatic transmission fluid) is a sign that new fluid is needed, so don’t hesitate to perform a full transmission service with all new filters, gaskets, and fluids.
Basic Step #2
An easily accessible speed sensor should be removed and cleaned. This costs almost nothing, and if you find the sensor tip is excessively dirty once removed, you might be able to clean your problems away, literally. Use brake cleaner and a cloth to ensure a clean pickup on the sensor. Dirt and/or metal shavings here can interfere with sensor readings, so make sure yours is clean!
NOTE: Any signs of rubbing on the sensor may indicate improper spacing between the reluctor ring and the sensor. Most likely, the sensor has malfunctioned and is now hitting the ring. If a replacement sensor still doesn’t clear the issue, refer to the manufacturer’s procedures to adjust the gap between the sensor and the reluctor.
Basic Step #3
Test the sensor and its circuit. To test the sensor itself, you will need to use your multimeter and the manufacturer’s specific specifications to measure different electrical values between the sensor pins. A good tip is to perform these tests from the same wires but at the corresponding ECM or TCM connector pins. This will check the integrity of the involved harness as well as the sensor.