Technical Description of OBD-II Trouble Code
Intermittent Circuit Issue with Intermediate Shaft Speed Sensor A
What does that signify?
This is a generic diagnostic trouble code (DTC) that typically applies to OBD-II vehicles with automatic transmissions. This can include vehicles from various manufacturers such as Mazda, Toyota, Chrysler, Ford, VW, Dodge, Jeep, Mercedes, Lexus, Chevrolet, etc.
Although 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 the vehicle is in. In a manual transmission, the gear selector determines the speed, so there is no need to monitor the intermediate shaft speed.
However, in an automatic transmission, the gear is determined by the TCM (Transmission Control Module) using multiple sensor inputs to ensure smooth and efficient shifting. One of the sensors involved is the Intermediate shaft speed sensor. The TCM relies on this specific input to 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.) can be helpful as most speed sensors have similar designs.
The ECM (Engine Control Module) and the TCM (Transmission Control Module) may activate the P0794 code and associated codes (P0791, P0792, P0793) 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 functionality by adjusting hydraulic pressure, but this can vary significantly between manufacturers.
The code P0794 Intermediate Shaft Speed Sensor A Circuit Intermittent is set by the ECM (engine control module) and/or the TCM (transmission control module) when an intermittent or erratic signal is detected from the A speed sensor or its circuit. Consult a vehicle-specific repair manual to determine which part of the circuit corresponds to the “A” designation in your particular application.
NOTE: If multiple warning lights are illuminated (e.g., 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?
This fault can be considered moderately severe. As mentioned earlier, your automatic transmission may still function properly. However, it could also indicate more significant underlying issues. It is recommended to diagnose any transmission issue as soon as possible.
What are some of the symptoms of the code?
Symptoms of a P0794 trouble code may include:
What are some of the common causes of the code?
Possible causes for this P0794 engine code may include:
What are some P0794 troubleshooting steps?
Basic Step #1
If you are researching this code, it is assumed that you have already checked the transmission fluid level. If not, start by doing so. Ensure that the fluid is clean and at the appropriate level. Once the fluid is in good condition, 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. to gain access. Ensure that the sensor and its connector are in good condition and properly 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 the sensor’s pickup to be excessively dirty when removed, 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 cleanliness is important!
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 a replacement sensor still doesn’t clear the ring, 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’s 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.
