Error Code P0791 is defined as Intermediate Shaft Speed Sensor A Circuit. This is a generic error code that usually applies to OBD-II vehicles equipped with an automatic transmission. This includes vehicles from, but not limited to, Chevrolet, Chrysler, Dodge, Ford, Lexus, Mercedes Benz, Toyota, and Volkswagen. Specifications on the definition, troubleshooting, and repairs may vary from make, model, and powertrain configuration.
The intermittent shaft, also known as the layshaft, helps in distributing the rotational force from the input drive to the output shaft found inside the transmission. The intermediate shaft speed depends on which gear the vehicle is in. For vehicles with a manual transmission, the gear selector dictates the speed, which means there is no need for the monitor speed of the intermediate shaft.
For vehicles with automatic transmission, however, when set at the drive “D” mode, the gear is determined by the TCM (transmission control module). It uses multiple sensor inputs which all contribute to the vehicle’s smooth and efficient shifting. One of these sensors includes the intermediate shaft speed sensor. The TCM needs specific input to help in determining and adjusting hydraulic pressures, shift points, as well as patterns. There are different types of speed sensors, but experience in diagnosing one type (vehicle speed sensor or VSS, engine speed sensor or ESS, etc.) will help you in understanding, as most of these sensors are similar in design.
The PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes) works with the TCM (transmission control module) to activate the Check Engine light when they monitor this Error Code. Other associated error codes include:
The PCM determines these codes when it sees a fault within the intermediate shaft speed sensor or its accompanied circuits. Other times, when the sensor fails, the TCM will use another speed sensor within the transmission and determine a “backup” hydraulic pressure to keep the automatic transmission functional. This, of course, can vary between vehicle manufacturers.
Error Code P0791 is set when by PCM or TCM when there’s a general malfunction within the A speed sensor or its circuit. To pinpoint the “A” part of the circuit, refer to the repair manual of the vehicle.
Note: Make sure you record other codes active in other systems if there are multiple warning lights being activated (ABS, VSC, Traction Control, etc.).
- Poor performance and drivability
- Hard shifting
- Erratic engine speed
- Multiple instrument panel lights illuminated
Potential causes for this code include:
- Faulty intermediate shaft speed sensor
- Faulty input speed sensor
- Faulty output speed sensor
- Malfunctioning shift solenoids
- Dirty transmission fluid
- Defective engine temperature sensors (or other drivability sensors)
- Damaged or faulty electrical components (connectors and wires)
- Defective PCM (rare)
How to Check
When checking for this code, chances are you have already checked the transmission fluid level. If not, then start with that. Make sure the fluid is at the right level and clean. If the fluid checks out good, find the intermediate shift speed sensor. This is usually mounted straight to the transmission housing.
You can also access the sensor from under the hood, which may require removing one or more components, such as air cleaner, box and wires, to get access to the sensor. Check whether the sensor and its connector are in good condition and are fully connected.
Tip: Any burnt smell on the ATF (automatic transmission fluid) is a sign of fluid replacement. Get a full transmission service and have the filter, gasket, and fluid replaced with new ones.
Speed sensors (which are easily accessible) should be removed and cleaned thoroughly. If the sensor is excessively dirty when removed, you may simply have to wash the problem away (literally). Use a rag and a brake cleaner to ensure a clean sensor pickup. Any dirt or shavings in this area can alter the readings in the sensors, so make sure it’s always clean.
Note: Signs of rubbing may indicate not enough spacing between the reluctor ring and the pickup. If this is the case, then there’s a good chance the sensor malfunctioned and is hitting the ring. If, after replacing the sensor, the ring still will not clear, then you need to refer to the manufacturer’s service manual for the exact procedures of adjusting the gap between the pickup and the reluctor.
Test both the sensor and its circuit using a multimeter. Use the specific specifications set by the manufacturer. Measure the different electrical values between sensor pins. One easy way to do this is to test from the same wires but at the corresponding PCM or TCM connector pin. This can verify the integrity of both the harness and the sensor involved.
How to Fix
- Replacement of intermediate shaft speed sensor
- Replacement of input and/or output speed sensor
- Replacement of damaged electrical components
- Replacement of shift solenoids
- Addition or replacement of transmission fluid
- Replacement of drivability sensors
- Replacement of PCM
This code is moderately severe, and an automatic transmission may still function. However, leaving this problem for a long time can lead to more serious problems in the transmission. Thus, it is best to diagnose and address any issues in the transmission as soon as possible.