P0793 code definition
An incorrect signal to the Powertrain Control Module (PCM) from the transmission intermediate shaft speed sensor circuit causes the storage of a P0793 trouble code and the Check Engine Light to illuminate. This in turn causes the transmission to fail to provide a proper shift strategy, leading to problems with shifting into gear while driving. Associated codes that are sometimes stored with a P0793 trouble code include P0791, P0792, and P0794.
What the P0793 code means
When shifting gears, the transmission shaft speed increases gradually until the shift point is reached and shifting into the next gear. When the speed is erratic or fails to increase as it should, a P0793 trouble code is stored to the PCM. While the Check Engine Lights illuminates immediately upon storing this code in many models of vehicles, some models might require multiple failure cycles before the indicator lamp comes on.
What causes the P0793 code?
A defective intermediate shaft speed sensor, or either the output or input speed sensor, is usually the cause of a P0793 trouble code. In addition to these causes, damaged or shorted wiring or connectors can also cause the storage of this code. Some additional causes of a P0793 trouble code includes faulty shift solenoids, dirty transmission fluid, faulty engine drivability sensors, a defective valve body, and, while rare, PCM failure.
What are the symptoms of the P0793 code?
Symptoms of a P0793 trouble code commonly include problems with shifting, a wildly fluctuating speedometer, and a decrease in the fuel mileage of the vehicle. In addition, the speedometer can fail altogether, the vehicle might stall when stopping, and the engine could misfire or hesitate while in operation. When diagnosing this code, the mechanic should look for an accompanying input speed sensor code stored on the PCM.
How does a mechanic diagnose the P0793 code?
Diagnosing a P0793 trouble code requires a mechanic to have an advanced scanner on hand, in addition to a digital volt/ohmmeter, a manufacturer's wiring diagram, and an oscilloscope.
To begin, the mechanic needs to examine all wiring, connectors, and components. The mechanic is looking for any damage, corroded, or shorts while performing this inspection.
The mechanic should also connect the advanced scanner to the diagnostic connection and download any stored trouble codes and the freeze frame data. This can help the mechanic with his diagnosis, especially when trying to determine if the problem is an intermittent condition or not.
If the code returns upon clearing, check the reference voltage and ground signal coming from the intermediate shaft speed sensor.
If both detect as open, check the continuity and resistance of both circuits using the digital volt/ohmmeter. The mechanic should first make sure that any related control modules are disconnected before proceeding. Failure to do so could result in controller damage.
Using the manufacturer's wiring diagram and the volt/ohmmeter, the mechanic also needs to test any related circuits, as well as the sensor, for continuity, comparing their findings to the manufacturer's specifications.
If all of the system circuits test as good, the mechanic should connect the oscilloscope to the transmission and observe the data coming from the intermediate shaft speed sensor while the vehicle is running.
If everything checks out, disconnect the PCM and test it for fault.
Clear the code and retest the system to see if the P0793 trouble code comes back.
Common mistakes when diagnosing the P0793 code
One particular mistake made by mechanics when diagnosing a P0793 trouble code is to assume the problem lies with one of the many vehicle speed sensors, with replacement failing to fix the problem and the code returning. Some other mistaken diagnoses of a P0793 trouble code is identifying the problem as an engine misfire, a faulty fuel system, problems with the transmission, and a vehicle speed or driveline malfunction.
How serious is the P0793 code?
In addition to the usual transmission shifting problems associated with this code, a P0793 trouble code can also produce lower fuel economy, engine misfires, and even vehicle stalling. The transmission needs the correct data from the Intermediate Shaft Speed Sensor Circuit in order to function properly, otherwise the vehicle could have trouble determining the proper gear to shift into.
What repairs can fix the P0793 code?
To repair a P0793 trouble code requires a mechanic to take the following action:
Replace the PCM if it is at fault.
Need help with a P0793 code?
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