Fix P0793: Mechanic’s solutions that WORKED

The P0793: Intermediate Shaft Speed Sensor “A” Circuit No Signal code signifies a problem with the intermediate shaft speed sensor “A” circuit, indicating that no signal is being detected. This sensor plays a crucial role in informing the vehicle’s computer system about the transmission shaft speed, which is vital for proper gear shifting and transmission performance. A fault in this sensor or its circuit can lead to issues with transmission operation, potentially affecting the vehicle’s ability to shift gears correctly.

P0793 quick overview

Description	P0793: Intermediate Shaft Speed Sensor “A” Circuit No Signal
Is it serious?	This code can be serious as it may affect the vehicle’s transmission behavior and overall drivability.
Possible causes	– Faulty intermediate shaft speed sensor – Damaged or corroded wiring to the sensor – Poor electrical connections in the sensor’s circuit – Transmission mechanical issues affecting the sensor – Possible ECU/PCM malfunctions or programming needs
How to diagnose?	Check Sensor and Location: Verify the intermediate shaft speed sensor’s location and condition. Inspect Wiring and Connectors: Look for any damage, disconnection, or corrosion in the sensor’s wiring and connectors. Test Sensor Functionality: Use diagnostic tools to test the sensor’s functionality and signal output. Examine ECU/PCM: Check if the vehicle’s computer system needs updates or reprogramming. Investigate Mechanical Issues: Explore any possible mechanical problems within the transmission that could affect the sensor.

How to fix P0793 – learn from mechanics

I looked into P0793 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.

Case #1: Toyota

Vehicle Information: 2010 Toyota Highlander 2.7L

Issue + Repair History: Vehicle setting codes P0717 and P0793. Attempted repair following a TSB that involved replacing the speed sensor assembly on top of the valve body and performing a relearn, but the codes persisted. Checked reluctor rings in transmission, measured resistance of wire connector to TCM, and reviewed scan data showing both cluster and turbine speeds within specs. The TC sensor showed open resistance on two new sensors, and the code P0793 set without exceeding 15.5 mph. Despite clearing codes, the vehicle would initially shift well but then revert to setting codes and entering default mode.

Suggestions to Fix Issue:

• Scope NC+ & NC- at the PCM for detailed analysis.
• Thoroughly inspect the harness for any looseness or incorrect placement.
• Conduct a circuit integrity test to rule out wiring issues as the root cause.

Final Fix: The resolution came from addressing poor grounds at the driver’s side cylinder head and body. Correcting the grounding issue resolved the erratic transmission speed readings and eliminated the ticking noise. The vehicle was successfully repaired.

Case #2: Lexus

Vehicle Information: 2004 Lexus RX330 3.3L

Issue + Repair History: Transmission problems, including slipping, delayed shift, and code P0793. After transmission rebuild, the same issues persisted. The NC and NT sensors were new. Electrical connections to ECU and sensors were verified as good.

Other OBD2 Codes + Test Results:

• Code P0793 sets when NC sensor RPM drops below 300rpm for five seconds while vehicle speed is over 31 mph.
• NC sensor RPM drops to 250 rpm at 30 – 35 mph.

Suggestions to Fix Issue:

• Check if the torque converter was replaced.
• Examine the transmission fluid temperature sensor.
• Perform line pressure measurements and ensure ATF Type IV is used.
• Investigate any vehicle history of accidents that could impact wiring or grounding.
• Confirm the quality and correctness of the NC sensor, especially if it is an OEM sensor.

Final Fix: The issue was resolved by replacing the aftermarket NC sensor with an OEM sensor from the dealer. The aftermarket sensor was faulty, and once replaced with the correct dealer part, the vehicle functioned properly without setting any codes.

Case #3: Chevrolet

Vehicle Information: 1999 Chevrolet Tahoe LS 5.7L

Issue + Repair History: The vehicle exhibited hesitation, surging, cutting out, and misfire. During the problem, the voltage reading on both front O2 sensors dropped to zero. Multiple tests and replacements were done, including changing the fuel filter, checking the alternator, and replacing the O2 sensors. The transmission also displayed issues like flaring and lugging.

Other OBD2 Codes + Test Results:

• P0793: No specific details provided.

Suggestions to Fix Issue:

• Check fuel pressure and consider replacing the fuel pump if needed.
• Consider if a weak alternator could cause voltage fluctuations affecting various systems.
• Examine for any transmission-related issues, possibly requiring an overhaul.
• Inspect and ensure proper electrical grounding, especially at critical points like the thermostat housing and engine block.
• Consider the possibility of a faulty ignition switch or related electrical issues.
• Disconnect and evaluate the impact of aftermarket stereo installations on vehicle systems.

Final Fix: The vehicle’s issues were resolved by replacing the aftermarket NC sensor with an original equipment manufacturer (OEM) sensor. Additional grounding was provided to the ground wires from the VCM, which improved engine performance. A new computer (PCM) was also installed, which required programming and crankshaft sensor relearn. The final fix involved addressing both engine performance and transmission shifting issues, indicating that multiple factors contributed to the vehicle’s problems.

Case #4: Scion

Vehicle Information: 2005 Scion tC 2.4L

Issue + Repair History: The car was experiencing an issue with no overdrive after driving. The check engine light would come on, and overdrive would not engage in subsequent drives. Both transmission speed sensors were replaced due to splitting and fluid leakage.

Other OBD2 Codes + Test Results:

• P0717: Turbine Speed Sensor circuit no signal.
• P0793: Intermediate Shaft Speed Sensor “A”.

Suggestions to Fix Issue:

• Verify live data from both sensors to ensure they are providing accurate readings.
• Use a lab scope to check if there is a signal from the speed sensors to the ECM connector.
• Inspect pin connectors at the ECM for damage or discoloration.
• Consider the possibility of the sensors being installed incorrectly or reversed.
• Evaluate the condition of the ECM and consider the possibility of ECM malfunction.

Final Fix: As of the last update, the vehicle was not fixed. The technician was certain that a good PCM (Powertrain Control Module) would correct the problem but was unable to replace or repair the original ECM due to the customer’s inability to leave the vehicle for a sufficient duration. The technician had tried two used ECMs, which led to other problems not present with the original ECM.

Case #5: Toyota

Vehicle Information: 2004 Toyota Solara SE 2.4L

Issue + Repair History: The vehicle initially had issues with the transmission defaulting to 2nd gear and displaying check engine light. It had codes P0717 and P0793, indicating speed sensor problems. After replacing a broken speed sensor, the codes persisted. Extensive testing showed an issue with the S2 solenoid waveform. The customer supplied a used computer which was reflashed, but the vehicle then lacked 4th gear shift. The S2 solenoid had a waveform at all times, and the S1 solenoid was functioning correctly. The vehicle had been in a front-end collision and had undergone repairs, including the underhood fuse box.

Other OBD2 Codes + Test Results:

• P0717
• P0793
• Transmission temperature code (later development)

Suggestions to Fix Issue:

• Verify solenoid waveforms and functionality.
• Consider the possibility of a faulty replacement computer.
• Check for additional collision-related damages affecting transmission or sensors.
• Inspect and test the transmission temperature sensor and related wiring.

Final Fix: The final solution involved replacing the transmission temperature sensor, which is part of the internal wiring harness. After this replacement, the transmission functioned correctly. The issue with the sensor was not setting consistent codes, complicating the diagnosis. This fix was the last part of the puzzle, following extensive testing and repairs necessitated by the previous collision and damage.

Common Advice	Diagnostic Step Description
Check for defective sensors	Inspect speed sensors or related components for damage or malfunction.
Verify wiring and connections	Inspect and test wiring harnesses and connections for continuity and damage.
Perform comprehensive waveform testing	Use a scope to analyze waveforms from sensors and solenoids.
Inspect for mechanical transmission issues	Examine the transmission for any mechanical faults or signs of wear.
Consider ECU/PCM issues	Evaluate the vehicle’s computer for faults or the need for updates/reflash.
Examine post-collision repairs	Review any repairs done after a collision to ensure they are not affecting transmission performance.