Vehicle diagnostics aren't always straightforward. Sometimes a vehicle arrives with symptoms that seem to point in one direction, only for testing to reveal a completely different root cause.

Recently, we worked on a 2019 GMC Acadia with 151651 miles that came in with complaints of hesitation, bucking while driving, and a noticeable lack of power. What initially appeared to be an engine performance issue ultimately led us down a different diagnostic path.

The customer reported that the vehicle was hesitating and bucking during acceleration. A scan of the vehicle revealed several diagnostic trouble codes:

• P0700 – Transmission Control Module Requested MIL Illumination
• P0716 – Input Speed Sensor Performance
• P0717 – Input Speed Sensor No Signal
• P219A – Air/Fuel Ratio Imbalance

At first glance, the P219A code and drivability concerns suggested there could be an engine-related issue contributing to the problem.

During our inspection, we noticed fuel trim data indicating an air/fuel imbalance. The vehicle also exhibited a rolling idle, and we discovered that a camshaft solenoid had been replaced by another repair facility just one day earlier.

Because of the recent repair work, we carefully inspected:

• Intake manifold connections
• Vacuum lines
• PCV system components
• Wiring harnesses and connectors
• Intake gaskets and sealing surfaces

We also found oil present inside the intake manifold and portions of the PCV system, which warranted further inspection.

After cleaning components, inspecting the intake system, reinstalling the manifold with properly cleaned gasket surfaces, performing an idle relearn procedure, and completing injector balance procedures, the vehicle ran significantly better in the service bay.

At that point, it appeared we may have addressed the concern.

However, during the road test, the symptoms quickly returned. When stopped at a traffic light, the vehicle struggled to accelerate. There was virtually no takeoff power, and the transmission's Drive indicator began flashing. Most importantly, the P0717 code immediately returned. This shifted our diagnostic focus away from the engine and toward the transmission.

The P0717 code indicates that the Transmission Control Module is not receiving a signal from the transmission's Input Shaft Speed Sensor.

This sensor plays a critical role in transmission operation. The control module uses its signal to determine:

• Shift timing
• Gear ratios
• Torque management strategies
• Overall transmission performance

Without a valid input speed signal, the transmission may enter a protective mode that significantly reduces vehicle performance.

Using scan tool data, we monitored:

• Engine RPM
• Transmission Input Shaft Speed
• Transmission Output Shaft Speed

With the engine running, the transmission input shaft speed remained at zero RPM.

The output shaft speed reading was expected to be zero while stationary, but the input shaft speed should have been providing a signal.

Further testing revealed:

• Wiring and external connectors appeared intact
• Grounds tested properly
• The input speed sensor showed no visible physical damage
• The reluctor wheel did not show obvious signs of failure during visual inspection

Following GM's diagnostic procedures, the evidence pointed toward either:

• A failed input shaft speed sensor
• An internal valve body-related issue affecting the sensor circuit

One factor that supported our diagnosis was parts availability and failure history. After speaking with a local Chevrolet dealership, we learned that they regularly stock and sell a significant number of input shaft speed sensors compared to valve body assemblies. In many cases, high dealer inventory and frequent part movement can indicate a commonly encountered failure. Combined with our test results, replacing the input shaft speed sensor became the most logical next step before considering more extensive transmission repairs.

Upon removing and inspecting the newly installed input shaft speed sensor, we discovered that the tip of the sensor had become physically damaged. This damage was not caused by the new sensor itself. Instead, it indicated that an internal transmission component had made contact with the sensor. The most likely cause was internal transmission failure resulting in damage to the reluctor assembly or related rotating components that interact with the input shaft speed sensor. At that point, the transmission had progressed beyond a sensor failure and into internal mechanical damage.

Due to the internal transmission damage, replacement of the transmission assembly was recommended. While the initial fault codes pointed toward a speed sensor issue, the damaged replacement sensor confirmed that the underlying problem was much more severe. The failed sensor was actually a symptom of a larger internal transmission failure.