What is the first step to properly diagnose an improperly shifting transmission?

To properly diagnose an improperly shifting transmission, verifying proper engine operating condition should be the first step. This is critical because the transmission relies heavily on the engine's performance to operate correctly. If the engine is not running smoothly or is compromised in any way—such as issues with fuel delivery, air intake, or spark—it can affect how the transmission receives signals for shifting.

An engine that is misfiring, running rough, or experiencing any significant mechanical issues can cause the transmission to behave erratically, leading to improper shifting. Ensuring that the engine is in good condition first helps eliminate it as a source of the shifting problem, allowing the technician to focus on the transmission itself with a clearer understanding.

The other options, while also important in the diagnosis process, are secondary to ensuring that the engine is functioning optimally. Checking the throttle cable adjustment and torque converter operation or verifying line pressures are all relevant steps that might be taken later in the diagnostic process, after confirming that the engine itself is operating properly.