If you are investigating a clunk during a gear change, save every stored and pending code before clearing anything. Scan transmission modules for codes and compare commanded versus actual gear where supported. Record the engine temperature and load alongside the reading; the same value can mean different things in different conditions.
A basic reader may not access every module. Check for warning lights and recent drivetrain work; do not repeatedly provoke a hard shift. Use the exact vehicle’s service information to interpret the result.
Quick educational note
For information, not a diagnosis or repair instruction. Vehicle-specific checks are still required. Consult the manufacturer's guidance and a qualified technician.
How to use this information
This article explains general principles and does not establish what is wrong with your vehicle. Read the content limitations.