A transmission-side fault. Modern automatics use solenoids, speed sensors, and a transmission control module (TCM). Many shifting-quality complaints (harsh shifts, slipping, no engagement) trace back to one of these subsystems. Most transmission codes warrant a shop visit promptly — continued driving with a slipping or overheating transmission can turn a $400–$800 solenoid into a full rebuild.
Causes are listed roughly in order of how often they're seen on Canadian independent-shop tickets. Always confirm with a scan-tool freeze-frame and a circuit/sensor check before replacing parts.
