I have a transmission tale from 20 years ago.
I had a Honda Accord with about 140k miles. The torque convertor began being intermittent in whether it locked up or not.
Eventually, it just didn't lock up, and the torque convertor just remained in play full time.
I tried to take it to Honda for a diagnosis. The service writer, after I told him the symptoms, simply stated "You need a new transmission. $2500."
This was without even looking at, and certainly without test driving the vehicle.
So......I went to a well recommended transmission shop. The tech drove the car, and told me that there was nothing discernable wrong with it, near as he could tell.
The problem with my car became a subject of conversation with the guys I worked with at a coffee break. I mentioned that before the problem, as designed, the torque convertor did not lock up until the engine was fully warmed up. One of the guys said, "You know, the temperature gauge in new cars doesn't reflect the actual engine temperature anymore. It just gets to operating range, and indicates the same temperature, even if it isn't actually fully warmed up."
The light in my head came on, I bought a thermostat, and installed it that night. Viola!
The design of that thermostat included a rubber seal for the moving plate to seal on. It had deteriorated, and the thermostat was leaking by a small amount.....Enough so that the engine was staying a few degrees lower than design.
Ever since, whenever I service the cooling system on any vehicle I own, I replace the thermostat with an OE one.
And........I never just replace major components without fully understanding why they are defective.