All of my locomotives are probably well past warranty stage. Many of them have been in storage a long time.
I now have a couple of non-runners, slow-runners, and spastic runners.
So, whether I ever want to see them go roundy-round again, or sell them, I must take the dive into N-scale surgery.
I've recently purchased a soldering iron, a tweezers set, a set of hobby -size tool kit.
I think I have what I need tool-wise.
Now, I just need courage.
Tell me your wisdoms!
Number 1 is always identify the true root cause issue before heading off in the wrong direction with unnecessary tinkering that does more harm tham good. Treat the Cause not the Symptoms...
Is it mechanical or electrical? Often as not, all thats actually needed is a good clean and maybe (sparingly) lubrication...
Probably beneficial if you give a bit more detail case by case - what loco, Symptoms etc.