To be fair and only fair on an as-is you only need to go as far as the door.
That said I always try to help if they contact with any issue in the first few months.
I would give advice but seldom free parts it is just too much at the price point they are sold at.
If it is very recent after purchase I would go as far as to drive local to them and help with repair if I was adept. The most I have ever offered is a new Motherboard do to reset issues shortly after purchase. I already had one tested and working and felt bad for issues shortly after move.
Not obligated but it is a nice gesture to your customer. IMO
Open and honest about everything that you know about. The rest is just electronics and normal situations for 30 year old electrical.