Seller should have provided receipts by licensed contractors showing the work was completed and is guaranteed. A home warranty may help, but there is generally a co-pay. Your first step would be to contract your agent and see if he/she can work with listing agent to get the seller to handle the issue post closing. If that does not work, go to the home warranty company. Read the fine print on the warranty, they can be great and I alway recommend them to my buyers, but there is a lot of fine print and if the systems are old, some home warranties will cover some parts, but not all. If you do not have a home warranty, ask yourself how much do you want to fight for this, because your next step is to repair the issue and take the seller to small claims. This step rarely satisfies due to the time and effort spent dealing with the claim and collection (assuming judgement is in your favor).
Its better to walk through a few days ahead of closing and closely review receipts, call contractors if necessary, make sure the seller lives up to the contractual agreement while you still have leverage - closing can be delayed a day or two if necessary.
Best wishes to you - its always a "buyer assumes risk" situation post closing.