The company issuing you the warranty is responsible if under the terms of the warranty--which is a contract, and which is therefore binding like, and can be enforced as, any other contract--they have to make the repair. If you believe that under the terms of the warranty, they should take care of this issue but they do not, you could sue them for "breach of contract."The seller would be liable if you can show that the seller knew of a problem with the A/C, or logically under the facts of the situation must have known, but despite knowing, did not disclose it. A failure to disclose a known problem is fraud, and would provide a basis to sue for compensation (e.g. repair costs).If the seller did not know of the problem and the warranty does not in fact cover this, when it would be your cost.