There don't appear to be any Santa Clara-specific laws on the matter, so California law (including this) would govern this situation. A landlord generally has an obligation to maintain the premise in habitable condition (can't stick you with the bill for repairing the water main), and has to fulfill the obligations of the lease (if the lease says that a working washing machine is part of the premise, the landlord has to fix it if it breaks). An AC is not part of what makes a unit "habitable" in the legal sense. You should have to scrutinize exactly what the lease says about the AC, but saying that it is provided "as is" indicates that the landlord is disclaiming any obligation to fix it if it breaks. You are allowed to use it, but if it breaks, he won't fix it.
The fact that he has no obligation to fix it does not relieve you of your duty to care for his property (irrespective of the fact that it was abandoned by a prior tenant – there's a notification procedure regarding abandoned real property, which I assume the landlord followed so it is his AC). Your obligation to compensate the landlord for damaging his property is not triggered by his legal obligation to maintain the property, it is triggered by the fact that it is his property. So you are legally on the hook: under §1929, "The hirer of a thing must repair all deteriorations or injuries thereto occasioned by his want of ordinary care". However, the size of the hook is not clear: the cost of replacement or repair could be vastly higher than the actual value of the unit. There is a legal concept of "unjust enrichment" that could be applicable, if the landlord plans to bill you $500 for a new AC which he got for free, but you'd probably need to hire a lawyer to make a solid legal argument in court.