Do I have a lawsuit against my landlord if she refuses to replace my mercury thermostat?

First of all, do you want to create such a big issue over this when you are leaving in a month? If you actually want to move forward with this complaint (after living there for 4 years and never taking action), you need to pay your rent into an escrow account, along with any late fees that would be charged according to the lease. If you just fail to pay, then it looks like you just don't have the money to pay and you are withholding the rent for that reason. If you put the money into the escrow account, you are showing good faith. Once you put the money into the escrow account, with the late fees, you should send the landlord a letter requesting the the thermostat be changed. With all that said, I'll be completely honest in saying that it certainly looks like you just don't want to pay your rent for this month. It has been 4 years and you have never put your money into an escrow account and demanded this problem be fixed. You aren't going to come out looking good on this one, in my opinion. Lastly, I am not aware of any legislation that disallows the use of mercury thermostats in rental homes. There is, however, legislation that bans the sale of the thermostat and the use of it in schools.