You may withhold a portion of your rent if:
You have appealed to your landlord in writing to make the necessary repairs; or

Your local Board of Health has inspected your apartment and found health code violations and notified your landlord; or

You are current in your rent up until the time your landlord learns of the problem, you are not the cause of the problem, and the unsanitary conditions do not require the apartment to be vacated to make repairs.

Source