The IRS does not retract an extension if you don't pay, but you begin to pay failure-to-pay penalties as well as interest from the date that your taxes are due.
In New Jersey, if you don't pay at least 80% of the tax that is ultimately due when you file your extension, then you will accrue both failure-to-pay and failure-to-file penalties. It is the failure-to-file penalties that you are typically hoping to avoid when you file the extension - so it's pretty much like you didn't file it if you don't pay at least 80%..
