Not much! A recent Washington Post article tells us:
The law...severely limits the ability of the IRS to collect the penalties. There are no civil or criminal penalties for refusing to pay it and the IRS cannot seize bank accounts or dock wages to collect it. No interest accumulates for unpaid penalties.
So how can the IRS enforce the mandate? Scary letters and threats to withhold tax refunds.
The law allows the IRS to withhold tax refunds to collect the penalty, and most filers get refunds. This year, 77 percent of the 135 million individual income tax returns processed by the IRS qualified for a refund. The average refund: $2,707.
For those who don’t qualify for a refund, a stern letter from the IRS can be effective, even if it doesn’t come with the threat of civil or criminal penalties, said Elizabeth Maresca, a former IRS trial attorney who supervises the Tax & Consumer Litigation Clinic at the Fordham University law school.
"Most people pay because they're scared, and I don't think that's going to change," Maresca said.