The controversial decision pushed the rule's implementation to after the 2014 midterm election. The Obama administration has explained that it was done to quell concerns from businesses that wanted more time to comply with the law.
Critics of the Affordable Care Act have jumped on the delay as an admission that the law in its entirety is unworkable and needs to be scrapped.
The letter follows a previous letter to the Treasury and multiple hearings in the House investigating the decision to push back the mandate.
Lawmakers have not received answers to their previous questions, they said in the new letter, and want a response.
Signing the letter was Rep. Fred Upton (R-Mich.), the committee’s chairman, as well as Reps. Tim Murphy (R-Pa.), Joe Pitts (R-Pa.), Marsha Blackburn (R-Tenn.) and Michael Burgess (R-Texas).
The lawmakers want a reply by Sept. 6.