Taxation of discriminatory POP plan

[Guest pinsall]

125 plan fails 25% concentration test for 1999 calendar year.

It is a premium only plan (POP) with medical premiums taken out on pre-tax basis of ee's pay

What is HCE taxed on , amount of pre-tax ee premiums withheld or maximum they could have withheld under highest option?

What are the mechanics of correction? amended 1999 W-2 including above amount and an amended 1999 1040?

Thansk