new payroll company forgot to withhold 401k

Company switched payroll providers. New payroll company has not withheld 401k from the 5 participants for 2 months.

What's the best (and cheapest) way to correct? Can this be self-corrected? Must employer make up lost earnings? Can the DOL calculator be used?

It can be self-corrected.

Use EPCRS.

Some people say to use the DOL calculator, some say no.

I was always under the impression, that the calculator could only be used if you are filing under VFCP for  late deposits.

However, I know many people in the industry use it even if they aren't filing the correction with the DOL.

See EPCRS, App. A, section .05(9)(a) for the special safe harbor correction method for employee elective deferral failures that do not exceed three months. The failure can be corrected without a QNEC if the conditions are satisfied.

According to the DOL, the DOL calculator can only be used if you file under VFCP.

If you don't satisfy the conditions for the safe harbor correction method and you need to calculate the earnings rate, EPCRS is clear on the earnings rate to use. Assuming the plan allows investment direction, EPCRS (App. B, section 3.01(3)) says to use the rate applicable to the employee's investment choices for the period of failure. Most of the time, this is relatively easy to get from the investment provider. For administrative convenience, if most of the affected employees are NHCEs, you can use the rate of return of the highest-performing fund for the period of the failure. And if the employee failed to make investment choices, then you can use the rate of return for the plan as a whole.