Correction for excluding a portion of compensation when doing 401(k) d

[John A]

What is the best way of correcting the following?

Due to a payroll service not having the definition of compensation set properly, 401(k) plan deferral amounts and associated match are incorrect (too small)over a few months. Some of the participants that had incorrect amounts withheld have terminated, others are still active. Would APRSC be acceptable, or is VCR better in this case? Can the active participants be "corrected" by withholding additional amounts out of future compensation as a "catch-up." Does the plan sponsor need to correct this by making QNECS equal to all shorted 401(k) and match amounts plus earnings? What's the best way to get this corrected?