As a plan sponsor, hiring one of the two biggest national payroll providers as your 401(k) plan’s third-party administrator (TPA) sounds like a great idea, except it isn’t. As an ERISA attorney who does quite well in practice in fixing errors for plan sponsors who fired these payroll providers, I can assure you that you should forget about hiring ADP or Paychex as your 401(k) TPA. This is based on 25 years of experience as an ERISA attorney.