29 C.F.R. § 4041A.25

(a) Annual insolvency determination. The plan sponsor of a plan that has been amended to eliminate all benefits that are subject to reduction under section 4281(c) of ERISA shall determine in writing whether the plan is expected to be insolvent for the first plan year beginning after the effective date of the amendment and for each plan year thereafter. In the event that a plan adopts more than one amendment reducing benefits under section 4281(c) of ERISA, the initial determination shall be made for the first plan year beginning after the effective date of the amendment that effects the elimination of all such benefits, and a determination shall be made for each plan year thereafter. The plan sponsor of a plan under which no benefits are subject to reduction under section 4281(c) of ERISA as of the date the plan terminated shall determine in writing whether the plan is expected to be insolvent. The initial determination shall be made for the second plan year beginning after the first plan year for which it is determined under section 4281(b) of ERISA that the value of nonforfeitable benefits under the plan exceeds the value of the plan's assets. The plan sponsor shall also make a solvency determination for each plan year thereafter. A determination required under this paragraph shall be made no later than six months before the beginning of the plan year to which it applies.

(b) Other determination of insolvency. Whether or not a prior determination of plan solvency has been made under paragraph (a) of this section (or under section 4245 of ERISA), a plan sponsor that has reason to believe, taking into account the plan's recent and anticipated financial experience, that the plan is or may be insolvent for the current or next plan year shall determine in writing whether the plan is expected to be insolvent for that plan year.

(c) Benefit suspensions. If the plan sponsor determines that the plan is, or is expected to be, insolvent for a plan year, it shall suspend benefits in accordance with § 4281.41.

(d) Insolvency notices. If the plan sponsor determines that the plan is, or is expected to be, insolvent for a plan year, it shall issue notices of insolvency or annual updates and notices of insolvency benefit level to the PBGC and to plan participants and beneficiaries in accordance with part 4281, subpart D.