Under 20 CFR 404.991a and 416.1491, the Appeals Council (AC) may revise a determination or decision after the applicable time period specified in Hearings, Appeals, and Litigation Law (HALLEX) manual HA 01390.030 or HA 01390.040 expires if the Social Security Administration (SSA) begins an investigation into whether to revise the determination or decision before the time period expired. SSA may begin the investigation either based on a request by the claimant or on its own initiative. The investigation is the process of SSA gathering facts after reopening a determination or decision to determine if a revision of the determination or decision is appropriate.

Diligent pursuit means that in light of the facts and circumstances of a particular case, SSA undertakes the necessary action and carries out the action as promptly as circumstances permit.

Diligent pursuit is presumed to be met if SSA concludes the investigation and, if necessary, revises the determination or decision within 6 months from the date SSA begins the investigation.

If SSA diligently pursued the investigation to its conclusion, the AC may revise the determination or decision. The revision may be favorable or unfavorable to the claimant.

If SSA did not diligently pursue the investigation to its conclusion, the AC will only revise the determination or decision if a revision is favorable to the claimant. The AC will not revise the determination or decision if it will be unfavorable to the claimant.