The Chicago-based airline said testing will be done on 52 777s during the next 36 hours. Spokeswoman Jean Medina said 12 planes had been inspected and cleared to fly by midmorning.
The carrier has about 460 aircraft.
Delays were being reported in Japan and Hawaii Wednesday morning and the airline warned passengers to be prepared for other delays and cancellations as it proceeded with the inspections.
United, a subsidiary of UAL Corp., said a review of maintenance records showed a test on one of five bottles in the fire suppression system hadn't been performed. The airline alerted authorities.
The planes, which have a so-called "intuitive" self-diagnostic system that would have detected any malfunction with the fire suppression system, mostly fly international routes and from the carrier's major hubs.
United carried out unscheduled maintenance on seven of its Boeing 747 jets last month but found no safety-related issues.
The Federal Aviation Administration has been checking maintenance records at all domestic airlines after revelations surfaced about missed safety inspections at Southwest Airlines Co.