During the outage, the company said it would work on the boiler, precipitator and scrubber. The company said it moved the outage up one week due to unplanned fan repair work.
The company did not say in the report when the unit would likely return.
Electricity traders guessed the outage would last a couple of months.
The Pirkey station, which entered service in 1985, is located in Hallsville in Harrison County about 140 miles east of Dallas.
One MW powers about 500 homes in Texas.
AEP's regulated Southwestern Electric Power Co subsidiary operates the station for its owners: Northeast Texas Electric Co-op Inc (12 percent), Oklahoma Municipal Power Authority (2 percent) and Southwestern Electric Power Co (86 percent).
AEP, of Columbus, Ohio, owns and operates more than 38,000 MW of generating capacity, markets energy commodities, and transmits and distributes electricity to more than 5 million customers in 11 states.