PECG spokeswoman Lisa-Marie Burcar said union officials aren't yet sure what form the legal challenge will take - lawsuit, grievance or some kind of arbitration.
But the union representing engineers said a third furlough day will stall government infrastructure projects needed to create jobs and bolster California's economy.
"Telling people to stay home a third day a month slows economic recovery," PECG president Mark Sheahan said in a statement announcing the lawsuit.
Sheahan expressed concern that with state engineers furloughed three days a month, work may have to be outsourced to private companies "at more than twice the cost."
"This executive order mandates the waste that the governor says he wants to eliminate," Sheahan added.
Sheahan noted that 95 percent of PECG's members are paid through special funds, such as the gas tax, federal funds, and other sources - not the general fund.
"Cutting our pay won't help the general fund, but it will cost the taxpayers more than twice as much (if the state has to outsource)," Sheahan added.
PECG's prior lawsuit and grievance to stop the Schwarzenegger administration's initial two-day furloughs are working their way through the courts and administrative process.
Prior posts about the ongoing legal dispute can be read by clicking here and here.