Stock investors are banking on the Federal Reserve doing something to juice the economy.

But after two rounds of quantitative easing, "Operation Twist" coming to an end and Treasury yields already at record lows, what exactly does the Fed have left in its arsenal?

Dan Greenhaus, chief global strategist at BTIG, is convinced Ben Bernanke & Co. will come up with something. Whether it solves anything remains the big mystery.

"As long as there is air to breathe, and assets that pay interest, the Fed is not out of bullets in an effort to boost growth," Greenhaus says. "There is technically no constraint on what the Fed can purchase to stimulate economic activity."

He says a third round of asset purchases is "now more likely than ever," and he sees it being enacted sometime in the third quarter. "The FOMC will probably have to wait a bit more to ensure that what we've seen is actual slowing rather than a statistical playbook," Greenhaus says. "If that view is confirmed, the Fed will have to get into motion."