Sebastian Vettel was quickest for the second session in a row at the Circuit of the Americas.
But the Red Bull driver had to spend around half the session in the pits after a water leak was discovered on his car after just three laps.
Mark Webber, who had a similar problem to Vettel in the first session, was second-fastest but over three-quarters of a second slower than his team mate.
He was just eight-thousandths of a second faster than Fernando Alonso. Next were the two McLaren drivers, fourth-placed Lewis Hamilton having a 360-degree spin at one point.
As in the first session several drivers had spins and off-track moments as the track surface offered very little grip. Romain Grosjean and Vitaly Petrov spun in separate incidents at turn one.
The two HRT drivers did practically no running in the first half of the session, but did appear and set times in the second half.
As the session came to an end Pastor Maldonado crawled into the pits with an apparent engine problem, race engineer Xevi Pujolar telling him on the radio to stop the car and turn his engine off.
2012 United States Grand Prix
- F1 fans’ videos from the 2012 United States GP
- Top ten pictures from the 2012 United States GP
- Photozen wins USA Predictions Championship round
- Grosjean warns rivals “I won?óÔé¼Ôäót leave the door open”
- 2012 United States Grand Prix weekend in Tweets