Mars rover Spirit is being steered in reverse due to a "sticky" front wheel.
Spirit has now embarked on its journey up the Columbia Hills
Engineers have decided not to use the problem right-front wheel except where it is absolutely essential - such as driving up challenging, sandy slopes.
This wheel was drawing roughly twice the current of the other five and attempts to improve its performance have only been partially successful.
Efforts to warm and rotate the wheel to distribute lubrication had yielded a 25% increase in performance, Nasa said.
The sticky front wheel may be a sign of wear and tear.
Rover engineer Joe Melko told a news conference in Pasadena that the rover was designed to drive in reverse just as well as it drives forward.
Meanwhile, Spirit appears to have found its first outcrop of layered rocks on Mars, but as yet it is unclear whether these rocks have been deposited by the action of water, wind or volcanoes.
They may be the oldest rocks yet seen by Spirit, dating back three or four billion years.
On the opposite side of Mars, Spirit's twin, the Opportunity rover, has found that the element chlorine seems to increase in concentration the deeper it delves into Endurance Crater, the 130m-wide depression it is currently exploring for past evidence of water on the Red Planet.
Razorback may have been formed by flowing fluid - possibly water
Chlorine levels at the rover's current position are about three times higher than at a region examined by Opportunity on the crater rim.
But researchers are not yet sure what the chemical signal means.
Opportunity has also taken images of puzzling teeth-like structures lining a crack in the rock of Endurance Crater, which scientists have dubbed "Razorback".
Scientists think they may have been formed by fluid - possibly water - that flowed into cracks, depositing material along the sides.