One embodiment is a system operable to control entry of an oxygen sensor into a learning mode. Also provided are methods of making such urethane-containing structures, and the urethane structures made thereby. Fuel pressure from the refueling pump opens the normally-closed check valve. A masking layer is formed over the substrate to cover the base portions of the substantially upright silicon-comprising structures while leaving the end portions exposed. The generated electric energy can also be converted to power line frequency.