here's a couple of links to another place relating to problems with the same codes.
http://www.maseratilife.com/forums/coupe-spyder-gs/20612-code-p0491.html
http://www.maseratilife.com/forums/coupe-spyder-gs/16202-cel-error-code-p0492.html
What i can tell you from resent experience of the same sort of thing with my v6 Mercedes, is...
1. as you have both codes at once, the likelihood is it's a common item failure, rather than 2 issues, like 2 tubes leaking or as voicey said, unlikely to be both upstream lambdas, especially as you have no other issues
2. it does not have to be the air delivery system component per-say, for example it could be the relay that feeds the pump, or the fuse linked with said relay
3. it could also be the pump, and on mercedes this is not uncommon, and i have seen videos of people stripping them and cleaning all the carbon dust out and them working again
4. in general, if it is the pump, it's not a hard job by any standards, although pump prices can vary widely, i could have saved £200 simply by buying the bosch boxed item, compared to the mercedes boxed bosch item!!
i was lucky, in that the issue was a bad connector in the fuse box, which over time caused enough heat to melt that connectos plastic cover enough to disrupt the power supply and trip the warning light.
