The PO551 code indicates a loss of pressure in the power steering system, which is a problem but not THE problem. Check the power steering fluid level, and add the correct fluid if it is low. Check for a leak as well, under the car on the ground where you park and under the power steering unit itself. Power steering fluid is under high pressure and will run out very quickly if there is a leak. If it is leaking, get it repaired ASAP. The PS is not cheap and the fluid is just one more thing running off the roads into storm drains.
So likely the problem has to do with the code PO456. I would have replaced the gas cap first as well, since it is the cheapest and easiest solution and it makes sense.
But this code doesn't reference gas caps. Read the link http://www.obd-codes.com/p0456.