Good evening.....Code P0551 is commonly the p/s pressure switch as you mentioned. The switch is located on the steering rack itself on the back side on the drivers side.
Code P0455 has nothing to do with the EGR valve. It is a large EVAP leak which could be one of numerous possible causes....not just one common. It could be anything from a loose or bad gas cap....to a split evap hose....to a purge solenoid or leak detection pump issue. This isn't a code that can be checked very easily without the proper equipment. A basic visual inspection would be the first step for both codes to make sure no obvious issues are found such as damaged wiring or unhooked connectors.
To check the large evap leak...you would need a machine capable of either flow testing or smoke testing the system to locate and pinpoint the leak or leaks. As mentioned....do a basic check to see if you can find a simple cause...if nothing obvious then I would recommend replacing the p/s pressure switch and then also make sure the gas cap is tight. That will be about all that you can do with out all the proper test equipment. Once repaired....clear the codes and give them some time to see if they return.
If this answers your question then please press "ACCEPT "so that I may be paid for the time spent helping you. If you need more help then please let me know. Thanks
Serious....not really. The only case that could cause a serious issue would be if the large evap leak was at the fuel tank.....a leaking fuel tank can cause a large evap leak and cause a safety issue. I would look for signs of any fuel trails from the fuel tank and for anything wet underneath. Otherwise it most likely wont be anything serious as far as safety or leaving you on the side of the road.
Thats why used car dealers put a bad image on the industry...you can never trust them. Next time you take a long trip on the interstate for vacation.....pay attention to the cars broke down on the side of the road and see if you notice how many have a used car paper tag in the rear.....you might be surprised. Its sad...a little work and they wouldn't have to hide things from their customers such as none working check engine lights.
Here is the description of the p/s pressure switch operation:
A power steering pressure switch is used to improve the vehicle's idle quality. The pressure switch improves vehicle idle quality by causing a readjustment of the engine idle speed as necessary when increased fluid pressure is sensed in the power steering system.
The pressure switch functions by signaling the power train control module that an increase in pressure of the power steering system is putting additional load on the engine. This type of condition exists when the front tires of the vehicle are turned while the vehicle is stationary and the engine is at idle speed. When the power train control module receives the signal from the power steering pressure switch, it directs the engine to increase its idle speed. This increase in engine idle speed compensates for the additional load, thus maintaining the required engine idle speed and idle quality.
I would recommend replacing the switch...see if you feel a change and also do a basic suspension and steering check to make sure nothing is loose or bent for example. If your having issues in one direction only.....the switch most likely wont be related. You may have issues with the rack and pinion or a suspension component. Being that its used....and the issue with the light being disabled.....you never know the history such as minor or major body damage.