Hi. P1684 is an informational code that means you have disconnected the battery recently. This code doesn't hurt anything, and won't turn the engine light on. If your engine light is on you have some more codes stored. If you don't have a code reader:
Cycle the key from off to on three times ending with it on and watch the digital odometer display. The codes will pop up here, you should see more than one come up. Let me know what you see.
Ok great. P0551 is for a failure of the power steering pressure switch. This switch is on the stering rack under the car. It senses pressure in the power steering system, when the pressure comes up when you are at full lock it raises the idle speed. This switch does sometimes fail.
With the car up secured on jack stands slide under it. On the left side from behind the engine cradle find the steering rack where the shaft comes down into it from the column. Just under the shaft you'll see a 2 wire pressure switch, the only thing on the steering rack that is electrical. Unplug the switch and you're almost gauranteed to find power steering fluid has leaking into it. Either way, replace the switch. If there is fluid in the connector get it out with brake cleaner or electrical contact cleaner. When you're done either clear codes with a reader or disconnect the battery and the light will go off.