Subaru p0137 and p0138

So i have a 2002 subaru legacy L.
The car is in decent condition (i think, not to car savvy ), but last summer, and this summer, but only during the summer the check engine light would constantly come on, doesn't stay on forever, and not every time i drive. but recently its been staying on the entire time, so i had the code read.
I don't understand how it can read High and Low. Does anyone have any thoughts as to what i can do to avoid paying loads of money that I don't have being the college student I am? The gas mileage seems to be decent, at about 400 miles ish per tank (about 14.5 gal per tank) if it matters.
Thanks in advance.

P0137 Rear O2 Sensor Circuit Low Voltage
P0138 Rear O2 Sensor Circuit High Voltage

Sound like it might be a loose connection either under the car on the left side right by the back of the trans or possably at the ecm which is under the carpet of the passenger feet youll need to pull up the carpet and remove a metal shield that has 4 ten MM bolts some times just unplugging these and plugging them back in will fix the connection. good luck