In broad terms It’s sort of self explanatory . This is only an educated interpetation;
P1155: Some Oxygen sensors (especially widebands) need to be artificially heated. If it not, it will think it cold and run the car rich thus giving crummy mileage.
P0171 say not enough coolant to create heat. Same deal, it not reading the heat required to heat the O2 sensor enough to make the car right. Same deal, to rich, ergo bad mileage.
P0171 (system too lean) makes no sense to me because the other code and your mileage point to a car running rich. It may just be a code thrown out because other things are screwy and the computer doesn’t know how to take it. Or, maybe one bank is running so rich, the other compensating by leaning way out.
Anyway, this is just horse sense but I’d say you have a problem with an o'2 sensor or wiring to it, or your low on coolant is low (unlikely) Both the same symptom to the computer.
Hope this broad analysis and helps you.
Regards, PK