my expedition is giving me codes P0171 and P0174 which means system too lean on bank1 and bank 2

PO171 And PO174 means it's running lean. This means your manifold air pressure sensor is saying X amount of air entering motor yet your O2 sensor is saying XY amount of air is exiting engine. The computer reads this as extra air is getting in somewhere. This is usually a vacuum leak from your PCV vacuum lines. Start your engine, let it idle. Spray starting fluid around your PCV valve and all vacuum lines connected to it. You should hear you idle pick up. Then trace those lines with your finger and you will find a hole that rotted through. go to the following links for more help and videos. https://www.facebook.com/martin.kamenar.92 and https://www.youtube.com/watch?v=YaynnTca3tw&feature=youtu.be

Before you replace the heater core for a leak, check the OEM quick lock fittings on the coolant lines to the heater coar. These tend to leak and the antifreeze will run down the heater coar pipes into the cabin hvac box and cause a smell of coolant in the cabin. This happens 50% of the time. If the temperture control knob feels funky, it may have cracked, which causes it to not rotate the heater temp. Mine did that on a 1998 Expedetion. Simple fix. Advanceautoparts Fix It . 4 knobs for like $5 .