Ford F-150 - 1997-2003 :: P0136 Code - Horrible Fuel Mileage

2002 F-150 4x4 super crew w/5.4..So I've got the P0136 code, horrible fuel mileage (around 7 mpg just driving to work without hitting very many lights) and the check engine light comes on intermittently, and I get that code. I have borrowed my friends scanner ("Determionator Scan System") and I can see the live data from the 02 sensors. Today I drove around for a while and watched the scanner and the H02 data, and the downstream sensor on bank 1 shows at most, .09 volts, and it also says its detecting a lean mixture. The code P0136 is for this sensor in particular, and I have heard that an 02 sensor should be around 1 volt. The downstream sensor on the drivers side showed around .75 volts at max.

I believe that it is working correctly, or close to it, and I do not get a code for that sensor. So my question is, is the downstream sensor on the passenger side (bank 1) causing the P0136 code and the poor fuel mileage? also, could an exhaust leak cause the sensor to throw a code and not be bad at all? I do have some very minor exhaust leaks, on both sides of the truck..I'm leaning towards replacing that one 02 sensor. The wires and plug connection to that sensor look fine, plus I would think that if there was a bad wire/connection it would not register any voltage at all.. I have to add, the truck runs fine overall after I replaced the intake manifolds last week. It does still idle high, around 1,000 rpm, after coasting to a stop, then after a few seconds goes back down to a normal idle of around 650 rpm. but idles normal upon starting or in park.