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. so 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.

 

Code 136 is a 'circuit' fault indicator for the rear Ox sensor bank 1 pass side.
It is a cat efficiency monitor and does not control fuel injection.
Only the front Sensor controls fuel.
The fault could be the harness wiring or plug is open or shorted.
Or the sensor is open internally.
If you see a reading from that sensor, it is not open.
It's reading does not change much in a good system.
Look at both long term fuel tables to see if they are to far away from normal.
You have to see a fault before changing any parts that would correct it or your just guessing.
If the exhaust system has large leaks before the front Ox sensors, it could detect excess Ox and run the engine rich.
A stuck open thermostat can do the same..
A CHT circuit fault can also do the same.
Compare both banks to see if possibly the cats are broken and blocking exhaust in either bank. That will cause low power and excess throttle application to move the truck.
Find the fault before changing parts.
Good luck.

 

what is a good long term fuel trim? my short term is within the +-10%, but iirc, the long term still gets up to around 20%. I will have to check back tomorrow with the scanner to read the LTFT and I will post on here again what I found.