so Kevin doesn't have to retype all that over and over... which I (and anyone else that needs an explanation) really do appreciate,
first is to note that you can not clear codes if whatever set the code is related to the foot pedals (so no brake light switch codes or accel pedal sensor codes) can be cleared with 'the dance'.

ed-tharp dot is-a-geek dot org/pedal_dance dot jpg

change the "dot" to a real period ".", add your own http :/ and remove the white spaces.
you should recognize that this version is different than in the book, as this version's "time line graph" is drawn with respect to the electrical signal, and the book shows a drawing made with respect to the foot pedal position, which is the opposite of the electrical signal.
now on to the O.P.'s question about hard set codes.
E26= an O2 sensor error, does not mean the sensor went bad, it means the sensor had bad readings. which it will do if it is running badly, which the operator may not be able to notice, as this truck runs pretty well until it is running on only 1 or 2 cylinders. How many hours are on this machine, and has anyone ever done a tune up if over 5000 hours?
if the answer is no, I would be considering it time for a full tune up including changing the coil packs (^$).
Codes in history will show when the key is first turned on, but will show the hour meter once the truck is running, active codes will not allow you to see the hour meter reading, until whatever set the code is fixed.
if you are seeing the MIL and code as soon as the truck starts, then you still have the error that caused the code, especially with this code, as it takes the O2 sensor a few seconds to warm up to the temp it can read at (sometimes called "closed loop", with "open loop" being that the 02 sensor is not accurate enough to use until it becomes warm enough [it is why there are Heated O2 sensors]).