Tech who worked on this truck did not get the O2 sensor connector snapped to lock, that is what set the code.
We DID NOT have to tap dance at all today.
We used our SST that we normally apply to our Komatsu trucks, minus the adapter harness that matches the OBDII to the Komatsu diagnostic plug.
When we turned on the SST and ran the diagnostic menu, we saw right off "code P0031", Heated Oxygen Sensor open circuit.
So, Mitsubishi E-26 is P0031 using our Hitachi SST and our Nissan K-21 service manuals.
We also used the SST to erase the code and truck is fine now.
Thanks to all.

Just my word on O2 sensors........I have OVER two hundred trucks (CAT NISSAN MITSU's) at my various customers that I service with the Nissan K21/K25 Engine....some with 14,000 Hrs PLUS and have NEVER replaced an O2 Sensor...it is more than likely set because the connector was unplugged when it was running or when your repairs were made some how the wiring pigtail to the sensor was laying on the exhaust and melted the wiring......

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]).

Can try doing "the pedal dance" to clear codes but if it's an active code it might not clear...can do a search in the forum I have posted SEVERAL times how this is done...if the truck has an automotive style OBD II connector under the cup holder or dash area anyplace can use an automotive scan tool to clear them also...or unhook battery for ten minutes or so both positive AND negative.....

One more thing. When a problem causes E-26 to appear, does that "hard set" the code into ECM memory where the code must be erased, or will the code (and MIL) disappear when the cause or defect is corrected?
I ask because I can't be 100% certain the tech working on the truck might not have have turned the key on to check seat switches operation before the O2 sensor was reconnected when he worked on the truck.
If he did that, the MIL and E-26 code might just need to be erased, and problem solved???

The O2 sensor did get unplugged, but during R&R of the lower trans cooler hose (at the radiator).
The truck was not run any until the seat was back in place.
We will get back in there and check the condition of the wires in both connectors that serve the O2 sensor today and I'll let you know what we find.
Thanks guys.

Check to see if the connector for the O2 sensor got unplugged during the cushion change and see if the harness was maybe laying on the exhaust and burning thru the wiring........

Check your O2 sensor wiring for shorts to frame/engine, broken O2 wires or wire chaffing and shorting to another wire. If this does not pan out then it seems a service call with a sst is in the making. The O2 sensor itself maybe at fault but i have rarely seen it.