ECU for a 7afe in a 5sfe 1996 manual celica Options

[thebestofall007]

Hello all, I found a surprise in my 1996 Celica: It has an ECU for a 7afe manual transmission engine, however, mine is a 5sfe manual. The engine runs well, but gas mileage is not as good as it is supposed to despite replacing the cap, rotor, plugs, wires, and I get a P0141 code despite replacing the O2 sensor with an OEM Denso and checking the wiring connection between the sensor and the computer. The cables don't appear to be all spliced up, either.

Why would someone do this, and what would I have to do to replace it with the original (if this would fix the gas mileage and sensor issues)?

This post has been edited by thebestofall007: Oct 21, 2016 - 5:00 PM

[thebestofall007]

I did some looking at the wiring schematics of one out of 7afe m/t, 7afe a/t, and the 5sfe m/t I have, and the 5sfe m/t and 7afe a/t ECUs appear to have similar wiring diagrams, but the 7afe m/t has a different plug layout compared to my 5sfe. Something is telling me that someone got into this ECU or refurbished it at some time and mismatched the cover or something. I found some numbers written on it as well.