Fpr\ Check engine light?

[Specter]

I had installed an after market fpr, removed the old fpr solenoid and the car was running fine. That same day the car started sputtering and i got a cel, i checked all the grips, and one injector grip was loose, so i addressed that issue, the cel was gone and the car was great again. The next day i got cel again, but no sputtering, the car runs same as before, no problems in boosting. When i installed the fpr, i was hoping that my mileage would increase as the car was running very rich before (had black smoke out the exhaust too), but there was no good effect after fpr installation. I get almost the same kind of shitty mileage. I have plugged in the grip of the stock fpr solenoid but the cel is still there. I do not have any kind of diagnostic equipment. Could this be coincidence that i got this cel after i had my fpr work done but is not related to it in any way? I am thinking that the 02 wires might be bad, or the 02 sensor itself considering the mileage i am getting, but i need advices on what else to look for. I have not checked what cel code is the ecu throwing, i will check it as per vfaq, but i do not know which wire to check.

-P.S: It is a rvr ecu, same pins format as an evo III ecu.
Thanks!