Hi I have a 2001 bugeye wrx its got turbo back exhaust/3port solenoid/panel filter/ngk 8 plugs/alloy rad/forge d.V./Uprated fuel pump/ & been mapped by v. Good mapper, now for last 9months or so it drives like its standard then once in a while it will go absolutely great as it ment too, when I had a map tweak last time the mapper ask had I used a different fuel or done something else to it as he got another 3degree's of timing & car went fantastic then next day it drove crap again, it idles fine there's no cel at mo, I did get cel for front o2 sensor & replaced with denso sensor thought this was going to solve the problem but still no joy, there is one other cel I got the day before I first got it remapped which was 0420 but mapper said no probs as he was turning off the rear sensor, but it got to a point were its only getting 20mpg max on a run taking it real easy when I was getting real good mpg, I just dont no what to try next help please?
Disable the 02's and run a standalone fuel management system. If the its throwing an 02 code its because there is a problem with the fuel that it is metering/burning. Check the voltage to the sensors and make sure that the sensors are both reading correct ohm's so you know they are good.
The tune itself could of caused everything. Switch back to the old tune and see what happens. Make sure to run the same amount of boost and timing that you were running on the old map.