or its from a bad o2… if your o2 is telling the ecu to add fuel when it doesnt need it bang thats your bad mpg problem.
then if you cel is on for a lean condition that could be fixed by the same o2 sensor. once the o2 craps out it will not read correctly. so a bad sensor could have your engine throwing in too much fuel and it will still think its running lean.
newer cars use the o2 to fatten the tables or lean them out. depends on the reading the o2 sends to the ecu