Sudden MPG drop

[RZNT4R]

Originally Posted by johnnybaboon

Update: It was very likely a bad O2 sensor. I took it to another mechanic and he told me the same as the dealer that the computer didn't show anything wrong. .

The computer might not throw a code, but if the O2 sensor has just skewed instead of outright failing, you'd see normal g/sec on the maf, but fuel trims way up to satisfy the bad O2 sensor, and at +30% fuel, the exhaust would stink.

Saw this a week ago on a Kia Forte. Failed O2, fuel trims way high, just letting the car idle it stank up the shop with all garage doors open. Even without a gas analyser, the exhaust can tell part of the story: That kia was so rich it stank, saw cars glow the cats red they were so lean. In those case, it's easy tell.

Sometimes though, it can be a pain to pinpoint, everything's just's barely out of whack enough to be suspect, and nothing you touch makes a difference and you're looping the diag flowchart feeling like an imbecile.