Sudden increase in fuel burn very often (with 2 cars in my case) comes from the fuel control staying in "default" mode, that is, the mode it is when you start the engine and that will stay just until engine temperature warms up to 140°F and then turns the computer to feedback mode (that is, using the O2 sensor to precisely calculate the correct fuel injection). Soooooooooo....a failed upstream O2 sensor will keep the engine in default mode...but that will normally light up the SES light.

 

Yes, this is true. I had this case, but interestingly, the light went off again after a few days, but the fuel consumption did not go back to the original level!

 

If it really was a sudden increase (and not gradual), then I would take it in. Could be lots of things and a pro scanner is probably required to troubleshoot it.

 

I would suggest a bottle of Techron Fuel System and CRC intake cleaners for starters.

 

That's really low for a full tank. With Active Eco on, my 2014 2.4L gets around 360 miles in the city. On highway I get anywhere from 380 to 400 miles on a tank. That's 611 to 643 KM