horrible gas mileage cause?

[chengsta]

My '99 eclipse spyder gst has been getting horrible gas mileage withing the last 2 weeks. While the car is cruising at a coast, it would slightly shake and sputter like it was misfiring. I had a check engine light: P0136 Bank 1 Sensor 2 which would be the 2nd o2 sensor by the cat. So I replaced that o2 sensor and put new NGK BPR7ES plugs(gapped at .030) figuring it would solve the sputter/misfiring but still no luck.

I reseted and checked the ECU again for any new codes and it through this code now:

-P0125 Insufficient Coolant Temperature for Closed Loop Fuel Control

I've done some research and found out this code might be caused by a bad FRONT o2 sensor. Would this be the cause for my bad gas mileage?