I have a 99 OBW, 2.5 auto. It has 187,000 and starts good and runs normal. I’ve been getting 12 MPG or less in all around town driving. It used to get more like 17 MPG around town. This has been going on for awhile, not just a new thing with cold outside temperatures. From reading, this could be lots of things, including mass air sensor, air or water temperature sensors, or bad injectors among other things. Is there any preferred order to check into these things? Does anybody have any good troubleshooting methods?.

I would start by knowing your basics are good.......... How good are the spark plugs, plug wires? Has the EGR valve been cleaned/changed? Does motor warm up normally? If not, the water temp sensor or thermostat may be bad. Any of these things can cause poor mpg.

Plugs and wires may be a good place to start, They were done about 45,000 miles ago with genuine subaru parts. The EGR moves when you blip it and the EGR ports were checked clear in the last year. The temp gauge comes up pretty quick and heater works well, but the water temp sensor for the computer could still be bad? Appreciate the insight.

Starts fine, runs fine, bad mileage, probably a bad front O2 sensor.

If you put in copper plugs last time they could probably stand to be replaced by now, but I've never had spark plugs make a huge difference in fuel mileage unless they were causing misfires.

Definitely replace the front O2 sensor with an OEM unit. I think the 99 came with a Bosch. They are different parts for auto and stick. should cost around $100 but at 14mpg you it will pay for itself pretty quickly. I would also change the plugs too.... they might be a little black and sooty

Do you have a CEL (check engine light on)? If so, pull the engine codes to learn the problem. Once fixed, your mpg should improve.