The mileage on my 98 Legacy L (AT, 106k miles) has been poor for a while. 20 mpg in a commute that's 95% highway. The last remaining culprit is the oxygen sensor, since I've changed the plugs, wires, oil, tire pressure, and had the tranny rebuilt (because it was failing, not for better mileage!). There are no engine error codes. To my knowledge the O2 sensors are the originals.

 

But, there are 2 O2 sensors, in the front & rear, and 1st Subaru parts lists the price for both at $91 each. Is one more likely to fail than the other, or should I bite the bullet and replace both? Thanks!

Only the front O2 sensor is used to control the engine, so it may have an effect on your fuel ecomony. The rear one is used to monitor the catalytic converter function, it will have no effect on your fuel economy.

Great, thanks! That saves me $90 .

I've had a failing O2 sensor with no error code before. It happens

You might want to test it before replacing it. Voltage is supposed to rise from .1 when cold to .9 or something after idling for 2 minutes.

 

Check fuel system as well. Did it degrade suddendly ? If so, one of sensors - fuel pressure regulator and MAF sensor, coolant temp sensor, throttle position sensor. If slowly, air filter (easiest), fuel injectors - additives for cleaning them etc.

You might want to test it before replacing it. Voltage is supposed to rise from .1 when cold to .9 or something after idling for 2 minutes.

 

Check fuel system as well. Did it degrade suddendly ? If so, one of sensors - fuel pressure regulator and MAF sensor, coolant temp sensor, throttle position sensor. If slowly, air filter (easiest), fuel injectors - additives for cleaning them etc.

 

I think the mileage has been poor for a while. The car only became my daily driver a few months ago, before that my wife just drove it around town. I've had the fuel pressure checked and it has a new radiator thermostat, air filter, and fuel filter. I used a bottle of Redline fuel injector cleaner. I'm pretty confident it's the O2 sensor, or failing that I guess I'll have to check out the MAF sensor.

 

I ordered the O2 sensor from oxygensensors.com for $61.