98 Mistique OBDII code 1131

I have a OBDII code of 1131 Lack of HO2S1 Switches-Lean (Bank 1 sensor 1). The check engine lite comes and goes. This is a 4 cylinder engine in the Mistique. Any good Ideas as where to check? MAF sensor was just cleaned. I'm getting poor gas mileage too, around 19MPG VS: 28MPG not too long ago.

does your car smell like rotten eggs? or hear a slight rattle under the car? when the engine like comes on its usually emissions. signs of a catalytic converter going bad is gas mileage and your engine light coming on/ flashing while driving

2001jettavr6 (http://www.automotiveforums.com/vbulletin/member.php?u=527726) ...nope no smells or loss of power....Cat is not rattling and doesn't seem to be plugged. Does anyone know what sensors would be involved in this problem?

Its the upstream 02 sensor. If your running lean, I would guess youve got an intake leak somewhere. I would rule the cat out.