Code P1131 after O2 sensor replacement

[Mac2002]

My 1999 Taurus, DOHC, 183,000 miles with all original O2 sensors, gave me a P0153 code last week (bank 2, sensor 1 not switching fast enough, I think), so I replaced the sensor and cleared the codes. 3 days later, she throws another code at me. This time, P1131 (bank 1, sensor 1 lean). My question: can my replacing only one upstream sensor cause the other to finally go out of range. I'll go hunt for vacuum leaks now, but this just seems to coincidental.

Thanks you

[65comet]

How is it running? Does it need to down shift on hills that it didn't need to in the past? If so at that mileage it might be the catalytic converter finally giving up the ghost and the codes are telling you about that problem. And of course it could be those sensors are just finally failing. Is it burning oil? Oil through the exhaust loves to ruin the sensors.

[Mac2002]

65Comet, she seems to run perfectly. No loss of power. But she leaks a little oil and I can't tell if the oil level drops due to that or from her drinking it. I'm sure it's a little bit of both. Oil consumption is about a quart in 2000 miles of commuting in rush hour. No smoke in the exhaust, though.

Thank you for the help.