My ES300 had same issue, only I have 190K on the clock. Car started getting bad mileage and eventually check engine light came on. Did a lot of reading, in the end I replaced both front and rear 02 sensor univeral bosch types from autozone and problem solved car now gets 22 mpg and light went out. After problem solved I did replace PCV air filter and fuel filter just for good measure. Next I plan on removing the bosch platnum spark plugs and replacing them with bosch super. I did this in my 85 944 replaced platnum with regular bosch super and my 944 went from 20 MPG to 24 MPG. I'll let you guys know if this helps mileage.