Will a "weak" or old Oxygen Sensor impact gas mileage? Should they be changed on a regular routine basis? ('98 LS400--120,000 miles).

 

Not sure particularly on the LS, but oxygen sensors on the ES tend to get lazy after 100,000-125,000 miles, thus lowering the gas mileage. They may or may not trigger the check engine light, but I'd say it's a key part of preventive maintenance to replace the O2 sensors after that kind of mileage.

 

it definitely will, i just had my sensor go out on bank2 , sensor2 and i noticed it has my mileage off by about 1MPG. when the sensor gets lazy the computer cant adjust the ratio properly