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Mileage mystery

I have a Honda 2000 Civic 3 door. Recently I had to replace the oxygen sensor after the ‘check engine’ light came on. My mileage had dropped off just before this happened from 37 to 39 mpg down to 31 mpg. With the new sensor, the mileage kept dropping. It’s now getting 27 to 29 mpg. My mechanic hooked up his computer & drove around with me for awhile, but no luck. He can’t find any problem.


It would help greatly if the exact code that was pulled were known. There are many codes for the O2 sensors. And depending on the code for the O2 sensor, it could be reflecting on the operating conditions of the engine, and not with a problem with the O2 sensor itself.


It appears from the invoice that the code may have been “P0135 02 heater sensor failure” They first “cleared the code” (i.e. rebooted th car’s computer), but the check engine light came on again in a few days. Then they replaced the 02 sensor.