RAYThOM

After two winters of intermittent check engine lights indicating O2 related problems (below c. 40°F) I replaced the original sensors in February. (Auto Zone/universal, spliced, single wire connection type.) No more lights, no more raw gas smell, instant start up. Oddly, for the longest time I was getting approx. 21 mpg. With the new sensors, however, my mpg has dropped to approx. 17.

Do sensors have a long breaking in period and/or is there Honda specific sensors -- contrary to what the counter guy sold me? Any reply will be appreciated. Thanks.