My 98 Civic DX is throwing a code 63 - "Secondary oxygen sensor". My first assumption is that it is the sensor after the CAT. Before I buy and replaced it can someone confirm which is the "Secondary" sensor?

It's been like that for about a year. My gas mileage is down from 41 to 34 mpg. With gas prices so high I want to fix it now.

Thanks in advance for your help.

 

yes the secondary is the one in the cat