2002 Toyota Echo oxygen sensor fault

yes, the oxygen sensor is the likely problem and is very easy to replace by your self. you can get the part from your local auto parts reatailer (kragen, shucks, checker, napa, autozone, o'reileys, advance auto... depending on where you live in the country.) If you live outside of california and the oxygen sensor does NOT fix the problem, the a new catalytic converter is in order. easy enough, again, to replace provided you know how to weld. probably not, seeing as how your on here asking the question. the part is again availible at your local retailer for about $80 and just about any mechanic or muffler shop can replace it for you. BUT if you live IN california, like me, your screwed and it'll cost about $400 or more to have replaced and you won't find the part in ANY store. good luck.