My thoughts exactly. Those things aren't cheap!Generally the rule-of-thumb is 100,000 miles for 02 sensors. Now, can they last longer... sure. Should you replace them without a code... I wouldn't, but you could. The idea that O2 sensors were just maintenance items was back in the day when they only costed $40-65 bucks to replace. It was generally considered part of the 100k mile service tune-up and a good idea to keep the a/f ratio perfect. But when your front O2 sensors are $175, I would wait until there's a really good reason to change them out.