Appreciation post: the 'replace both O2 sensors' advice I kept hearing is a waste of money

River Harris · 2026-10-09T06:25:03.866095+00:00

I keep seeing folks in the shop say you should always swap both upstream and downstream sensors at the same time, but last week I read a service note from a Toyota tech in Reno that said the downstream sensor on most 2005 and newer cars only checks cat efficiency and rarely fails on its own. That surprised me because I have been quoting pairs to customers for years without really thinking about it. The upstream sensor is the one doing the fuel trim work, so if your code is P0133 or a heater circuit code, just do the one that's bad. Anyone here have real world data on whether the downstream ones actually fail more often than I think?

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Appreciation post: the 'replace both O2 sensors' advice I kept hearing is a waste of money | Auto Mechanics - Free To Speak