Drive with caution — fix soon.
This code means the bank 2 sensor 3 oxygen sensor responds too slowly to changes in the exhaust, taking longer than expected to swing between rich and lean. This usually happens as the sensor ages or becomes contaminated and can no longer react quickly. The fix is almost always replacing the oxygen sensor, though an exhaust leak or contamination from oil or coolant should be ruled out.
$150 – $400
Varies by vehicle and root cause.
Usually yes, because a slow-responding rear sensor mainly affects emissions monitoring rather than how the car drives. Short trips are fine, though you may see slightly worse fuel economy. Plan to replace the sensor within a couple of weeks, as slow response usually means it's wearing out.
Most repairs run $150 to $400, covering the oxygen sensor and about an hour of labor. Sensors that are hard to reach on certain vehicles can cost a bit more. Since the cause is almost always a tired sensor, replacement usually resolves it cleanly.
It's a moderate concern. The vehicle stays drivable and safe, but the aging sensor won't help the engine run efficiently and will fail an emissions test. Replacing it promptly restores accurate monitoring and fuel control.
Over tens of thousands of miles, the sensor's sensitive tip gets coated with carbon and combustion byproducts, and the internal element degrades. As that happens it can no longer flip quickly between rich and lean, which is exactly what a slow-response code like P0166 detects.