P0420 is rarely the sensor — what the rear O2 is actually saying

The catalyst stores oxygen while it works. The rear O2 sensor exists to watch that storage: while the cat is healthy, the front sensor switches constantly and the rear one stays comparatively flat and lazy. P0420 sets when the rear starts mirroring the front — oxygen is passing straight through.

So the sensor reporting the failure is usually doing its job, and replacing it usually buys a few weeks until the monitor runs again. Before any parts, two checks:

First, what killed the efficiency. Catalysts rarely die of old age alone — sustained misfires load them with unburned fuel, lean running overheats them, burned oil or coolant coats them. If there is a misfire or fuel-trim code current or in history, fix that first, or the replacement cat inherits the same death.

Second, exhaust leaks ahead of and around the rear sensor. A leaking flange or a cracked flex section pulls in outside air and makes a working cat read as failed.

Then look at the rear O2 live. Flat around 0.6–0.7 V on a warm steady cruise is a healthy catalyst; a rear trace switching in step with the front is the real failure the code describes.

A word on the spacer trick sold everywhere: it moves the sensor out of the exhaust stream so the monitor passes. It does not make the exhaust clean, and it fails the honest version of any inspection.