P0421 (2025 Update)

Googling it looks like it means something is amiss between the reading of the upstream O2 and the downstream while it's warming up. So, yes, it could be. Clare here is a retired mechanic, he'll probably see it and give you better info.

Since a cat is expensive, a doublecheck test would be nice to confirm that it's really the cat, even if that's the most likely thing.

Is there a way to doublecheck the cat using the real time readout on the scanner to look at some particular value?

Same way the ECM checks. The pre-cat clocks. If the post-cat doesn't clock, the cat's purring. If ther post-cat clocks, the cat's dead. If the post-cat just twitches, the cat's on life support.

another functional test - pyro ahead and behind - outlet has to be hotter than the inlet if it's working. NOT easy to test accurately this way.

replying to DForth, Dman wrote: You should be replacing both O2 sensors. Check your plugs. Fouled plugs sometime cause this. Also clean your MAF sensor with MAF sensor cleaner. DO NOT use carb or any other cleaner for the MAF sensor. Good luck. After replacing your O2 sensor, did you reset the original error with the OBD reset option?