What should I check first for a P0420 catalyst efficiency code on my Camry?

[Eleanor_W]

I just got a P0420 code on my 2008 Camry and I’m trying to figure out what to check first. The car runs fine, no weird smells or sounds, but the light is on and it failed its emissions test because of the catalyst efficiency.

[DonaldQS]

I get it. The code points to the cat not doing its job, but could be a sensor or an exhaust leak too. I started simple: checked for obvious exhaust leaks around the manifold and hangers, then checked the O2 sensors and made sure the downstream sensor was actually switching. If nothing obvious shows up, I’d suspect the cat or a lazy sensor, but I didn’t rush to replace the converter.

[AaronR]

Same car, similar symptoms. No smells, no noise, light stays on. I tried clearing it once, and it came back after a few drives. A shop suggested the cat, but I wasn’t convinced and bailed on the replacement after price.

[Tyler.S]

I wonder if the real culprit is the upstream sensor or a misfire at idle that logs bad trims. The engine ran smooth, so it felt off to chase the cat first. I did a couple of highway runs and watched fuel trims, they stayed kind of odd though not dramatic.

[StellaW]

Have you checked for pending codes or tried a long drive to clear it? P0420 shows catalyst efficiency below threshold, but that can hide a bad sensor or a leaky exhaust more often than a dead converter?