The check engine light came on in my 2015 Toyota Camry yesterday. The car seems to be running fine—no rough idle, no loss of power, and fuel economy seems normal. I borrowed a basic OBD2 scanner from a friend, and it's showing a P0420 code, which I think is related to the catalytic converter or oxygen sensors. Before I take it to a mechanic and potentially get hit with a huge bill, I'm wondering what the most likely culprits are for this code on a relatively low-mileage car. Could it be something simpler like a faulty oxygen sensor or an exhaust leak, or is the catalytic converter usually the problem? What's a logical diagnostic sequence to follow?
You're not wrong to be cautious. P0420 means catalyst efficiency is below threshold. On a relatively low-mileage 2015 Camry, the culprits are more often a faulty downstream O2 sensor or a small exhaust leak than a dead cat. Start by logging live data: compare LTFT and STFT for the upstream (pre-cat) sensor versus the downstream (post-cat) sensor. If the post-cat sensor barely moves or tracks oddly while the pre-cat is healthy, you’re likely chasing a sensor issue. Do a quick visual exhaust check for leaks at clamps, gaskets, and the cat inlet. Even a tiny crack or loose clamp can throw readings off. Then swap the downstream O2 sensor (cheap and common) and clear the code. If it returns, test for misfires (P0300-series). If misfires aren’t present and all sensors look good, the converter may be failing—though that’s less common at low mileage.