My 2018 SUV's check engine light came on yesterday, and while the car seems to be running normally, I'm nervous about ignoring it. I borrowed a basic OBD2 scanner from a friend, and it's giving me a P0420 code, which points to a catalyst system efficiency issue. I'm fairly handy and have done basic maintenance, but I'm not sure how to proceed with this diagnosis. Could it be something simple like an oxygen sensor, or is this a sign of a failing catalytic converter? For those who have dealt with a P0420, what was your step-by-step diagnostic process to pinpoint the problem without just throwing expensive parts at it? I'd like to try some troubleshooting myself before taking it to a shop.
P0420 can be caused by a failing catalytic converter or by the oxygen sensors or even a leak in the exhaust, so here's a practical DIY approach: start by confirming there are no other codes, then check for exhaust leaks around the manifold, turbo, or cat that could fool the sensor readings. Use live data: watch the upstream O2 sensor (sensor 1) as it should cycle quickly between about 0.1–0.9 V; the downstream sensor (sensor 2) should be relatively flat and sit around mid-volt values without following every wiggle of the upstream sensor. Note your LTFT and STFT; persistent positive trims suggest fueling issues or a vacuum leak, while large negative trims point to an overly rich mix. If the sensors look healthy but the downstream signal is still erratic or converges with the upstream, the cat is suspect; if you have a suspected faulty sensor, swapping it can help confirm. If in doubt, test drive with a known-good sensor or a shop test, but avoid throwing parts blindly. After any fix, clear codes and drive for a bit to ensure P0420 doesn’t return.