12-24-2025, 06:51 AM
The check engine light in my 2015 Honda Civic came on last week, and while the car seems to drive fine, I'm hesitant to just ignore it. I borrowed a basic OBD2 scanner from a friend and it's showing a P0420 code, which points to a catalyst system efficiency issue. I'm a fairly handy DIYer but new to emissions-related problems. For those who have dealt with this code, what was the most common culprit you found, and what's a logical diagnostic sequence to follow before throwing expensive parts like the catalytic converter or oxygen sensors at it? Are there any specific tests with a multimeter or visual inspections under the car I should perform first to narrow it down?