2017 Honda Accord - P0420 Catalyst System Efficiency Below Threshold

Question

My Accord's check engine light came on yesterday. Scanned it and got the dreaded P0420. The car drives totally fine, no loss of power, no smell, and no weird noises. It has 110k miles.

I really don't want to drop $1k on a new catalytic converter if it's just a sensor. Is there a reliable way to check if the cat is actually bad or if it's just the O2 sensor being lazy?

AudiAlex22 · Accepted Answer

✅ Confirmed fix: Found a small pinhole leak in the exhaust manifold weld just before the primary cat. This was allowing oxygen to enter the stream, tricking the downstream sensor into thinking the catalyst wasn't doing its job. Tig welded the crack, cleared the code, and it's been off for 5,000 miles. Always check for exhaust leaks before replacing the cat!

WesternStarWill · Answer

Use a scan tool to graph the upstream vs downstream O2 sensors once the car is closed-loop. If the downstream sensor is mimicking the upstream (oscillating rapidly), the cat is dead. If it's a steady line around 0.6-0.7V, the cat is fine and you have a sensor or exhaust leak issue.

2017 Honda Accord - P0420 Catalyst System Efficiency Below Threshold

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