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Vehicle
2015 Honda Civic LX · 92,400 mi
Check engine light came on yesterday. Scanner says P0420. Car still drives fine. Is it safe and how bad is this?
P0420 means your catalytic converter isn't cleaning exhaust as efficiently as it should — usually a slow-developing issue, not an emergency.
Severity
Safe to drive short term. Will fail an emissions test and may worsen fuel economy.
Probable Causes
- • Aging catalytic converter (most common above 90k mi)
- • Faulty downstream O2 sensor giving misleading readings
- • Exhaust leak before the catalytic converter
- • Engine misfire contaminating the cat
DIY Feasibility
O2 sensor swap is beginner-friendly (~$40 part, 30 min, basic wrench). Catalytic converter replacement is shop-recommended.
Estimated Cost
O2 sensor
$60 – $180
Cat replacement
$650 – $1,400
Next Steps
- 1Drive normally for a week — note if MPG drops noticeably.
- 2Replace the downstream (post-cat) O2 sensor first — cheapest fix, often resolves it.
- 3Clear the code, drive 50–100 mi, rescan. If P0420 returns, the catalytic converter is the culprit.
- 4Get 2 shop quotes before approving a cat replacement — prices vary widely.
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Disclaimer: This example is illustrative. Always confirm critical repairs with a qualified mechanic.