Quick answer: Honda Accord catalytic converter problems usually store P0420, and on the Accord the most frequent real cause is a degraded downstream oxygen sensor or an aging original converter past 150,000 miles. Honda’s factory converters are comparatively sensitive, and owners widely report that budget aftermarket units re-trigger the code within weeks.
The Accord’s long service life means many are still on the road well past the point where the original converter loses efficiency. This guide covers what is specific to the Accord and how to avoid buying the wrong part twice.
Why the Accord Logs P0420
P0420 is set when the downstream oxygen sensor stops seeing the expected difference from the upstream sensor. On the Accord, that is most often caused by a drifting downstream sensor or a genuinely tired original converter. Because Honda’s OEM catalyst is closely matched to the ECM’s expectations, replacing it with a low-cost universal unit frequently sets the code again even when the part is new.
For the full explanation, see our main guide: P0420 – Catalytic Converter Efficiency Explained.
Most Affected Accord Years
- 2003-2007 (K24): Original converters and O2 sensors reaching end of life.
- 2008-2012: Very common P0420 range on high-mileage examples.
- 2013-2017 (2.4L Earth Dreams): Usually sensor-related rather than a failed catalyst.
Symptoms on the Accord
- Check engine light with a stored P0420.
- Slight fuel economy loss.
- Occasional sulfur smell from the exhaust.
- No noticeable change in power in most cases.
Common Causes (Cheapest First)
- Downstream oxygen sensor – the single most common false trigger.
- Exhaust leak near the manifold or flex pipe.
- Unresolved misfire or oil consumption – these damage a converter over time.
- Worn original converter – replace with OEM or a high-quality CARB unit.
Check These Yourself First
- Check for other codes; fix misfire or lean codes before touching the converter.
- Inspect the front exhaust for leaks and soot.
- Check oil level – oil reaching the exhaust degrades the catalyst.
Estimated Costs (2026)
| Job | Parts ($) | Labor ($) |
|---|---|---|
| Downstream O2 sensor | $50 – $160 | $60 – $120 |
| Exhaust leak repair | $25 – $100 | $80 – $160 |
| OEM catalytic converter | $500 – $1,200 | $160 – $320 |
Note: 2026 US estimates. On the Accord, spending more on an OEM or CARB-compliant converter usually costs less than replacing a cheap one twice.
Frequently Asked Questions
Why did my Accord’s P0420 come back after a new converter?
Two reasons dominate: either the underlying cause (worn sensor, exhaust leak, misfire) was never fixed, or the replacement converter does not meet the specification the ECM expects. Both are common on this car.
Should I replace the O2 sensor first?
It is usually the right order. A downstream sensor is a fraction of the converter cost and is the most frequent cause of a false P0420 on the Accord.
Can I pass an emissions test with P0420?
Not typically. A stored P0420 with the check engine light on will fail inspection in most regions.
Related: P0420 full guide · P0171 lean condition · P0300 misfire