P0420 means your catalytic converter isn't cleaning exhaust gases efficiently enough. The downstream O2 sensor is detecting that converter performance has dropped below the acceptable threshold set by your vehicle's ECU.
DTC/P0420·Honda
P0420 on Honda: Causes, Symptoms and Fix Cost
Quick Answer
Moderate SeveritySafe to drive (short-term)Honda
P0420 on a Honda means your catalytic converter isn't cleaning exhaust gases efficiently enough. The downstream O2 sensor is detecting that converter performance has dropped below the acceptable threshold set by your vehicle's ECU.
Repair cost on Honda€90 - €1620
What does P0420 mean on a Honda?
Honda Civic
2006–2024On 2006–2011 Civics with the R18A engine, P0420 is frequently caused by a failing rear oxygen sensor rather than the converter itself. Always test the downstream O2 sensor first before committing to a converter replacement. Exhaust manifold cracking is another common issue on earlier models.
What causes P0420 on a Honda?
Beyond the generic causes listed on the main P0420 page, these are the Honda-specific patterns we see most often:
Honda CivicRear O2 sensor degradation common on R18A engines
Honda CivicExhaust manifold cracking on 2006–2009 models
How to diagnose P0420 on a Honda with OBD2
Follow these steps to pinpoint the root cause of P0420:
- Read freeze frame data to identify conditions when code was set
- Check for other codes (misfires, O2 sensor codes) that may be the root cause
- Inspect exhaust system for leaks before and after the catalytic converter
- Monitor O2 sensor waveforms with live data. Downstream sensor should show steady voltage if converter is working
- Compare upstream vs downstream O2 sensor readings. If nearly identical, the converter is failing
How much does P0420 cost to fix on a Honda? (EUR)
Estimated repair costs on a Honda (Honda parts and labour typically run 10% below the average for this code).
Prices estimated as of May 2026. Costs vary by region, vehicle, and shop.
Related codes that often appear with P0420 on Honda
These codes commonly cluster with P0420 on Honda vehicles:
FAQ: P0420 on Honda
Can a P0420 code clear itself?
Yes, if the underlying issue was temporary (like a tank of bad fuel), the ECU will clear it after several successful drive cycles. If it persists after 3–4 drive cycles, the converter or sensor likely needs attention.
Will P0420 cause my car to fail emissions?
Yes. P0420 directly relates to catalyst efficiency, which is a primary emissions test parameter. You'll need to resolve this code before passing an emissions inspection.
Can I just clear the code and ignore it?
You can clear it, but it will return if the root cause isn't fixed. Ignoring it long-term can lead to further converter damage and reduced fuel economy.
Is it the catalytic converter or the O2 sensor?
About 60–70% of P0420 cases are the converter itself, but the downstream O2 sensor is a common misdiagnosis. Always test the O2 sensor response before replacing the converter, as it's much cheaper.
Looking for the full P0420 reference (all makes, full diagnosis flow, complete repair cost matrix)?
See the main P0420 guideSkanyx · Honda
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