P0420 on Honda: Causes, Symptoms and Fix Cost
Quick Answer
Safe to drive (short-term)
P0420 on a Honda means the catalytic converter on Bank 1 (the engine side with cylinder 1) is no longer cleaning the exhaust efficiently enough, based on the computer comparing the oxygen sensors before and after it. The most common cause is a worn, aged catalytic converter. Usually there are no driveability symptoms beyond the warning light and slightly worse economy, but at a periodic inspection the lit warning light can be recorded as a deficiency. A converter replacement typically costs 400 to 1800 EUR or more.
- Severity
- Moderate · Emissions
- Typical repair cost, all makes
- €100-€1800
Connect a standard OBD2 adapter (Bluetooth LE on iPhone) to your Honda and Skanyx tells you what P0420 means on that engine, whether it is safe to keep driving, and what the fix should cost. Free on iPhone and Android.
Get Skanyx freeWhat does P0420 mean on a Honda?
The Quick Answer above is the generic definition. What follows is the model-specific detail for the Honda models that log P0420.
Honda Civic
2006-2024
On 2006-2011 Civics with the R18A engine, P0420 is frequently caused by a tired rear oxygen sensor rather than the converter itself, so test the downstream O2 sensor first before committing to a converter. Exhaust manifold cracking on earlier models can also pull in air and trigger the code.
What causes P0420 on a Honda?
Beyond the generic causes listed on the main P0420 page, these are the patterns noted for Honda models:
Rear O2 sensor degradation common on R18A engines
: Honda Civic
Exhaust manifold cracking on 2006-2009 models drawing in air
: Honda Civic
How to diagnose P0420 on a Honda with OBD2
- Read freeze frame data to see the engine load, temperature and fuel trims when the code was stored
- Scan for other codes first, misfire (P0300 series), O2 sensor or fuel-trim codes usually point to the real root cause and should be fixed before touching the converter
- Inspect the exhaust from the manifold back, look and listen for leaks at gaskets, flex pipes and welds before and around the converter
- Watch live O2 sensor data, the upstream sensor should swing rapidly while a healthy downstream sensor stays nearly flat
- Compare upstream versus downstream waveforms, if the downstream sensor swings almost as much as the upstream one, the converter is failing
- Check fuel trims and the MAF reading, rule out a rich or lean condition or a dirty MAF that could be contaminating the converter
- Clear the code and complete a drive cycle, confirm whether it returns and re-test catalyst monitor readiness
How much does P0420 cost to fix on a Honda? (EUR)
Estimated repair costs for P0420 in EUR, compiled across makes. Honda parts and labour usually land towards the lower end of each range.
| Repair | Parts Cost | Labor Cost | Total Estimate | DIY Difficulty |
|---|---|---|---|---|
| Replace catalytic converter | €300–€1500 | €100–€300 | €400–€1800 | Professional |
| Replace downstream O2 sensor | €50–€150 | €60–€120 | €110–€270 | Moderate |
| Fix exhaust leak | €20–€100 | €80–€200 | €100–€300 | Moderate |
Prices estimated as of October 2026. Costs vary by region, vehicle, and shop.
Codes related to P0420
These codes are often seen alongside P0420 or point to related issues, on any make:
Looking for the full P0420 reference (all makes, full diagnosis flow, complete repair cost matrix)? See the main P0420 guide