Honda Check Engine Code
P0420 on Honda
Catalyst System Efficiency Below Threshold (Bank 1)
Also covers: Acura
P0420 on Honda: what makes it different
P0420 on Hondas is most commonly the downstream O2 sensor — Honda cats are well-engineered and tend to outlive the chassis on properly maintained vehicles, but the rear O2 ages predictably and triggers P0420 at 130–180k miles.
Most-affected engines
- K20/K24 (Civic Si, Accord, CR-V, Element)
- J35 V6 (Accord V6, Pilot, Odyssey, Ridgeline)
- R18/R20 (Civic, CR-V)
- L15 1.5T (Civic, CR-V, Accord 1.5T)
Common model years: 2001–present (most common on 130k+ mi vehicles)
Most likely cause on Honda
Aged downstream O2 sensor
Known Honda engine-family issues
The 2008–2013 J35 V6 with VCM (Variable Cylinder Management) is well-documented for excessive oil consumption that coats the catalytic converter. Owner workarounds include the VCM Tuner II ($150–$200) which keeps all 6 cylinders active and dramatically reduces oil consumption — but only the cat replacement fixes the existing P0420 if the substrate is contaminated.
Honda-specific causes (most common first)
- Slow downstream O2 sensor past 120k miles — replace with Denso or NTK (both OE suppliers for Honda)
- Ignored upstream misfire history — past misfires (P030x) thermally damage the cat and eventually trigger P0420
- Oil consumption on early VCM J35 V6s (2008–2013 Accord/Pilot/Odyssey V6 with VCM) — VCM-related oil burning coats the cat
- Exhaust leak ahead of the rear O2 (common at the flex pipe on older Civics and Accords)
- Aftermarket cat without OE precious-metal loading
- L15 1.5T (2017+ Civic, CR-V, Accord 1.5T) with the documented oil-dilution issue — cylinder washing eventually contaminates the cat
Honda-specific diagnostic tip
Before condemning the cat, check for stored or cleared misfire codes in the PCM's history. If the vehicle has a history of P0301–P0306 (cylinder misfires), the cat may be thermally damaged from past raw-fuel events. A cat replaced without fixing the underlying misfire cause will fail again.
Symptoms drivers report
- Check engine light
- Slight loss of power or fuel economy
- Rotten-egg smell (severe cases)
- Emissions test fail
Typical repair cost on Honda
Most Honda owners fix P0420 for between $60 and $1400, depending on which underlying cause turns out to be at fault. Start with the most-likely cause for your vehicle — Aged downstream O2 sensor — before throwing parts at it.
Honda P0420 FAQ
My Civic has 200k miles and just threw P0420 — is the cat shot?
Not necessarily. Try the downstream O2 sensor first (Denso 234-4356 or 234-4622 depending on year, ~$70 from RockAuto). At 200k miles, the rear O2 is almost certainly past its useful life and sluggish enough to trigger P0420 even on a healthy cat. Civic cats often last 250k+ miles when the engine has been maintained.
Related codes
Drive a different make? See the general P0420 guide for cross-vehicle causes and symptoms.
Confirm P0420 on your Honda in 60 seconds
AXLY.pro is a free iPhone app that pairs with any Bluetooth OBD2 adapter to read P0420 along with freeze-frame data and live engine readings. No subscription.