P0420 on Honda Accord
Catalyst System Efficiency Below Threshold Bank 1
Methodology: Reviewed against the SAE J2012 generic DTC definition and NHTSA owner-complaint patterns, then scoped to Honda Accord context without treating complaints as confirmed diagnoses.
Sources: SAE/OBD-II definitions, manufacturer service information, and public reliability reports (including NHTSA where applicable).
- SAE J2012 Diagnostic Trouble Code Definitions — Generic diagnostic trouble code definition reference for P0420.
- NHTSA recalls, complaints, and manufacturer communications — Used to check public owner complaint context, recall status, and safety-related manufacturer communications by VIN or vehicle.
Limits: Guidance is informational and may not match every model-year or revision. Confirm with the official manual and diagnostics before replacing parts. For safety-critical or electrical work, use a qualified technician.
What P0420 Means on Your Honda Accord
P0420 is a diagnostic trouble code indicating: Catalyst System Efficiency Below Threshold Bank 1. This code relates to the emission controls system. The vehicle's computer detected a condition outside normal operating parameters and stored this code.
Real Honda Accord Owner Reports
Source: NHTSA Complaints Database
"I just bought the car and had it 1 week and i was driving to work when the check engine light came on suddenly and car started misfiring. this happened right when i was stopped at a light and turned on my a/c. 3 codes came on which were p0420, p0139, and p2647."
"In 2008, i purchase a new honda accord v6 paid with financing $25,000. i purchase a brand new vehicle to not have to be bothered with someone else problems. when my car reached 100 thousand miles in the winter, i heard a loud knocking sound under the hood and i was loosing a lot of oil."
"My 2010 honda accord has had a check engine light for a p0420 code at 110k miles. for someone that knows something about cars, a catalytic converter should not be going out this soon."
"After service at honda service station at rosenberg, tx... i had issues with the temperature sensor in the front of the grill, also a code showed up 2 days later, along with check engine light... when scanned it was "p0420"..."
All reports filed by vehicle owners directly with the U.S. Department of Transportation.
📊 Complaint Trend by Model Year
Common Causes on Honda Accord
Based on NHTSA complaint component analysis for this vehicle.
What To Do Next
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1
Check for recalls on YOUR VIN
Go to nhtsa.gov/recalls and enter your VIN. If your vehicle is covered, repairs are free.
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2
Get a proper diagnosis
A code alone doesn't tell you the exact failed part. A diagnostic at a shop ($50–$150) pinpoints the root cause before you spend money on parts.
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3
Compare repair quotes
Get 2–3 quotes. Dealer vs. independent shop prices often differ 30–50% for the same repair.