P0420 means the PCM judged Bank 1 catalyst oxygen-storage performance below threshold; it does not identify what degraded the catalyst. Save freeze data and readiness, repair misfire or mixture faults, check oil/coolant consumption and exhaust leaks, then compare upstream A/F and downstream O2 behavior under the correct Honda procedure. Two Accord-specific exceptions require VIN verification. Bulletin 14-002 covers narrow 2010 V6 VIN ranges where an A/F Sensor 1 cap may fall off and damage a warm-up catalyst. A separate historical notice covers certain 2006–2007 Accord Hybrids. Neither branch applies to ordinary four-cylinder, turbo or other-year Accords.
Match the symptom
Select the closest observation. The result changes the first test; it does not name a part to replace.
Preserve monitor evidence and complete prerequisite tests before parts.
Before authorizing a catalytic converter
Ask the shop to document three things: the companion-code status, exhaust integrity, and the evidence that Bank 1 sensor behavior fails the VIN-correct catalyst test. A quotation that begins only with 'P0420 means converter' skips the root-cause work that protects the replacement. Review misfire history, fuel trims, oil use and coolant loss before approving an expensive part.
Confirm the proposed converter matches the truck's model year, engine, wheelbase/exhaust configuration and underhood emissions certification. Federal and California-emissions applications may not use the same part. Ask whether new seals, fasteners or sensors are required, what warranty applies, and whether the estimate includes correcting the condition that damaged the original unit.
After repair, verify there are no leaks, rising misfire counts or abnormal trims. Complete the applicable Ford drive cycle so the catalyst monitor can run. A permanent P0420 cannot simply be erased; it clears after the OBD system observes enough passing operation. Keep the invoice and part certification for future emissions inspection or warranty questions.
What can set P0420—and how to separate it
The order below is evidence-driven, not a universal probability ranking. Vehicle year, repair history, companion codes and the operating condition in freeze frame change where diagnosis should begin.
| Possible cause | Evidence that supports it | First useful test |
|---|---|---|
| Bank 1 catalyst deterioration or physical damage | Prerequisites pass and Honda catalyst monitor test fails | Engine-specific catalyst evaluation |
| Misfire or rich operation overheated catalyst | P030x history, flashing lamp, negative trims or deposits | Correct ignition/fuel cause first |
| Oil/coolant contamination | Consumption, smoke, coolant loss or deposits | Mechanical/cooling diagnosis |
| Exhaust leak near monitored catalyst/sensors | Cold tick, soot or leak-test evidence | Cold inspection and safe leak test |
| A/F or downstream O2 sensor/circuit problem | Controlled response or electrical test fails | Graph sensors and test circuit |
The 2010 V6 bulletin describes physical sensor damage
Honda states that one or both A/F Sensor 1 caps may fall off and bore a hole through the affected warm-up catalyst. This is not generic sensor aging.
Verify the Accord's VIN against the published range and follow the retrieval/repair procedure. A shared model year without the matching V6/VIN is insufficient.
A catalyst can be the victim
Repeated misfire, excessive fuel, oil burning or coolant entry can damage a replacement rapidly. Review historical codes and fluid consumption even if the engine is smooth today.
A flashing-MIL event under load is especially important. Do not erase it from the story just because P0420 is now the only confirmed code.
Hybrid and conventional monitor conditions differ
The gasoline engine in a hybrid may stop while the vehicle remains operational, changing catalyst temperature and monitor opportunities.
Use Honda's hybrid-specific service mode and safety information. A generic steady-speed drive cycle may not produce a valid result.
Save these scan-data clues
Freeze frame is a snapshot of conditions when the PCM matured the fault. It can distinguish an idle-only problem from a load, cold-start or monitor-completion problem. Clearing first throws that context away.
- RPM, load, speed and coolant temperature
- Misfire and fuel-trim history
- A/F Sensor 1 and O2 Sensor 2 data
- Oil/coolant consumption
- Exhaust leak, rattle and restriction evidence
- VIN, engine, calibration and emissions label
A practical diagnostic workflow
- 01Preserve all emissions evidence
Save confirmed, pending and permanent codes plus readiness before clearing.
- 02Identify engine, bank and catalyst
Four-cylinder and V6/hybrid exhaust layouts use different bank and converter arrangements.
- 03Resolve catalyst killers
Diagnose misfire, rich/lean control, oil consumption and coolant entry first.
- 04Inspect the exhaust cold
Check flex sections, flanges, sensor bungs and shields without burn risk.
- 05Apply VIN-specific Honda branches
Use 14-002 or the hybrid notice only when every applicability detail matches.
- 06Complete the monitor
After repair, follow the Honda drive/monitor procedure and confirm no pending P0420.
Do not replace the downstream O2 sensor or converter from one code. Bulletin 14-002 requires the listed 2010 V6 VIN range and its physical failure condition. The 2006–2007 Hybrid notice was a time/geography-limited historical program; verify current VIN status rather than promising coverage.
Why the model year changes the test
Accord diagnostics span older F-series four-cylinders and J-series V6 engines, K-series four-cylinders with rocker-arm oil-pressure monitoring, Earth Dreams direct-injection engines, 1.5L and 2.0L turbo powertrains, and multiple hybrid generations. Cylinder/bank layout, VTEC/VTC oil control, EVAP strategy, catalyst monitoring and fuel delivery differ. Confirm model year, VIN, engine, transmission, hybrid status, emissions label and current Honda bulletin/campaign eligibility before choosing a test or part.
- 2010 V6: bulletin 14-002 lists narrow VIN ranges and possible P2243/P2247 companion codes from A/F Sensor 1 cap damage.
- 2006–2007 Accord Hybrid: bulletin 18-022 documentation describes a historical catalyst/software warranty action with exclusions and deadlines.
- A four-cylinder has one bank; J-series V6 engines have two and require correct Bank 1 identification.
- Turbocharged 1.5L/2.0L engines add boost, direct-injection and oil-dilution history to upstream diagnosis.
P0420 FAQ
Does P0420 prove the catalyst is bad?+
No. Engine, exhaust and sensor prerequisites must be tested first.
Can I drive with P0420?+
Usually briefly with a steady lamp and normal operation; stop for flashing, strong odor, rattle or power loss.
Will an oxygen sensor fix it?+
Only if response and circuit testing proves that sensor or wiring faulty.
Does bulletin 14-002 cover every 2010 Accord?+
No. It specifies V6 body/transmission configurations and narrow VIN ranges.
Is the old Hybrid extension still active?+
Do not assume so; verify VIN and current campaign/warranty status with Honda.
Can oil use cause P0420?+
Yes. Oil ash can contaminate catalyst material, so repair the consumption source.
Why did P0420 return after replacement?+
An upstream fault, exhaust leak, incorrect part or sensor/circuit problem may remain.
What confirms repair?+
Healthy fuel/misfire control and a completed catalyst monitor without pending P0420.
Ford diagnostic references
These documents explain Ford monitor operation. Exact pinpoint tests, connector views and specifications must be selected for the truck's VIN and model year in current service information.
01Honda Service Bulletin 14-002 — 2010 Accord V6 P0420/P0430Narrow 2010 Accord V6 VIN ranges only; an A/F Sensor 1 cap may damage the affected warm-up catalyst↗02Honda Service Bulletin 18-022 notice — 2006–2007 Accord Hybrid catalystHistorical 2006–2007 Accord Hybrid campaign/warranty notice with geographic and time limitations; verify VIN and current status↗03Honda ServiceExpress technical informationHonda's service publications, wiring information and model/year repair procedures↗04Honda Accord authorized manualsHonda directs owners and technicians to ServiceExpress for electronic service publications↗