Honda Accord P0420: Catalyst System Efficiency Below Threshold (Bank 1)

Severity: medium Honda Accord Most affected: 2008–2017 (2.4L K24 / 3.5L V6)

Quick answer: On the Honda Accord, a P0420 code is frequently caused by the rear (downstream) oxygen sensor rather than the converter — Honda’s rear O2 sensors are a common P0420 source. Test or replace the rear sensor before the converter, which is significantly more expensive.

TL;DR

Honda Accord P0420: often the rear O2 sensor, not the converter. Test/replace the downstream sensor first (cheaper). If fuel trims are good and the sensor checks out, then suspect the converter. Shop cost: $200 (sensor) to $1,500 (converter).

Why Honda Accord owners get P0420

Accords commonly set P0420 due to a degraded downstream oxygen sensor whose signal starts mirroring the upstream sensor. Because the converter is costly, the model-specific best practice is to verify the rear sensor and fuel trims first; many Accord P0420s clear with a sensor for a fraction of the converter price.

Most likely causes on the Accord

Likely cause	Probability	Notes
Faulty downstream (rear) O2 sensor	45%	Common Accord cause — check first
Worn catalytic converter	40%
Exhaust leak near the sensor	15%

Honda Accord-specific known issues

Rear O2 sensor degradation is a frequent P0420 trigger on Accords — test before replacing the cat.
Use a Honda/Denso-grade O2 sensor; cheap sensors can re-trigger the code.
A cheap universal converter may re-set P0420 — fit a CARB-compliant direct-fit unit.

Honda Accord P0420 repair cost

Typical shop cost (Accord)	$200–$1,500
DIY range (generic)	$40–$250

Ranges vary by region, engine and parts grade.

Owner tips

Have the rear O2 sensor tested first; it is far cheaper than the converter.
Prefer OEM/Denso sensors and CARB-compliant converters for lasting fixes.
Clear the code and re-drive after a sensor change to confirm before approving a converter.

Confirm the code yourself: OBD-II scanners →

Full P0420 diagnostic guide

For the complete step-by-step diagnosis, all causes, and repair walkthrough that applies to every vehicle, see the main guide: P0420 — Catalyst System Efficiency Below Threshold (Bank 1).

Honda Accord P0420 — FAQ

Can the O2 sensor cause P0420 on a Honda Accord?

Yes — a degraded downstream O2 sensor is one of the most common P0420 causes on Accords. Test it before replacing the converter.

How much to fix P0420 on an Accord?

Around $200 if it is the rear O2 sensor, up to roughly $1,500 if the catalytic converter needs replacing.

Can I keep driving my Accord with P0420?

Short-term yes, with a steady light and no misfires. You will fail emissions until repaired.