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

Severity: medium Honda Civic Most affected: 2006–2015

Quick answer: On the Honda Civic, P0420 is frequently the rear oxygen sensor rather than the catalytic converter — a known Honda pattern. Test or replace the downstream O2 sensor first; on high-mileage Civics with good fuel trims and a verified sensor, then suspect the converter.

TL;DR

Honda Civic P0420: often the rear O2 sensor (Honda pattern), not the converter. Test the downstream sensor first — far cheaper. Shop cost: $200 (sensor) to $1,400 (converter).

Why Honda Civic owners get P0420

Civics share the Honda tendency for downstream O2 sensor degradation to set P0420 before the converter actually fails. Because the converter is the costly part, verify the rear sensor and fuel trims first; many Civic P0420s clear with a Denso/OEM sensor.

Most likely causes on the Civic

Likely cause	Probability	Notes
Faulty downstream (rear) O2 sensor	45%	Check first on Civics
Worn catalytic converter	40%
Exhaust leak near the sensor	15%

Honda Civic-specific known issues

Rear O2 sensor is a common Civic P0420 trigger — test before the converter.
Use a Honda/Denso-grade O2 sensor; cheap sensors can re-trigger the code.
Fit a CARB-compliant direct-fit converter if replacement is needed.

Honda Civic P0420 repair cost

Typical shop cost (Civic)	$200–$1,400
DIY range (generic)	$40–$250

Ranges vary by region, engine and parts grade.

Owner tips

Have the rear O2 sensor tested before approving a converter.
Prefer OEM/Denso sensors for a lasting fix.
Re-drive after a sensor change to confirm before spending on 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 Civic P0420 — FAQ

Is P0420 on a Civic the catalytic converter?

Often it is actually the rear O2 sensor, which is much cheaper. Test the sensor and fuel trims before replacing the converter.

How much is P0420 on a Honda Civic?

Roughly $200 if it is the rear O2 sensor, up to about $1,400 for a catalytic converter.

Can I keep driving my Civic with P0420?

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