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P0420 on a 2017 Honda Civic

Catalyst Efficiency Below Threshold

P0420 on a 2017 Honda Civic indicates catalyst efficiency below threshold. It usually stays drivable short-term but should be diagnosed promptly. The most common cause is worn or contaminated catalytic converter (bank 1) (typically $600–$2,400). Confirm the root cause before replacing parts.

Severity: moderate Safe to drive (short term) Compact Sedan 2015-2019 Honda Civic

Reviewed by MECH AI Editorial · Last verified · Includes 20 active NHTSA TSBs

What does P0420 mean on a 2017 Honda Civic?

P0420 is set when the engine control module (ECM) compares the upstream and downstream oxygen sensor readings on Bank 1 and concludes the catalytic converter is no longer storing and releasing oxygen efficiently. In a healthy system, the downstream O2 sensor signal stays relatively flat compared to the switching upstream sensor. When both signals look similar, the ECM infers the catalyst has lost its ability to convert hydrocarbons, carbon monoxide, and oxides of nitrogen into less harmful byproducts.

This guide covers P0420 across the 2015-2019 Honda Civic generation — the symptoms, causes, and diagnostic steps below apply to every model year from 2015 through 2019.

Is it safe to drive a 2017 Honda Civic with P0420?

In most cases a 2017 Honda Civic stays drivable for short trips with P0420 active, but diagnose and repair it promptly. This is a moderate-severity code — ignoring it can lead to further damage or a failed emissions test.

What are the symptoms of P0420 on a 2017 Honda Civic?

What causes P0420 on a 2017 Honda Civic?

Cause Likelihood Estimated repair (USD)
Worn or contaminated catalytic converter (Bank 1) Most common root cause once the vehicle is over 100,000 miles. Most common $600–$2,400
Failing downstream (post-catalyst) oxygen sensor Slow-switching O2 sensor mimics a dead catalyst. Common $150–$450
Exhaust leak upstream of the downstream O2 sensor Common $100–$500
Engine misfire or rich/lean fuel trim contaminating the catalyst Resolve any P0300-series codes before condemning the catalyst. Occasional $200–$1,200
Coolant or oil entering the exhaust (head gasket, valve seals) Phosphorus and silicon poison the catalyst substrate. Rare $1,200–$3,500
Aftermarket / off-brand catalytic converter that does not meet OEM spec Occasional $600–$1,800

How to diagnose this on a 2017 Honda Civic

  1. Pull all stored and pending codes first

    Use an OBD-II scanner to read every stored, pending, and permanent code. If misfire codes (P0300–P0308), fuel trim codes (P0171/P0174), or upstream O2 codes (P0130-series) are present, fix those first — they will set P0420 as a downstream symptom.

    Tools: OBD-II scanner with live data capability

  2. Inspect the exhaust between the engine and the rear O2 sensor

    Cold-start the engine and listen for ticking from the manifold, gaskets, flex pipe, and weld seams. A pinhole leak upstream of the downstream O2 sensor pulls in fresh air, fools the sensor, and sets P0420 with a perfectly good catalyst.

    Tools: Mechanic's stethoscope or rubber hose, Flashlight

  3. Compare upstream and downstream O2 sensor voltages in live data

    Bring the engine to operating temperature and hold ~2000 RPM. The upstream sensor should switch between roughly 0.1 V and 0.9 V several times per second. The downstream sensor on a healthy catalyst should hold steady around 0.6–0.8 V. If the downstream sensor mirrors the upstream sensor's switching, the catalyst is no longer storing oxygen.

    Tools: Scan tool with live PID graphing

  4. Measure short and long-term fuel trims

    Fuel trims outside ±10 % at idle or cruise indicate an unrelated fuel mixture problem that may be cooking the catalyst. Resolve the trim issue before replacing the converter.

    Tools: Scan tool with fuel trim PIDs

  5. Inspect for physical damage and rattle

    Tap the body of the catalytic converter with a rubber mallet. A rattling sound indicates the substrate is broken — the converter is finished and must be replaced.

    Tools: Rubber mallet

Known Technical Service Bulletins for the 2015-2019 Honda Civic

Manufacturers publish Technical Service Bulletins (TSBs) when a known issue affects a specific vehicle. These bulletins come from the NHTSA database for your Honda Civic.

+14 more TSBs available in MECH AI's TSB explorer for this vehicle.

NHTSA complaints & recalls for the 2017 Honda Civic

Owner-reported safety complaints and official recalls filed with the U.S. National Highway Traffic Safety Administration for the 2017 Honda Civic. Use these to gauge how common a problem is on your specific vehicle before you start chasing Honda Civic diagnostics.

569 owner complaints
22 involved a crash
6 involved a fire
11 reported injuries
  • ENGINE 65
  • STEERING 161
  • UNKNOWN OR OTHER 160
  • ELECTRICAL SYSTEM 99
  • SERVICE BRAKES 35

6 active recalls

  • STEERING:ELECTRIC POWER ASSIST SYSTEM Sep 2018

    Honda (American Honda Motor Co.) is recalling certain 2017-2018 Honda Civic and CR-V vehicles. The magnet that controls the torque sensor output signal for the electronic power steering system may not be properly secured, allowing the magnet to become dislodged. During a full…

    NHTSA campaign 18V663000
  • EQUIPMENT:OTHER:OWNERS/SERVICE/OTHER MANUAL Nov 2018

    Honda (American Honda Motor Co.) is recalling certain 2017-2018 Honda Civic Hatchback and Civic Type R vehicles. The owners guide in these vehicles may not have been included or if included, the owner's guide may not have been properly provided required information. As such, t…

    NHTSA campaign 18V817000
  • AIR BAGS Apr 2018

    Honda (American Honda Motor Co.) is recalling certain 2017 Honda Civic Hatchback and Civic Type R vehicles. Driver and front passenger seatback pads sold as replacement service parts were made without slit openings for the seat-mounted side air bags. In the event of a crash nece…

    NHTSA campaign 18V266000
  • POWER TRAIN:DRIVELINE:DRIVESHAFT Nov 2017

    Honda (American Honda Motor Co.) is recalling certain 2017 Honda Civic Sedan and Coupe vehicles. The right halfshaft may have been improperly heat treated, reducing its strength.…

    NHTSA campaign 17V706000

How do I fix P0420 on a 2017 Honda Civic?

About the 2015-2019 Honda Civic

The 2015-2019 Honda Civic was commonly sold with the following powertrains: 2.0L I4, 1.5L Turbo I4, 1.8L I4. Common trims include LX, Sport, EX, Touring.

What does P0420 actually mean?

P0420 is one of the most common emissions-related diagnostic trouble codes (DTCs) in modern vehicles. It does not mean your engine is about to fail — most cars with this code drive normally for thousands of miles. What it does mean is that the catalytic converter on Bank 1 (the side of the engine containing cylinder #1) is no longer scrubbing exhaust gases the way it should.

The code is set by the engine control module after running a diagnostic monitor that compares the upstream and downstream oxygen sensor signals over a defined drive cycle. When the downstream sensor’s signal starts to oscillate like the upstream sensor — instead of staying smooth — the ECM concludes the converter has lost its oxygen-storage capacity.

Why P0420 matters

While the vehicle remains drivable in almost all cases, ignoring P0420 has real consequences:

When to replace the catalytic converter

Replace the cat only after ruling out every cheaper cause: scanner-confirmed exhaust leaks, slow O2 sensors, misfires, and fuel trim problems. A genuine OEM converter on a daily driver will typically last another 150,000 miles; a cheap universal cat may set P0420 again within 12 months.

P0420 on a 2017 Honda Civic: frequently asked questions

What does diagnostic trouble code P0420 mean on a 2017 Honda Civic?

P0420 is set when the engine control module (ECM) compares the upstream and downstream oxygen sensor readings on Bank 1 and concludes the catalytic converter is no longer storing and releasing oxygen efficiently. In a healthy system, the downstream O2 sensor signal stays relatively flat compared to the switching upstream sensor. When both signals look similar, the ECM infers the catalyst has lost its ability to convert hydrocarbons, carbon monoxide, and oxides of nitrogen into less harmful byproducts.

What are the symptoms of P0420 on a 2017 Honda Civic?

Check Engine Light is illuminated (steady, not flashing). Mild loss of fuel economy. Faint rotten-egg (sulfur) smell from the exhaust. Slight reduction in power under hard acceleration. Vehicle will fail an OBD-II emissions / smog test. Typically no noticeable drivability issue at idle

What causes P0420 on a 2017 Honda Civic?

Worn or contaminated catalytic converter (Bank 1) (most-common). Failing downstream (post-catalyst) oxygen sensor (common). Exhaust leak upstream of the downstream O2 sensor (common). Engine misfire or rich/lean fuel trim contaminating the catalyst (occasional). Coolant or oil entering the exhaust (head gasket, valve seals) (rare). Aftermarket / off-brand catalytic converter that does not meet OEM spec (occasional)

Is it safe to drive a 2017 Honda Civic with P0420?

In most cases a 2017 Honda Civic stays drivable for short trips with P0420 active, but it should be diagnosed and repaired promptly — this is a moderate-severity code. Ignoring it can lead to further damage or a failed emissions test.

Related diagnostic codes

P0420 on other Honda Civic model years