P0420 on Honda: How to Diagnose & Fix
Catalyst System Efficiency Below Threshold (Bank 1)
7 Honda models · 11 documented issues · 4 critical · $81-$3,500 repair
P0420 on Honda by model: symptoms, diagnosis and fix
Gasoline mixes with engine oil in the 1.5L turbocharged engine, causing the oil level to rise above the full mark. The issue is most prevalent in cold climates where short trips don't allow the engine to reach full operating temperature. Unburned fuel bypasses the piston rings and enters the crankcase, diluting the oil and reducing its lubricating properties.
Common Symptoms
- Oil level reading above full mark on dipstick
- Strong gasoline smell from engine oil
- Check engine light illumination
- Reduced engine power
- Engine stalling
- Poor fuel economy
How to Fix
Honda released a software update for the engine and transmission control units (with an oil change, and in some cases a replacement A/C control unit) to help the engine warm up faster in cold conditions. For 2017-2018 models, Honda also extended warranty coverage on the camshaft, rocker-arm assemblies and spark plugs by one year, to 6 years from purchase with unlimited miles. Owners should change oil more frequently (every 3,000-5,000 miles) in cold climates.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- Genuine Honda 15400-PLM-A02 engine oil filterOEMHonda 15400-PLM-A02
Oil-filter consumable when an owner performs the shorter oil-change interval described by the repair guidance Fitment/availability: 2017-2023 U.S. CR-V 1.5T within this record; cataloged broadly for the CR-V and shown specifically for th…
Owner tips & cautions
- WarningThis is a high-severity issue - ignoring it can lead to costly repairs or safety concerns. Address it promptly.
- TipGet a proper diagnosis before replacing parts - similar symptoms can have different causes
- TipSearch Honda CR-V forums and owner groups for real-world experiences and DIY guides
- TipGet multiple quotes from independent mechanics - dealer prices can be 2-3x higher for the same repair
Sources for this issue (4)
Variable Cylinder Management (VCM) deactivates 3 rear cylinders during light load, but insufficient pressure allows oil to bypass the piston rings into the combustion chamber. Causes excessive oil consumption (1 quart per 1,000 miles), fouled spark plugs, catalytic converter damage, and premature piston ring wear. Honda settled a class-action lawsuit over the problem.
Common Symptoms
- Excessive oil consumption (1 qt per 1,000 miles or more)
- Engine misfire codes (P0300-P0304, typically cylinders 1-4)
- Fouled/oil-soaked spark plugs
- Rough idle and vibration
- Check engine light (misfire and catalyst codes)
- White smoke from exhaust on startup
- Failed catalytic converters (P0420/P0430)
How to Fix
Install VCM disable device (VCMuzzler, VCMTuner) which prevents cylinder deactivation and stops oil consumption. Replace fouled spark plugs after installing disabler. Honda settled a class action (Soto v. American Honda) covering 2008-2013 Odyssey with an 8-year warranty extension for engine misfire repairs, and announced a further warranty extension in June 2025 for certain 2013-2017 Odyssey. Severe cases may require piston ring replacement ($3,000-$5,000). 2014-2017 models still use VCM.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- Genuine Honda/NGK 9807B-5517W IZFR5K11 spark plug, eachOEMHonda 9807B-5517W / NGK IZFR5K11
Early-generation plug branch only after inspection identifies a fouled plug and exact catalog application Fitment/availability: 2005-2007 U.S. Honda Odyssey 3.5L applications in this issue; Honda catalogs 9807B-5517W across 2005-2010 but…
- Genuine Honda/NGK 12290-R70-A01 ILZKR7B11 spark plug, eachOEMHonda 12290-R70-A01 / NGK ILZKR7B11
VCM plug-fouling branch after code and plug inspection identifies replacement Fitment/availability: 2008-2010 U.S. Honda Odyssey EX-L/Touring with VCM; select 12290-R70-A01.
- Genuine Honda/NGK 12290-R71-L01 DILZKR7A11G spark plug, eachOEMHonda 12290-R71-L01 / NGK DILZKR7A11G
Fourth-generation plug-fouling branch after P0301-P0304 and physical plug diagnosis Fitment/availability: 2011-2017 U.S. Honda Odyssey all trims; sold individually and six are required for a full set.
As an Amazon Associate, we earn from qualifying purchases; other part links may also earn au7o a commission. Confirm fitment by VIN before buying.
Owner tips & cautions
- TipCheck oil level every fuel fill-up on VCM-equipped models - carry a quart of oil in the van
- WarningDon't ignore misfire codes - prolonged misfires destroy catalytic converters ($2,000+ repair)
Sources for this issue (2)
Variable Cylinder Management (VCM) causes excessive oil consumption (1 quart every 1,000-1,500 miles), engine misfires, and fouled spark plugs. The reciprocating effect creates vacuum that pulls oil past piston rings. Leads to premature failure of spark plugs, catalytic converters, engine mounts, and cylinder walls. The 2013 VCM class-action settlement (8-year warranty extension) covered only 2009-2013 Pilots, not these model years.
Common Symptoms
- Excessive oil consumption (1 qt per 1,000-1,500 miles)
- Engine misfires (P0300-P0306 codes)
- Fouled spark plugs
- Rough idle or hesitation
- Check engine light
- Excessive engine vibration when VCM activates
- Blue smoke from exhaust
How to Fix
Disable VCM using aftermarket devices: VCMuzzler II ($81), VCMTuner II ($150-200), or S-VCM Controller ($180-250). Check oil level every 500-1,000 miles and top off as needed. Use synthetic 0W-20 oil. Replace spark plugs every 30k-50k miles instead of 100k. Severe cases may need engine rebuild ($4,000-$8,000).
Owner tips & cautions
- TipDisabling VCM reduces damage to engine mounts ($800) and minimizes transmission torque converter wear
- TipCheck oil level every 500-1,000 miles and keep 2-3 quarts of oil in vehicle for top-offs
- WarningDo not ignore excessive oil consumption - running engine low on oil can cause catastrophic engine failure
Sources for this issue (2)
The PCM software may misinterpret sensor inputs as a deteriorated catalytic converter, and debris from the high-pressure pump or the injector machining process can cause internal wear or clogging inside one or more fuel injectors. P0420/P0430, P0300-P0306 and P219A/P219B may be stored. 2016-2017 Pilot (bulletin 20-100, with software update) and 2017-2019 Pilot (bulletin 21-010, without software update), all trims, eligibility by VIN.
How to Fix
Have a Honda dealer verify the VIN and stored codes against 20-100 (2016–2017 Pilot, with software update) or 21-010 (2017–2019, without that update); the bulletins require at least 5,000 miles. With only P0420/P0430, perform the cylinder air-fuel test and replace the injectors only if it fails; 20-100 also requires its software update. With the specified misfire or air-fuel imbalance codes, follow the injector-replacement branch and any applicable software update. For a 9-speed automatic under 20-100, also confirm its A/T software is current. Use the complete specified fuel-injector kit (06160-RLV-305 in these bulletin versions), including all must-replace parts, rather than only the six injectors pictured. Honda’s extension is 10 years from original purchase or 150,000 miles, whichever comes first, with eligibility confirmed by the dealer. These sources do not direct automatic catalytic-converter replacement. If none of the listed codes is present, continue normal troubleshooting.
Call to confirm they can assess your vehicle and perform the stated work. Search results do not confirm qualification, eligibility or coverage.
Sources for this issue (4)
- Honda Service Bulletin 20-100, Warranty Extension: MIL Comes On with DTC P0420/P0430, P030x, and/or P219A/B (With Software Update) (Version 2, February 11, 2021)tsb
- Honda Service Bulletin 21-010, Warranty Extension: MIL Comes On with DTC P0420/P0430, P030x, and/or P219A/B (Without Software Update) (February 11, 2021)tsb
- Honda owner letter: Warranty Extension (Without Software Update) for 2017-19 Pilot fuel injectorstsb
- Honda dealer notice: Warranty Extension: 2016 Pilot V6 Fuel Injectortsb
The braided flex section of the exhaust front pipe develops holes in its mesh and cracks, producing a loud exhaust drone/roar, a rattle, and exhaust odor that can enter the cabin. Failure is accelerated by worn/broken engine mounts that let the engine rock and stress the flex joint. The OEM front pipe has been discontinued, pushing owners to aftermarket (Walker/Bosal) replacements.
Common Symptoms
- Loud exhaust drone or roar
- Rattle from under the vehicle
- Exhaust smell in the cabin
- Hissing/ticking that is louder on cold start
- Failed emissions or downstream catalyst codes
How to Fix
Replace the cracked front pipe with a Walker or Bosal aftermarket unit, or have a muffler shop weld in a proper stainless flex section. Address worn engine/transmission mounts at the same time to prevent premature re-failure. Expect rusty flange bolts that may snap and need to be cut/replaced.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- Walker 50313 front exhaust Y-pipe with flex sectionOEMWalker 50313
Verify federal/local emissions configuration and flange condition before ordering.
- Genuine Honda 18210-S9V-A02 front exhaust pipeOEMHonda 18210-S9V-A02
2005-only complete front-pipe branch Fitment/availability: 2005 U.S. Honda Pilot EX, EX-L and LX 3.5L with KA 5AT; not the 2003-2004 or 2006-2008 pipe.
- Genuine Honda 18210-S3V-A02 front exhaust pipeOEMHonda 18210-S3V-A02
2006-2008 complete front-pipe branch Fitment/availability: 2006-2008 U.S. Honda Pilot 3.5L, listed 2WD/AWD trims with KA 5AT; supersedes 18210-S3V-A11.
As an Amazon Associate, we earn from qualifying purchases; other part links may also earn au7o a commission. Confirm fitment by VIN before buying.
Sources for this issue (3)
The 8th generation Accord V6 (3.5L J35Z2) with Variable Cylinder Management (VCM) deactivates 3 cylinders at highway speeds, causing excessive oil consumption (1 quart per 1,000-1,500 miles), misfires, and accelerated engine vibration that damages engine mounts. When VCM deactivates cylinders, insufficient ring pressure allows oil to bypass rings into combustion chamber. The class action lawsuit (Soto v. American Honda, filed 2012) covered about 1.6 million VCM vehicles. Honda settled in 2013, providing an 8-year, unlimited-mileage extended warranty for repairs related to VCM-caused misfires and spark plug fouling. Many owners in the Drive Accord community disable VCM immediately upon purchase.
Common Symptoms
- Oil level dropping about 1 quart every 1,000-1,500 miles
- Engine misfires (P0300-P0306 codes)
- Fouled spark plugs with carbon and oil deposits
- Excessive vibration felt through steering wheel and seats
- Check engine light with misfire and/or catalyst codes
- Catalytic converter efficiency codes (P0420, P0430)
- Blue smoke from exhaust at startup
How to Fix
Install VCM disabler device: VCMuzzler II (~$81, simplest install, no power wire), VCMTuner II ($150-200, more features), or S-VCM Controller ($180-250, automatic temperature adjustment). Check oil every 500-1,000 miles until VCM disabled. Replace fouled spark plugs with NGK DILKAR7B11 ($5-8 each) every 30,000-50,000 miles. Honda's 2013 class action settlement provided an 8-year extended warranty for misfires with fouled plugs - contact Honda with VIN (the 8-year term has now run out on most cars). Severe cases (catalytic converters destroyed): $2,000-3,500 replacement.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- VCMuzzler II VCM suppression deviceOEMVCMuzzler II
Optional owner-selected VCM suppression after understanding the device's operating tradeoffs Fitment/availability: 2008-2012 Accord V6 automatic only; vendor explicitly excludes manual-transmission Accord. Select model year, model and pi…
- S-VCM ControllerOEMS-VCM Controller
Confirm connector/application with vendor.
As an Amazon Associate, we earn from qualifying purchases; other part links may also earn au7o a commission. Confirm fitment by VIN before buying.
Sources for this issue (2)
P0420 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0420 mean on a Honda?
P0420 stands for "Catalyst System Efficiency Below Threshold (Bank 1)." P0420 means the engine computer determined the catalytic converter on Bank 1 isn't cleaning the exhaust as efficiently as it should. The computer compares the upstream and downstream oxygen sensors; a healthy converter stores and releases oxygen, so the downstream sensor should read fairly steady, while a worn one lets its readings mirror the upstream sensor. When that efficiency falls below a set threshold, this code is set. While it most often points to a worn-out catalytic converter, it can also be triggered by upstream problems like a faulty oxygen sensor or an exhaust leak, so the converter isn't always the actual fault. On Honda it is documented across 7 models: CR-V, Odyssey, Pilot, Accord, Civic, Passport, Insight.
What are the most common causes of P0420?
The most common causes of P0420 are: Worn-out or failing catalytic converter (Bank 1), Faulty or aged downstream (or upstream) oxygen sensor, Exhaust leak before or near the oxygen sensors, Engine running rich or lean (fuel trim issues), Misfires or oil/coolant contaminating the converter, Damaged or aftermarket low-quality catalytic converter, Faulty oxygen sensor wiring or connectors, Software/calibration needing update (on some vehicles). The specific cause varies by vehicle.
How much does it cost to fix P0420 on a Honda?
Published repair estimates on Honda range from $81 to $3,500 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
Looking for P0420 on a different make?
View P0420 across all makes →Join 700+ subscribers following Au7o for vehicle insights and updates.
Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.