P0420 on Subaru: How to Diagnose & Fix
Catalyst System Efficiency Below Threshold (Bank 1)
6 Subaru models · 8 documented issues · 3 critical · $250-$10,000 repair
P0420 on Subaru by model: symptoms, diagnosis and fix
Some early FB25 2.5L engines burn enough oil that owners have to top up between oil changes, and some get the low-oil warning light. Subaru's service bulletin 02-157-14R ties the problem to the oil-control piston rings. Subaru changed the rings' surface treatment and put redesigned rings into production. On the Outback, the bulletin covers 2013-2014 models: every manual-transmission car, plus 2013 CVT cars below VIN D*321435 (engine number below 151602). Cars built after the ring change are outside the bulletin. Wikipedia lists the FB25 in the North American Outback from 2013 to 2019. Subaru first fixed these engines with new piston rings under bulletins 02-143-13R, 02-144-13R and 02-145-13R, then switched to replacing the whole short block. Owners in the Yaeger v. Subaru of America class action (D.N.J.) said the piston rings were defective and that ring replacement did not reduce the oil use. Subaru denied the engines were defective but settled. The settlement extended oil-consumption coverage to 8 years/100,000 miles. Subaru's bulletin also says some oil use is normal. Under hard driving, towing, long idling or extreme temperatures, the owner's manual figure Subaru quotes is up to 1 quart per 1,200 miles.
Common Symptoms
- Oil level drops between scheduled oil changes and needs topping up
- Low oil level warning light comes on well before the next oil change is due
- Burning oil smell
- Blue smoke from the exhaust
How to Fix
1) Check the oil level correctly first. Park on level ground and wait at least 5 minutes after shutting the engine off. Use the correct oil grade; Subaru's bulletin lists 0W-20. Under the bulletin, a dipstick reading at or below the add mark 4,000 miles or more after the last oil change, or a low-oil light 5,000 miles or more after it, does not require an oil consumption test. 2) If the level drops sooner than that, ask a Subaru dealer to run a Vehicle Coverage Inquiry on your VIN for bulletin 02-157-14R. On the Outback, it applies to 2013-2014 manual-transmission cars and to 2013 CVT cars below VIN D*321435. 3) The dealer runs an oil consumption test. The car qualifies if it burns more than 1/3 quart (10.6 oz) in 1,200 miles. 4) The fix for a qualifying engine is a replacement short block, not a new set of rings. For the 2013-2014 Legacy/Outback the bulletin lists part 10103AC600, now superseded by 10103AC650. 5) Coverage: the extension pays for oil-consumption repairs for 8 years/100,000 miles from the original warranty start date. For 2013-2014 cars that period has now ended for nearly every vehicle, so a repair today will probably be paid out of pocket unless the dealer's coverage check shows otherwise. The settlement's cash-reimbursement claim deadline was October 1, 2016. 6) For Outbacks the bulletin does not cover, or that pass the test, look at the conditions the bulletin says raise normal oil use first: the wrong oil grade or viscosity, hard driving, heavy loads or towing, long idling, and extreme temperatures.
Call to confirm they can assess your vehicle and perform the stated work. Search results do not confirm qualification, eligibility or coverage.
Owner tips & cautions
- TipGet a proper diagnosis before replacing parts - similar symptoms can have different causes
- TipSearch Subaru Outback 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 (5)
- Subaru Service Bulletin 02-157-14R Engine Oil Consumption (revised 03/06/18)tsb
- Subaru Service Bulletin 02-157-14R Surface Treatment Change To Oil Control Piston Rings (revised 12/07/15)tsb
- Subaru retailer FAQ: Oil Consumption Class Action Law Suit (04/14/2016)tsb
- Subaru Oil Consumption Lawsuit May Be Settled (CarComplaints, 2016)news
- Subaru FB engine (Wikipedia)reference
2.5L Outbacks are prone to premature catalytic converter degradation that sets a P0420 (Catalyst System Efficiency Below Threshold) check-engine light, often well before 100,000 miles. On some 2015-2016 2.5L Legacy/Outback models the front exhaust pipe assembly (which houses the front catalyst) can crack at the flexible bellows — Subaru issued voluntary recall/campaign WUJ-95 to replace the assembly. Separately, cars with the FB-series oil-consumption issue can contaminate and prematurely poison the catalyst with burnt oil, accelerating the P0420 failure. Catalytic converters are covered by the federal 8-year/80,000-mile emissions warranty.
Common Symptoms
- Check engine light with P0420
- Reduced fuel economy
- Rotten-egg/sulfur exhaust smell
- Rattle from exhaust (broken catalyst substrate)
- Failed emissions/smog test
How to Fix
Read the code and verify it is a true catalyst-efficiency fault (compare upstream A/F sensor vs downstream O2 sensor switching) and rule out a lazy rear O2 sensor or exhaust leak first. Before paying out of pocket, check eligibility for recall/campaign WUJ-95 (2015-2016 2.5L) and the federal 8/80 emissions warranty — many owners get a free converter/front-pipe. If out of coverage, replace with a CARB-compliant OEM or OE-grade converter and address any underlying oil consumption first, or the new cat will fail again. Do NOT clear the code and ignore it — a degrading cat can overheat.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- Genuine Subaru 44620AD16B Front Exhaust Pipe and Converter
front converter/pipe only after catalyst or pipe failure is confirmed and coverage is excluded Fitment/availability: 2013-2014 Outback 2.5-liter FB25 branch; confirm VIN, transmission and emissions certification Fitment/availability: Add…
- Genuine Subaru 44620AD88B Front Exhaust Pipe and Converter
later front converter/pipe after campaign/coverage checks and diagnosis confirm replacement Fitment/availability: 2015-2019 Outback 2.5-liter branch; 2015-2016 owners must check WUJ-95 first, and all buyers must confirm VIN/emissions con…
- Genuine Subaru 44022AA123 Rear Exhaust-Pipe Gasket
one-time sealing gasket when the corresponding pipe flange is separated Fitment/availability: 2004-2021 Outback catalog range including the approved pipe branches; confirm VIN and exact flange position Fitment/availability: ships in 1-3 …
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)
On the Subaru boxer engine, Bank 2 is the driver's side (cylinders 2 and 4). P0430 sets when the driver-side rear (downstream) O2 sensor tracks the front sensor too closely, telling the ECM the catalytic converter is no longer scrubbing exhaust efficiently. On the Outback this most commonly appears on the 3.6R H6 (EZ36), which has a separate catalyst per bank; on 4-cylinder Outbacks, the Bank 1 twin code P0420 is more common than P0430. A well-known Subaru wrinkle: excessive oil consumption on some cars coked the O2 sensors and contaminated the converter, so a P0430 (or P0420) is frequently traced to a lazy/oil-fouled rear O2 sensor rather than a truly dead cat. After ~120k-150k miles the washcoat genuinely degrades and the converter itself fails.
Common Symptoms
- Check engine light
- Failed emissions/smog test
- Slight loss of fuel economy
- No obvious drivability change in most cases
- Occasional faint sulfur/rotten-egg smell
How to Fix
Confirm before condemning the cat: watch the driver-side rear O2 sensor voltage at a steady cruise — it should hold roughly 0.5-0.8V; if it swings with the front sensor, the converter has failed. If the sensor is old or oil-fouled, replace it first with an OEM Denso unit (aftermarket units are widely reported to cause repeat codes). On oil-consuming cars, address oil consumption/PCV as well. If voltage confirms a dead converter, replace the Bank 2 catalytic converter (OEM preferred to avoid re-triggering).
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- Genuine Subaru 22690AB06A Bank 2 Rear Oxygen Sensor
rear sensor only after heater/wiring/response testing proves it failed Fitment/availability: 2015-2019 Outback 3.6R H6, catalog position code 22690*B; confirm VIN and driver-side downstream position Fitment/availability: ships in 1-3 bus…
- Genuine Subaru 44620AC23A Driver-Side Front Exhaust Pipe and Catalyst
early driver-side catalyst only when this exact branch is confirmed Fitment/availability: one 2010-2014 Outback 3.6R H6 emissions/configuration branch; exact VIN and emissions label required Fitment/availability: ships in 1-3 business da…
- Genuine Subaru 44620AD06A Driver-Side Front Exhaust Pipe and Catalyst
alternate early driver-side catalyst only after branch confirmation Fitment/availability: alternate 2011-2014 Outback 3.6R H6 emissions/configuration branch; exact VIN and emissions label required Fitment/availability: ships in 1-3 busin…
- Genuine Subaru 44620AD980 Driver-Side Front Exhaust Pipe and Catalyst
later driver-side catalyst after diagnosis and coverage check Fitment/availability: 2015-2019 Outback 3.6R H6 driver-side branch; current replacement for 44620AD98A, confirm VIN/emissions Fitment/availability: ships in 1-2 business days …
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 2.0L FB20 engine in first-generation Crosstreks (2013-2017) shares the same oil consumption defect as the FB25 used in other Subaru models. Defective piston rings fail to maintain proper oil control, leading to oil burning at rates of 1 quart every 1,500-2,500 miles. Subaru covered this under an 8-year/100,000-mile powertrain warranty extension through the Yaeger v. Subaru class-action settlement (2013 CVT and 2013-2015 manual Crosstreks below set VINs). The issue is most prevalent in 2013-2015 models.
Common Symptoms
- Oil level drops 1+ quart between oil changes
- Low oil pressure warning light illumination
- Blue-grey exhaust smoke especially on cold start
- Fouled spark plugs from oil contamination
- Catalytic converter failure from excess oil burning (P0420)
How to Fix
Perform Subaru's official oil consumption test (OCT) at a dealership. If consumption exceeds 1 qt per 3,000 miles, piston ring replacement is the fix. The Yaeger v. Subaru settlement's 8-year/100,000-mile extension has run out on these model years, so ask the dealer about goodwill coverage under service bulletin 02-157-14R. Short block replacement (Subaru 10103AB830) may be offered for severe cases. Monitor oil level weekly and keep records of oil additions.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- Genuine Subaru SOA427V1310 Synthetic 0W-20 Engine Oil - 1 Quart
maintain the exact safe oil level during diagnosis; not a cure for oil consumption Fitment/availability: Top-off and controlled-test supply only for a 2013-2017 FB20 Crosstrek whose current owner manual/service information specifies 0W-2…
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 (6)
- Subaru of America retailer notice: Proposed Engine Oil Consumption Class Action Settlement (02/01/2016)tsb
- Subaru Oil Consumption Class Action (Yaeger v. Subaru)article
- Subaru Engines Models Years Burn Oil - Torque Newsmedia
- Is Your Subaru Burning Oil - A&A The Shopmedia
- FB20 Engine Tear Down repair for oil consumption - Subaru Crosstrek Forumsforum
- Oil Consumption Problem - Subaru Crosstrek Forumsforum
The FA20DIT engine can suffer from piston ringland failure, especially when tuned or running high boost. The ringland (area between piston rings) can crack, causing compression loss and engine damage. This is often related to detonation from improper tuning or low octane fuel.
Common Symptoms
- Loss of power
- Excessive oil consumption
- Blue smoke from exhaust
- Knocking or pinging under load
- Compression loss on cylinder test
- Check engine light with misfire codes
How to Fix
Always use 93 octane fuel. If tuning, use a reputable tuner with proper air/fuel ratios and timing. Consider forged pistons if building engine. Monitor for knock with an accessport or similar device. Catch early signs through oil analysis showing elevated metals.
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
Sources for this issue (1)
The 2013-2016 Legacy 2.5i uses the FB25 engine which is affected by the same oil consumption issue as other FB25-equipped Subarus. Defective piston rings allow excessive oil to bypass into the combustion chamber. The Legacy's higher average speeds mean oil consumption can be even more noticeable during highway driving. Subaru's oil consumption warranty extension and the Oakes v. Subaru settlement cover the 2013 Legacy 2.5i.
Common Symptoms
- Oil level drops 1+ quart between oil changes
- Low oil warning light
- Blue smoke from exhaust, particularly on cold start
- Oil fouling spark plugs causing misfires
- Catalytic converter premature failure (P0420 code)
How to Fix
Have dealer perform official oil consumption test (OCT). If consumption exceeds 1 qt per 3,000 miles, piston ring replacement or short block replacement is warranted. Check coverage under Subaru powertrain warranty extension (8 years/100,000 miles) or Oakes v. Subaru settlement. Use 0W-20 full synthetic oil and monitor levels weekly.
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 (5)
- Jalopnik: Subaru Owners Win Compensation And Warranty Boost For Oil-Burning Carsmedia
- Subaru Outback Forums: Subaru Settles Oil Consumption Lawsuitforum
- Subaru Forester Owners: 2011+ FB25 discussion about oil consumptionforum
- Subaru Outback Forums: Lawsuit charges Subaru engines may have defective piston ringforum
- List of Subaru transmissions (Legacy engine/CVT pairings by year)article
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 Subaru?
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 Subaru it is documented across 6 models: Outback, Crosstrek, WRX, Legacy, WRX STI, Forester.
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 Subaru?
Published repair estimates on Subaru range from $250 to $10,000 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.
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Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.