P0420 on Kia: How to Diagnose & Fix
Catalyst System Efficiency Below Threshold (Bank 1)
5 Kia models · 8 documented issues · 4 critical · $250-$6,000 repair
P0420 on Kia by model: symptoms, diagnosis and fix
Theta II, Nu, and Gamma engines in Kia Optima vehicles consume excessive amounts of oil between service intervals. On Theta II engines this issue is often linked to the same internal wear behind the Theta II engine recall, in which manufacturing debris and uneven machining could restrict oil flow within the engine. Many owners report using 1 quart or more of oil every 1,000 miles with no visible leaks.
Common Symptoms
- Low oil level between scheduled oil changes
- Oil consumption greater than 1 quart per 1,000 miles
- Blue smoke from exhaust
- Oil smell inside cabin
- Low oil warning light illumination
- Engine running rough
How to Fix
Oil consumption test at dealership (typically 1,000 mile monitored interval). Piston ring replacement if test confirms excessive consumption. Short block or complete engine replacement in severe cases. Switch to manufacturer-recommended 5W-30 oil weight.
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 (5)
The 2nd generation Forte with the Nu 2.0L GDI engine (2014-2018) can develop engine knocking and excessive oil consumption. Similar to the Theta II issues, Kia's campaigns for this engine target excessive connecting rod bearing wear, which produces knocking and can progress to engine failure. Kia added Knock Sensor Detection System (KSDS) software and extended engine coverage under Product Improvement Campaigns PI2105 (2016 Forte and Forte Koup 2.0 GDI) and PI2106 (2017-2018 Forte 2.0 Nu GDI built May 19, 2016 through May 7, 2018). Affected engines may consume 1+ quart of oil per 1,000 miles.
Common Symptoms
- Engine knocking at idle and under load
- Excessive oil consumption (1+ qt per 1,000 miles)
- Blue or white smoke from exhaust
- Check engine light with misfire codes
- Rough idle
How to Fix
Contact dealer for engine campaign eligibility check. On 2016-2018 cars the dealer installs the KSDS software free of charge; after it is installed, a continuously blinking check-engine light with limp-home mode signals rod bearing wear, and Kia warrants engine long block repairs for connecting rod bearing damage for 15 years/150,000 miles from the original in-service date, for new and used owners. Keep all oil purchase and change receipts.
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
- TipCheck oil level every 500 miles and document with photos. The oil consumption test at the dealer requires proof of consistent monitoring
Sources for this issue (7)
- Kia PI2106 - KSDS Product Improvement Campaign, 2017-2018 Forte 2.0L Nu GDItsb
- Kia PI2105 - KSDS Product Improvement Campaign renotification, 2016 Forte/Forte Koup 2.0L GDItsb
- 2017 Kia Forte specifications - Kia Mediaarticle
- Yet another Forte with Engine Knock - Kia Forte Forumforum
- Knock sensor 2014 2.0 GDI Looking for recall success stories - Kia Forte Forumforum
- 2.0 Nu engine blown 65K miles beware - Kia Forte Forumforum
- 2014 Kia Forte Engine Clicking/Knocking - CarComplaintsmedia
Forte 2.0L Nu cars that consume oil push unburned oil ash through the exhaust, which poisons and eventually plugs the washcoat on the close-coupled (manifold) converter and the underfloor converter. Owners see P0420 catalyst-efficiency-below-threshold, sometimes with a rotten-egg smell, sluggish acceleration, and a rattle from the exhaust as the ceramic brick breaks apart. Forte owners report replacing the easy-to-reach underfloor converter only to have P0420 return within weeks because the manifold-mounted primary converter is the one that is actually degraded. In advanced cases the fractured substrate raises backpressure enough to cause power loss and stalling, and debris can be drawn back toward the head.
Common Symptoms
- Check engine light with P0420
- Rattle from under the car
- Sulfur/rotten egg smell from exhaust
- Reduced power and poor acceleration
- Failed state emissions test
- Code returns shortly after converter replacement
How to Fix
Do an oil consumption test before spending money on converters - if the engine burns more than about a quart per 1,000 miles, a new converter will be destroyed again quickly, and the engine claim (Kia's engine coverage/settlement programs) should be pursued first. Verify with upstream/downstream O2 sensor live data and a backpressure test that the converter, not a lazy rear O2 sensor or an exhaust leak, is at fault, and confirm which of the two converters has failed. Federal emissions law requires an 8-year/80,000-mile warranty on the catalytic converter, so check coverage before paying. Out of warranty, dealer quotes for the assembly run about $1,500-$2,500; Forte owners report using a Dorman direct-fit unit (part 674-955 on the 2.0L) for roughly $500 plus labor.
Sources for this issue (4)
The 2.4L GDI Sorento (2012-2016) experiences premature catalytic converter failure, often linked to the Theta II oil consumption issue. Excess oil burning fouls the catalytic converter substrate, causing it to overheat and break down internally. Symptoms include a sulfur smell, reduced power, and P0420/P0430 codes. On vehicles with excessive oil consumption, the root cause must be addressed first or the replacement converter will fail again.
Common Symptoms
- Check engine light with P0420 or P0430 codes
- Sulfur or rotten egg smell from exhaust
- Reduced engine power and poor acceleration
- Rattling noise from under vehicle (broken substrate)
- Failed emissions test
How to Fix
Diagnose root cause of catalytic converter failure first. If linked to oil consumption (Theta II issue), address engine repair or replacement before installing new converter. OEM catalytic converter replacement (Kia part #28950-2G620) runs $800-1,500. Quality aftermarket options from Walker (part #16598) or MagnaFlow (part #52259) cost $300-700. Labor is 1-2 hours.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- Genuine Kia 28510-2G445 exhaust-manifold warm-up catalytic assemblyOEMKia 28510-2G445
Replace the failed warm-up converter/manifold after ENG160 diagnosis and root-cause correction Fitment/availability: 2013-2014 Kia Sorento 2.4L Theta-II catalog branch; verify the vehicle is GDI, match VIN, FWD/AWD and emissions label, a…
- Genuine Kia 28510-2GBB0 exhaust-manifold catalytic assemblyOEMKia 28510-2GBB0
Confirm VIN, GDI engine, FWD/AWD and ULEV/SULEV/test-group requirements bec
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
- WarningDo NOT just replace the catalytic converter without checking oil consumption first. If the engine is burning oil, the new cat will fail within 6-12 months
P0430 only applies to the V6 Sorento, since Bank 2 (the cylinder bank containing cylinders 2-4-6) exists solely on the 3.3L and 3.5L Lambda II V6 (2011-2020). The ECM sees the upstream and downstream oxygen-sensor signals tracking too closely on that bank, meaning the Bank 2 catalytic converter, monitored by the downstream (post-cat) O2 sensor, is no longer scrubbing enough pollutants. On the Sorento this is usually an aged/degraded catalytic converter or a lazy rear (downstream) O2 sensor on Bank 2, and occasionally an exhaust leak upstream of the sensor that skews the readings. There are rarely drivability symptoms — the illuminated check-engine light and a failed emissions test are typically the only signs.
Common Symptoms
- check engine light on
- failed emissions/smog test
- no noticeable drivability change
- possible faint sulfur smell
- slightly reduced fuel economy
How to Fix
Confirm no exhaust leaks near the Bank 2 sensors. Diagnostically swap the Bank 1 and Bank 2 O2 sensors — if the fault moves to P0420, the sensor is bad and should be replaced; if P0430 stays, the Bank 2 catalytic converter is the failed part and should be replaced (OEM or CARB-compliant on the V6). Verify long-term fuel trims and clear any misfire/rich conditions first, since raw fuel can prematurely kill the converter.
Sources for this issue (3)
NHTSA recall 21V259000 (Kia SC209) covers certain 2020-2021 Soul vehicles with 2.0L Nu MPI engines whose piston oil rings may not have been properly heat-treated. Kia communication 11017169/SST082 indexes a later PNSS vibration-inspection process to certain 2020-2023 Soul vehicles and describes ring-related cylinder-wall damage, increased oil consumption, abnormal noise, oil-pressure warning and P1327. That later bulletin does not make every 2022-2023 vehicle part of recall 21V259.
Common Symptoms
- Increasing oil consumption
- Abnormal engine noise or P1327
- Oil-pressure warning light
How to Fix
Check the VIN for SC209 or the current Kia PNSS program and monitor the oil level using the owner-manual procedure. A Kia dealer uses the VIN-driven KDS inspection; eligible recall vehicles receive inspection, PNSS software and engine replacement when necessary. Keep a log of how much oil you add between changes and ask the dealer about the PNSS inspection. Recall coverage does not extend to every 2022-2023 Soul, and adding oil or changing filters will not fix damaged rings.
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 (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 Kia?
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 Kia it is documented across 5 models: Optima, Forte, Sorento, Soul, Spectra.
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 Kia?
Published repair estimates on Kia range from $250 to $6,000 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.