P0420 on Hyundai
Catalyst System Efficiency Below Threshold (Bank 1)
P0420 on Hyundai vehicles indicates catalyst system efficiency below threshold (bank 1). Au7o has documented this code across 4 Hyundai models — most commonly on Elantra, Genesis Coupe, Palisade. 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. Typical repair costs on Hyundai range from $50 to $4,000, depending on the specific model and root cause.
Common Causes of P0420
- •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)
P0420 on Hyundai by Model
Hyundai Elantra(1 issue)
- P0420 — Catalyst Efficiency Below Threshold (Bank 1), Often Downstream of Nu 2.0L Oil Consumption2011-2022
P0420 on the Elantra means the downstream (post-cat) oxygen sensor sees the catalytic converter no longer scrubbing efficiently. On the 2017-2020 Nu 2.0L engine this frequently is not just an aged cat — it is a symptom of the well-documented Nu 2.0L piston-ring/oil-consumption defect (addressed by NHTSA recall campaign 21V722000 for certain 2019-2020 2.0L cars). Oil burned in the combustion chamber poisons and clogs the catalyst, so a converter replaced without fixing oil consumption is quickly re-destroyed. On older Beta/Nu cars P0420 is more commonly a simple worn converter or a lazy/failed downstream O2 sensor.
Hyundai Genesis Coupe(1 issue)
- Excessive Oil Consumption - 3.8L Lambda V62013-2016
The 3.8L Lambda V6 in the BK2 Genesis Coupe consumes oil at an excessive rate, often 1 quart every 1,500-2,500 miles. The issue is related to piston ring design and valve stem seal degradation. Some engines require a quart of oil every 1,000 miles. Hyundai acknowledged the issue but did not issue a recall.
Hyundai Palisade(1 issue)
- Oil Dilution / Gasoline Contamination in Engine Oil2020-2024
The 3.8L Lambda II V6 in the Palisade can suffer from fuel diluting the engine oil, particularly in cold climates with short trips. The GDI system can allow unburned fuel past the piston rings during cold starts. Oil level rises above the full mark and smells of gasoline. Hyundai released a software update to address the issue.
Hyundai Sonata(1 issue)
- P0420 — Bank 1 Catalytic Converter Failure from Theta II 2.4L GDI Oil Consumption2011-2019
On the 2011-2019 Sonata with the 2.4L Theta II GDI engine, P0420 (Catalyst Efficiency Below Threshold, Bank 1) is frequently a downstream symptom of the engine's well-documented excessive oil consumption. As worn rod bearings/rings let the engine burn oil, phosphorus and other contaminants coat and eventually melt the catalyst's honeycomb, killing its efficiency and setting P0420. Replacing only the converter without fixing the oil-burning root cause leads to rapid re-failure. This ties directly into Recall 132 (NHTSA 15V-568) for connecting-rod bearing wear and the KSDS software / extended 15yr-150k-mile short-block warranty.
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View P0420 across all makes →Frequently Asked Questions
What does P0420 mean on Hyundai?▼
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 Hyundai specifically, this code is documented across 4 models.
What causes P0420 on Hyundai vehicles?▼
Common causes on Hyundai: 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. Specific causes vary by model and year — see the per-model sections below.
How much does it cost to fix P0420 on a Hyundai?▼
Repair costs on Hyundai range from $50 to $4,000, depending on the specific model and root cause.
Which Hyundai models have P0420 documented?▼
Au7o has documented P0420 on 4 Hyundai models: Elantra, Genesis Coupe, Palisade, Sonata.