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 2 Hyundai models — most commonly on 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 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.
Looking for P0420 on a different make?
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 2 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 2 Hyundai models: Genesis Coupe, Palisade.