P0420 on Buick
Catalyst System Efficiency Below Threshold (Bank 1)
P0420 on Buick vehicles indicates catalyst system efficiency below threshold (bank 1). Au7o has documented this code across 1 Buick model — most commonly on Encore GX. 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.
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 Buick by Model
Buick Encore GX(1 issue)
- Incorrect Catalytic Converter Installed at Factory (Emissions Recall A202317281)2020-2023
Certain Encore GX vehicles with the turbocharged 1.2L (RPO LIH) engine left the factory with an incorrect catalytic converter that does not meet all emission system requirements, so the converter may not properly process exhaust gases. GM emissions recall A202317281 (NHTSA 10197318) covered 2020 Encore GX (paired with 2021 Chevrolet Trailblazer), and a separate later recall covered 2023 Encore GX units with the same wrong-converter problem. Affected vehicles can fail emissions testing.
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What does P0420 mean on Buick?▼
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 Buick specifically, this code is documented across 1 model.
What causes P0420 on Buick vehicles?▼
Common causes on Buick: 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 Buick?▼
Repair costs vary widely depending on the root cause and specific Buick model.
Which Buick models have P0420 documented?▼
Au7o has documented P0420 on 1 Buick model: Encore GX.