P0420 on Buick: How to Diagnose & Fix
Catalyst System Efficiency Below Threshold (Bank 1)
1 Buick model · 1 documented issue
P0420 on Buick by model: symptoms, diagnosis and fix
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.
Common Symptoms
- Check engine light / catalyst efficiency code
- Failed state emissions inspection
- Incorrect catalytic converter part number on inspection
How to Fix
Covered under GM emissions recall A202317281 (NHTSA 10197318) - dealers inspect the catalytic converter and replace it with the correct part (e.g. P/N 55515139) at no charge if the wrong unit was installed. Verify eligibility by VIN at nhtsa.gov.
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 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 it is documented across 1 model: Encore GX.
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 Buick?
Repair costs vary widely with the root cause and Buick model; each issue on this page carries its own estimate where one is published.
Looking for P0420 on a different make?
View P0420 across all makes →Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.