P0420 Code on a Buick: Catalyst Efficiency Below Threshold, Bank 1 (Buick Guide)

P0420 on a Buick is "Catalyst System Low Efficiency, Bank 1." The engine computer has measured how well the catalytic converter stores oxygen, and the result came out below its threshold. The reflex is to buy a converter. GM's own Buick documents say to slow down: on the 2013–2015 Encore, GM traced P0420 to a calibration error fixed by reprogramming, and on the 2024 Envision, GM's bulletin says to clear the code and "Do not make any other repairs to the vehicle." On older Buicks GM points at a cause almost nobody checks — a poorly crimped ignition coil ground. And if the converter really has failed, federal law gives it a longer emissions warranty than most owners realise.

What Does P0420 Mean on a Buick?

A healthy catalytic converter stores and releases oxygen as it cleans the exhaust. Your Buick's engine control module (ECM) measures that ability by comparing the upstream oxygen sensor, ahead of the converter, with the downstream sensor after it.

GM's 2008 OBD documentation for the Enclave's 3.6L V6 (LY7) shows how GM does it. The ECM runs the test at a valid idle — throttle closed, vehicle stopped, engine warm — and measures the difference in response time between the pre- and post-catalyst sensors. It compares that oxygen storage capacity against a "best failing" and a "worst passing" converter from calibration tables, giving a normalised ratio where 1 represents a good converter and 0 a very bad one. P0420 sets when the filtered ratio drops below 0.35. On that engine it is a Type A code — the check engine light can come on after a single failing test.

The test won't run with misfire, fuel trim, oxygen sensor, MAF, ECT and many other codes present. That is why P0420 is supposed to be the last code you fix, not the first.

For the general GM background, see our P0420 GM diagnosis guide. This page covers what GM has documented for Buick models.

Which Buicks Set P0420

Every Buick with a monitored catalytic converter can set P0420. Where it points depends on the engine:

• Four- and three-cylinder Buicks — Encore, Encore GX, Envista, Verano, Regal, Envision, LaCrosse 2.4L, Cascada: there is one bank, so P0420 covers the engine's main converter. There is no P0430.
• 3.6L V6 Buicks — Enclave, LaCrosse V6, Regal GS V6: bank 1 is the bank with cylinder 1, the rear bank nearest the firewall on these transverse engines. The front bank's converter sets P0430.
• 3800 V6 Buicks — LaCrosse, Allure, Lucerne 3.8L: GM's 2008 OBD documentation for the 3.8L lists P0420 but not P0430, so the whole engine reports through P0420.

The Software Fixes

2013–2015 Encore 1.4L turbo

GM first released bulletin 18-NA-331 in November 2018 for the 2013–2015 Buick Encore with the 1.4L turbo (LUV): the MIL on with P0420, caused by "a software anomaly," corrected with an ECM reprogram and no parts. In February 2020 GM replaced it with 19-NA-057, which describes the cause as a calibration error and adds two important instructions:

• Verify P0420 is not setting because of other faults — GM names air leaks and physical damage — before relying on the update.
• Reprogram the ECM and the transmission control module together to the latest calibrations.

If your 2013–2015 Encore has P0420 and nothing else, ask a dealer whether the ECM is at the current calibration before anyone quotes a converter.

2024 Envision

GM bulletin 25-NA-005 (January 2025) is about as blunt as GM gets. For the 2024 Buick Envision with a check engine light and P0420 but no driveability problems, GM says the cause may be a software issue, and the correction is: if no other DTCs are present, clear P0420. "Do not make any other repairs to the vehicle. Once the DTC is cleared, it should not return." GM even created a labor operation under emission coverage just for clearing the code.

That is a striking instruction, and it is the clearest example on any Buick of P0420 not meaning a bad converter.

The Coil Ground Tip

GM preliminary information PIP5232C (March 2016) covers "2016 and prior GM cars" and gas trucks where the catalytic converter has come apart or set P0420 and/or P0430 with no apparent reason. GM's recommendation: inspect the ignition coil ground terminal on the engine block for the bank with the concern for a poor crimp, corrosion or damage at the ring terminal, fasteners and grounding surface, and if all is well, tighten the ground bolt to specification.

The logic is simple. A bad coil ground causes weak or intermittent spark, unburned fuel reaches the converter and burns there, and the converter overheats. Replacing the converter without fixing the ground means the new one can fail the same way. NHTSA indexes this document to Buick models from the Century and LeSabre era through the Enclave, Encore, LaCrosse, Envision and Cascada.

A related GM bulletin, 06-06-01-010D, covers vehicles with close-coupled converters: if an engine has failed (broken valve or piston, or a severe overheat), debris or melted ceramic from the converter can be drawn into a replacement engine. GM tells technicians to inspect — and if damaged, replace — the converters when replacing an engine.

Recalls and Warranty

Emission recalls on the Encore GX. GM recalled certain 2020 Encore GX (A202317281) and 2023 Encore GX (A232398131) vehicles with the 1.2L three-cylinder (LIH) because they may have received an incorrect catalytic converter that does not meet all emission requirements. Dealers inspect and replace it if needed. These recalls do not name P0420, but if you own one of these Encore GX models, check for an open recall before diagnosing any converter code. See how to check for open Buick recalls using your VIN.

Federal emissions warranty. Under the EPA's federal emissions warranty, catalytic converters — along with the engine computer and the OBD system — are "specified major emission control components" covered for 8 years or 80,000 miles on 1995 and newer vehicles, for defects in materials or workmanship. Damage from misfire, oil burning or impact is a different matter, and some states have longer terms. Check your Buick's warranty booklet and ask the dealer before paying for a converter on a newer vehicle.

Symptoms of P0420

• Check engine light, usually with no driveability change
• Failed emissions or inspection test
• Rotten-egg (sulfur) smell from the exhaust
• Rattle under the vehicle if the converter substrate has broken up
• Loss of power or high exhaust back-pressure if the converter is plugged or melted
• Earlier misfire or fuel trim codes in history — often the real root cause

Causes and How to Confirm Each

How Is P0420 Diagnosed?

1. Read all codes and freeze frame. Fix misfire, fuel trim, O2 and MAF codes first.
2. Check the VIN for software bulletins (Encore, Envision) and recalls (Encore GX) before any parts.
3. Look for exhaust leaks between the engine and the rear oxygen sensor.
4. Review misfire history and inspect the coil ground for the affected bank on older Buicks (PIP5232C).
5. Watch both oxygen sensors warm at idle. The upstream sensor should switch; a healthy converter keeps the downstream signal much steadier.
6. Check oil and coolant consumption if the converter seems to have failed early.
7. Check the converter for damage — rattles, dents, restriction.
8. Clear codes and drive until the catalyst monitor completes. GM's test needs warm idle periods, so include some stops.

Parts and Cost

GM Documents That Name P0420 on Buick Models

Have a dealer check your VIN against current bulletins, recalls and emissions coverage rather than relying on any article, including this one. The check is free.

What Other P0420 Pages Get Wrong

• "P0420 means replace the catalytic converter." On the 2013–2015 Encore and 2024 Envision, GM's fix is software or simply clearing the code.
• Skipping the root cause. GM's PIP5232C points at coil grounds on the affected bank; misfire and fuel trim faults kill converters.
• Swapping the rear O2 sensor first. A sensor that sees an inefficient converter is doing its job.
• Ignoring exhaust leaks. GM's 19-NA-057 specifically says to rule out air leaks.
• Forgetting the emissions warranty. Federal rules cover catalytic converters for 8 years or 80,000 miles against defects.
• Talking about P0430 on a four-cylinder Buick. Encores, Veranos and four-cylinder Regals have only bank 1.

Frequently Asked Questions

Does P0420 always mean my Buick needs a new catalytic converter?

No. GM has documented software causes on the 2013–2015 Encore and the 2024 Envision. Exhaust leaks, misfires and fuel trim problems are also common causes. Replace the converter only after those are ruled out.

Is my catalytic converter under warranty?

Under the federal emissions warranty, catalytic converters are covered for 8 years or 80,000 miles against defects in materials or workmanship. A dealer can confirm coverage for your VIN.

My 2024 Envision has P0420. What should I do?

GM's 25-NA-005 says that if no other codes are present, the dealer should clear P0420 and make no other repairs. Ask your Buick dealer to check the bulletin against your VIN.

Can I drive my Buick with P0420?

Usually yes in the short term if it runs normally, but it will fail an emissions test. If the light is flashing, the engine is misfiring and you should stop driving — that can destroy the converter.

Why did my new converter fail again?

Usually because the cause was never fixed — misfire, a rich mixture, oil consumption or, per GM's PIP5232C, a bad coil ground on that bank.

Which converter is bank 1 on a Buick Enclave?

The converter on the rear bank — the side with cylinder 1, nearest the firewall on the transverse 3.6L.

Will clearing the code pass inspection?

No. Clearing it resets the readiness monitors, and most inspection programs fail a car whose monitors have not completed.