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

P0420 on a GMC is "Catalyst System Low Efficiency, Bank 1." The engine computer has measured how well the bank 1 catalytic converter stores oxygen, and the result came back below its threshold. The reflex is to buy a converter. GM's GMC documents say slow down. On Sierra HD and Savana work trucks, GM says to fix an exhaust leak behind the rear oxygen sensor — often caused by an aftermarket utility bed or hitch — before a converter is even considered. On 2014–2018 Sierras and Yukons, sand getting past a poorly sealed air filter can be the root cause. And on a low-mileage V8 truck, the converter may simply not have finished breaking in. The surprise: on one group of 2019 Sierra 2.7L trucks, GM's emission recall exists because the ECM might not set a code when it should.

What Does P0420 Mean on a GMC?

A working three-way converter stores and releases oxygen, which smooths out the switching you see on the upstream oxygen sensor. The downstream sensor, behind the converter, should read fairly steady. During its catalyst monitor, the ECM measures how much oxygen the converter can store. When bank 1's result falls below the calibrated threshold, it sets P0420.

Three things follow. First, the test assumes the rest of the engine is healthy — misfires, rich or lean fuel trim and oxygen sensor faults all skew it, so those codes come first. Second, anything that lets outside air into the exhaust near the rear sensor makes a good converter look bad. Third, a converter that did fail usually failed for a reason — oil, coolant, raw fuel from misfires, or debris — and a new one will fail the same way if that cause is still there.

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

Which GMCs Set P0420 — and Which Converter Is Bank 1

• V8 Sierra, Yukon, Yukon XL and Savana and the 4.3L V6: bank 1 is the driver's side, so P0420 is the driver's-side converter. The passenger-side converter is P0430.
• Acadia and Terrain 3.6L V6: bank 1 is the rear bank, by the firewall.
• Inline engines — Terrain 1.5T, 2.0T and 2.4L, Acadia four-cylinders, Canyon 2.5L and 2.7T, the 2004–2012 Canyon four and five, the Envoy 4.2L I6 and the 2.7L Sierra: the whole engine is bank 1, so P0420 is the only catalyst efficiency code these engines can set.
• Duramax diesels: P0420 refers to the diesel oxidation catalyst, a different system — see below.

What GM Found on Sierra, Yukon and Savana

Low miles or a new converter: break-in

GM's PIP3119P covers 2002–2018 GMC Savana, Sierra and Yukon models with a V8. GM says P0420 and P0430 are disabled for the first hour of engine run time on a new vehicle so the converter can break in. If the battery was disconnected during manufacturing, the test can run early; and after a converter replacement the hour no longer applies. Either way, a converter that hasn't finished breaking in can set a false P0420. GM's drive cycle:

1. Drive at sustained highway speed for 25 to 30 minutes.
2. Stop, shut the engine off for 1 minute.
3. Restart and drive at highway speed for another 10 to 15 minutes.
4. Stop, shut the engine off for 1 minute.
5. Restart and drive under the conditions that run the P0420/P0430 tests, and confirm they pass.

2014–2018 Sierra and 2015–2018 Yukon: sand past the air filter

GM bulletin 16-NA-111 covers the 2014–2018 Sierra (LV1, LV3, L83, L86) and 2015–2018 Yukon (L83, L86) with P0420 and/or P0430. GM's cause: sand and debris entering the air box and bypassing a poorly sealed air filter, damaging the oxygen sensor element and blocking the converter. GM's instruction: do not replace any parts until the air box has been validated as completely sealed — check the lid for missing tabs or fasteners, the filter seal, the housing for dirt past the filter, and the outlet duct for debris. Then repair the converter codes as needed. If you've only fixed the converter, you've only treated the result.

2011–2015 Sierra HD and Savana: the exhaust leak and the aftermarket bed

PIP5110C covers the 2011–2015 Sierra HD, Chassis Cab and Savana with the 4.8L L20 or 6.0L L96/LC8 that set P0420 with no other code. GM says to check for an exhaust leak behind the bank 1 sensor 2 oxygen sensor. If there's a leak — or the truck has an aftermarket utility bed or trailer hitch — check for at least 1.5 to 2 inches of clearance between the exhaust and those parts, then fit a new exhaust clamp. GM's words: this repair must be attempted before considering replacement of a catalytic converter.

A related GM special coverage, 15821, covered certain 2013 Sierra 1500s with P0420 from an exhaust leak where the bank 1 converter outlet joins the muffler inlet pipe, for 8 years or 80,000 miles; dealers replaced the clamp. That term has passed for practical purposes, and we could read only NHTSA's summary of it.

Any GMC gas engine: the coil ground

GM's PIP5232C covers 2016 and earlier GM cars and gasoline light- and heavy-duty trucks where a converter came apart or set P0420/P0430 for no apparent reason. GM says to inspect for a poorly crimped coil ground terminal on the engine block for the bank with the concern, check for corrosion or damage, and tighten the ground to the proper torque. A weak coil ground means weak sparks, partial misfires and unburned fuel cooking the converter.

Recalls that touch the converter

• N232412710 — emission recall for certain 2019 Sierra 1500 with the 2.7L L3B: the ECM may not properly diagnose, set codes or light the check engine lamp for certain converter faults. The fix is an ECM reprogram. If yours hasn't had it, a missing P0420 is not proof of a healthy converter.
• N232419720 — emission recall for certain 2022 Sierra and 2022–2023 Yukon with the 6.2L L87, where an incorrect converter service part may have been installed during servicing.
• 06-06-01-010D — GM's bulletin on inspecting, cleaning or replacing close-coupled converters when debris has entered the exhaust, for example after engine damage. It doesn't name a DTC, but it's why a converter should be inspected after an engine replacement.

Terrain Coverages and Recalls

• 2010–2011 Terrain 2.4L (LAF): special coverage 15810 covered converter thermal damage or face erosion that lights the check engine lamp, for 10 years or 120,000 miles. The dealer first confirms with a scan tool that the converter needs replacing. That term has passed for practical purposes.
• 2012–2013 Terrain 2.4L (Bin 4 emissions): special coverage N192287100 covered converter and oxygen sensor degradation — GM notes it can happen without a check engine light — caused by excessive oil consumption from piston ring wear (less than 2,000 miles per quart). The term was 10 years or 150,000 miles and has passed for practical purposes. A companion emission recall, N192268180, replaced the post-catalyst oxygen sensor on those Terrains; ask a dealer whether it was completed.
• 2022 Terrain 1.5L (LYX): emission recall N222379030 — a small number were built with the wrong converter. Dealers inspect the printed part number and replace the converter if it ends in 3459 or is illegible.

The Terrain 2.4L story matters for P0420 diagnosis: if the engine is burning oil, the oil is what's killing the converter. A converter replaced without fixing that will not last.

P0420 on a Duramax Diesel

On a diesel, P0420 is about the diesel oxidation catalyst (DOC), not a three-way converter. GM's PIP5085B covers the 2010–2015 Savana and 2011–2015 Sierra with the 6.6L Duramax (LML, LGH) setting P0420, P24A0 and/or P2463. GM asks technicians to record aftertreatment fuel injector quantity tests and regeneration history, and to inspect both faces of the DOC with a borescope for melting, powder or blockage — the inlet can look normal while the outlet is damaged. Gas-engine P0420 advice does not apply.

Symptoms of P0420

• Check engine light, usually with no driveability change
• Failed emissions or readiness inspection
• Rotten-egg smell in some cases
• Exhaust leak noise near the rear oxygen sensor (Sierra HD and Savana, PIP5110C)
• Rattle from the converter if the substrate has broken up
• Loss of power if the converter is plugged — GM's 16-NA-111 describes converters blocked with sand

Causes and How to Confirm Each

How Is P0420 Diagnosed?

1. Record all codes and freeze frame. Misfire, fuel trim and oxygen sensor codes come first.
2. Check for open recalls by VIN — especially a 2019 Sierra 2.7L, a 2022 Terrain 1.5L or a 2022–2023 Sierra or Yukon 6.2L.
3. Ask about mileage and history. A low-mileage V8 or one with a recent converter may need GM's break-in drive cycle.
4. Inspect the exhaust for leaks ahead of and around the rear oxygen sensor, and clearance to any aftermarket bed or hitch.
5. Inspect the air box on a 2014–2018 Sierra or Yukon for a bad seal and sand.
6. Check the coil ground for the bank with the code.
7. Look at sensor data. A downstream sensor mirroring the upstream one points at low oxygen storage.
8. Find out why it failed — oil use, coolant loss, misfire history — before replacing the converter.

Parts and Cost

Federal law gives the catalytic converter on light-duty vehicles an emissions warranty of 8 years or 80,000 miles, and some states and heavier trucks have different terms — check your warranty booklet and ask a dealer before paying.

GM Documents That Name P0420 on GMC Models

Have a dealer check your VIN for current bulletins, recalls and special coverages rather than relying on any article, including this one.

What Other P0420 Pages Get Wrong

• "It's the converter." GM tells technicians to fix exhaust leaks, air box seals and coil grounds first, and says a new converter may only need to break in.
• "Replace the downstream O2 sensor." None of GM's GMC P0420 documents fixes the code that way, except a specific Terrain recall that replaced the sensor for emissions accuracy.
• Ignoring aftermarket equipment. GM specifically calls out utility beds and hitches crowding the exhaust on Sierra HD and Savana.
• Assuming no code means no problem. GM recalled certain 2019 Sierra 2.7L trucks because the ECM might not flag converter faults.
• Treating a diesel the same. On a Duramax, P0420 concerns the diesel oxidation catalyst.
• Skipping the "why." On the 2012–2013 Terrain 2.4L, GM tied converter damage to oil consumption.

Frequently Asked Questions

Can I drive my GMC with P0420?

Usually yes in the short term if it runs normally, but it will fail an emissions test, and a plugged converter can cost power. A flashing check engine light means misfire — stop and fix that first.

Which converter is bank 1 on my Sierra?

The driver's side on the V8s and 4.3L V6. On a 2.7L four-cylinder Sierra, there's only bank 1.

My new Sierra set P0420 at 300 miles. Is the converter bad?

Maybe not. GM's PIP3119P says an incompletely broken-in converter can set a false code and gives a highway drive cycle to complete break-in.

I put a service body on my Sierra HD and now have P0420. Related?

It can be. GM's PIP5110C says to check for an exhaust leak behind the rear sensor and 1.5 to 2 inches of clearance to aftermarket beds and hitches before replacing a converter.

Is the converter covered on my Terrain?

The 2010–2011 and 2012–2013 Terrain 2.4L special coverages have passed for practical purposes, but a dealer VIN check is free, and a newer vehicle may be inside the federal emissions warranty.

Will a cheap aftermarket converter fix P0420?

It may not, and it may not be legal for your truck's emissions certification. GM's own recalls show how much the correct part matters. Order by VIN and certification.