P0420 on GMC: How to Diagnose & Fix
Catalyst System Efficiency Below Threshold (Bank 1)
4 GMC models · 5 documented issues · 2 critical · $600-$6,500 repair
P0420 on GMC by model: symptoms, diagnosis and fix
The 2.4L LAF and LEA four-cylinder engines in 2010-2013 Terrain are among the most reported excessive oil consumption issues in GM's lineup. Many owners consume 1 quart of oil every 1,000-2,000 miles. GM attributes it to piston ring wear that allows oil to bypass the rings into the combustion chamber. The problem was subject to class action lawsuits and GM Customer Satisfaction Programs. Burned oil causes spark plug fouling, catalytic converter damage, and oil pressure warnings. This is the same issue affecting the Chevy Equinox with the same engine.
Common Symptoms
- Oil level dropping 1+ quart every 1,000-2,000 miles
- Blue smoke on acceleration or on startup
- Spark plug fouling (oil on electrode)
- Oil pressure warning light
- Catalytic converter damage from oil burning
- Engine running rough (fouled plugs)
How to Fix
GM bulletin 13-06-01-003: after the ECM oil-life calibration update, the dealer runs an oil consumption test per bulletin 01-06-01-011. If consumption exceeds 1 qt per 2,000 miles, the pistons and rings are replaced, at no charge under the model-year special coverages (14159 for 2010, 15285 for 2011, 16118 for 2012, N192291100 for 2013) while they apply. Short-term: use Dexos1 Gen 2 full synthetic and check oil every 1,000 miles. GM's bulletin states the bores need no machining and engine replacement should not be necessary.
Sources for this issue (4)
GM says sand or debris can enter the air filter box and bypass a poorly sealing air filter, damaging the oxygen sensor element and blocking the catalytic converter with sand and debris particles, which turns on the check-engine light with P0420 and/or P0430. 2014-2018 GMC Sierra with the L83 5.3L V8, L86 6.2L V8 or LV1/LV3 4.3L V6, per GM bulletin 16-NA-111 (March 2019 version).
How to Fix
Have a technician inspect the air filter housing cover, the filter seal, the sealing surface and the outlet duct for dirt bypassing the filter, as bulletin 16-NA-111 directs, before replacing any sensor or converter. The bulletin says to replace the air filter or the air cleaner assembly if it is not sealing, then repair the P0420/P0430 as needed. Normal warranty rules apply; this is not a recall or a promise of free repair.
Call to confirm they can assess your vehicle and perform the stated work. Search results do not confirm qualification, eligibility or coverage.
Sources for this issue (1)
GM says sand or debris can enter the air filter box and bypass a poorly sealing air filter, damaging the oxygen sensor element and blocking the catalytic converter with sand and debris particles, which turns on the check-engine light with P0420 and/or P0430. 2014-2018 GMC Sierra with the L83 5.3L V8, L86 6.2L V8 or LV1/LV3 4.3L V6, per GM bulletin 16-NA-111 (March 2019 version).
How to Fix
Have a technician inspect the air filter housing cover, the filter seal, the sealing surface and the outlet duct for dirt bypassing the filter, as bulletin 16-NA-111 directs, before replacing any sensor or converter. The bulletin says to replace the air filter or the air cleaner assembly if it is not sealing, then repair the P0420/P0430 as needed. Normal warranty rules apply; this is not a recall or a promise of free repair.
Call to confirm they can assess your vehicle and perform the stated work. Search results do not confirm qualification, eligibility or coverage.
Sources for this issue (1)
On the 3.6L V6 Canyon (LFX in 2015-2016, LGZ from 2017), the catalytic converters lose efficiency and trigger P0420 (Bank 1) and/or P0430 (Bank 2), often a downstream consequence of oil consumption, misfires or exhaust leaks that contaminate the converter substrate and O2 sensors. Both converters can fail around similar mileage since they age under identical conditions.
Common Symptoms
- Check engine light with P0420 and/or P0430
- Reduced fuel economy
- Failed emissions/smog test
- Sulfur/rotten-egg exhaust smell
How to Fix
Diagnose before replacing: rule out a faulty downstream O2 sensor or exhaust leak, and correct any oil-consumption or misfire root cause first so the new converter is not re-poisoned. Replace the affected catalytic converter(s) once root cause is addressed.
Sources for this issue (2)
The 2007.5-2010 6.6L Duramax RPO LMM was GM's first Duramax with a diesel particulate filter, and the early calibration regenerates far too often. Owners on duramaxforum.com and dieselplace.com report the truck entering regen every 100-200 miles, and in trucks used for short trips, heavy idling or slow towing the regen cycle is repeatedly interrupted before completion. Soot then accumulates faster than it burns off, the differential pressure across the filter climbs, and the truck eventually refuses to self-regen at all and derates. The excess fuel injected late in the cycle for regeneration also washes down the cylinder walls and dilutes engine oil. This is an LMM-specific pattern; the pre-2007.5 LBZ has no DPF, and the LML added SCR/DEF which changed the regen strategy.
Common Symptoms
- Frequent regeneration cycles and repeated "Exhaust Filter Cleaning" messages
- "Reduced Engine Power" / derate
- Poor fuel economy
- Engine oil level rising and smelling of fuel
- Check engine light with DPF pressure codes
How to Fix
If the filter is not yet plugged solid, complete a dealer-commanded service regeneration and then drive 20-30 minutes at steady highway speed to finish it. A badly loaded DPF must be professionally bake-and-blow cleaned or replaced. Check for the upstream causes first: leaking injectors, a sticking VGT turbo, or a failed exhaust temperature sensor all cause repeat DPF loading. Shorten oil change intervals because of regen-driven fuel dilution. Emissions-delete hardware is illegal for on-road use and is not a repair.
Sources for this issue (3)
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 GMC?
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 GMC it is documented across 4 models: Terrain, Sierra 1500, Canyon, Sierra 2500HD.
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 GMC?
Published repair estimates on GMC range from $600 to $6,500 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
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Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.