Chevy Check Engine Code
P0420 on Chevy
Catalyst System Efficiency Below Threshold (Bank 1)
Also covers: Chevrolet, GMC, Buick, Cadillac, GM
P0420 on Chevy: what makes it different
P0420 is one of the most frequently reported codes on Chevrolet vehicles, particularly those equipped with the 2.2L Ecotec and 3.5L or 3.9L V6 engines. The code indicates that the downstream oxygen sensor is detecting catalytic converter efficiency below the acceptable threshold. Chevy trucks and SUVs with the Vortec V8s are also commonly affected, often due to oil consumption or exhaust leaks accelerating catalyst degradation.
Most-affected engines
- 2.2L Ecotec L61/LAP (Cavalier, Cobalt, Malibu)
- 3.5L LX9 V6 (Malibu, Impala, Uplander)
- 5.3L Vortec LM7/LC9 (Silverado, Tahoe, Suburban)
- 6.0L Vortec LQ4/LY6 (Silverado HD, Express)
Common model years: 2000–2016
Most likely cause on Chevy
Worn or poisoned catalytic converter on the front bank, often accelerated by slight oil consumption common to the 5.3L Vortec's AFM lifter system.
Known Chevy engine-family issues
The 5.3L Vortec engine with Active Fuel Management is well-documented for consuming oil through the valve stem seals and AFM lifter valley, which sends oil vapor into the exhaust and slowly poisons the catalyst. Disabling AFM via a range-of-motion module or tune before replacing the catalytic converter is strongly recommended, otherwise the replacement converter will fail prematurely. The 3.5L V6 had a known intake manifold gasket issue that could leak coolant internally, and coolant in the exhaust rapidly destroys catalyst substrate.
Chevy-specific causes (most common first)
- Catalytic converter degraded by oil consumption from Active Fuel Management (AFM) cylinder deactivation on 5.3L Vortec engines
- Coolant intrusion from a leaking intake manifold gasket on 3.5L and 3.9L V6 engines poisoning the catalyst
- Exhaust manifold or flex-pipe leak upstream of the catalyst confusing the downstream O2 sensor reading
- Faulty or lazy downstream (post-cat) oxygen sensor giving inaccurate efficiency readings
- Carbon buildup on the downstream O2 sensor causing sluggish response
- Cheap or undersized aftermarket catalytic converter installed as a prior repair
Chevy-specific diagnostic tip
Before condemning the converter on a 5.3L Vortec, check the engine oil consumption rate over 1,000 miles. If the engine is consuming more than a quart per 2,000 miles, the AFM system is likely introducing oil into the exhaust. Also inspect the spark plugs for oily fouling on cylinders 1, 4, 6, and 7 — the AFM cylinders — as a quick confirmation. Replacing the converter without addressing oil consumption will result in repeat failure within 20,000–40,000 miles.
Symptoms drivers report
- Check engine light
- Slight loss of power or fuel economy
- Rotten-egg smell (severe cases)
- Emissions test fail
Typical repair cost on Chevy
Most Chevy owners fix P0420 for between $150 and $900, depending on which underlying cause turns out to be at fault. Start with the most-likely cause for your vehicle — Worn or poisoned catalytic converter on the front bank, often accelerated by slight oil consumption common to the 5.3L Vortec's AFM lifter system. — before throwing parts at it.
Chevy P0420 FAQ
Can I pass emissions with a P0420 on my Chevy?
No. A stored P0420 will cause an immediate failure on any OBD2 emissions test. The code must be resolved and the readiness monitor must complete a full drive cycle before you can pass inspection.
Will disabling AFM fix my P0420 on a 5.3L Silverado?
Disabling AFM stops the root cause of oil consumption that poisons the converter, but it won't fix a converter that is already degraded. You'll likely need a new catalytic converter along with addressing the AFM system to get a lasting repair.
Is it safe to drive my Chevy with a P0420?
The vehicle will run normally and is not immediately dangerous to drive, but continued operation with a degraded converter can eventually damage the downstream O2 sensor and may worsen fuel economy over time. Address it before your next emissions test.
Related codes
Drive a different make? See the general P0420 guide for cross-vehicle causes and symptoms.
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Most common fix for P0420:
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