P0420 on Chevrolet
Catalyst System Efficiency Below Threshold (Bank 1)
P0420 on Chevrolet vehicles indicates catalyst system efficiency below threshold (bank 1). Au7o has documented this code across 4 Chevrolet models — most commonly on Equinox, Prizm, Silverado 2500HD. 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. Typical repair costs on Chevrolet range from $50 to $7,000, depending on the specific model and root cause.
Common Causes of P0420
- •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)
P0420 on Chevrolet by Model
Chevrolet Equinox(2 issues)
- Excessive Oil Consumption (2.4L Piston Ring Defect)2010-2017
The 2.4L Ecotec engine uses low-tension piston rings that fail to maintain proper tension to keep oil in the crankcase. Vehicles consume 1 quart of oil per 800-1,000 miles. This defect can cause premature engine wear, engine failure, and potential fire risk if oil levels drop critically low. GM acknowledged this defect through TSBs and class action settlements.
- Engine Oil Leak Fire Risk (Oil Filter Adapter)2010-2017
A defective oil filter adapter can cause engine oil leaks that may contact hot exhaust components, creating a fire risk. Combined with the excessive oil consumption issue, low oil conditions can exacerbate the problem.
Chevrolet Prizm(2 issues)
- Excessive Oil Consumption1998-2002
Toyota 1ZZ-FE engine (shared with Corolla) consumes excessive oil due to piston ring design. Can burn 1 quart every 1,000-2,000 miles. Toyota issued a TSB for Corolla with same engine.
- Oxygen Sensor Failure1998-2002
Oxygen sensors fail causing poor fuel economy and check engine light. Both upstream and downstream sensors prone to failure. Oil consumption issues accelerate sensor fouling.
Chevrolet Silverado 2500HD(1 issue)
- Exhaust Manifold Bolt Failure1999-2025
The exhaust manifold bolts on both the gas V8 (5.3L, 6.0L, 6.6L) and Duramax diesel engines in the Silverado 2500HD are notorious for breaking due to thermal cycling. The bolts snap flush with the cylinder head, causing an exhaust leak that produces a ticking or tapping noise on cold start that quiets as the manifold expands and seals when warm. Broken bolts can also cause exhaust fumes to enter the cabin through the HVAC fresh air intake. The passenger side manifold on V8 gas engines is the most commonly affected.
Chevrolet Sonic(1 issue)
- Persistent Check Engine Light and Electrical System Faults2012-2016
Early Chevrolet Sonic models are prone to persistent check engine light illumination with various electrical system faults. The 2012 model year in particular has a reputation for the check engine light activating with no clear mechanical cause. Issues range from faulty oxygen sensors and MAP sensors to ECM communication errors. The vehicle has been recalled 7 times for the 2012 model year alone across electrical and safety systems.
Looking for P0420 on a different make?
View P0420 across all makes →Frequently Asked Questions
What does P0420 mean on Chevrolet?▼
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 Chevrolet specifically, this code is documented across 4 models.
What causes P0420 on Chevrolet vehicles?▼
Common causes on Chevrolet: 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. Specific causes vary by model and year — see the per-model sections below.
How much does it cost to fix P0420 on a Chevrolet?▼
Repair costs on Chevrolet range from $50 to $7,000, depending on the specific model and root cause.
Which Chevrolet models have P0420 documented?▼
Au7o has documented P0420 on 4 Chevrolet models: Equinox, Prizm, Silverado 2500HD, Sonic.