P0420 on GMC
Catalyst System Efficiency Below Threshold (Bank 1)
P0420 on GMC vehicles indicates catalyst system efficiency below threshold (bank 1). Au7o has documented this code across 1 GMC model — most commonly on Terrain. 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 GMC range from $1,200 to $6,500, 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 GMC by Model
GMC Terrain(1 issue)
- 2.4L I4 Excessive Oil Consumption and Piston Ring Failure2010-2017
The 2.4L LEA and LE9 four-cylinder engines in 2010-2017 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. The root cause is inadequate piston ring tension combined with piston ring land fractures that allow oil to bypass 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.
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View P0420 across all makes →Frequently Asked Questions
What does P0420 mean on 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 specifically, this code is documented across 1 model.
What causes P0420 on GMC vehicles?▼
Common causes on GMC: 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 GMC?▼
Repair costs on GMC range from $1,200 to $6,500, depending on the specific model and root cause.
Which GMC models have P0420 documented?▼
Au7o has documented P0420 on 1 GMC model: Terrain.