P0420 on GMC TERRAIN — Catalyst System Efficiency Below Threshold Bank 1

Q: "What does P0420 mean on a GMC TERRAIN?"

"P0420 indicates: Catalyst System Efficiency Below Threshold Bank 1. This is a emission controls code. 3 GMC TERRAIN owners have reported this to NHTSA."

Q: "What are the most common causes of P0420 on GMC TERRAIN?"

"Based on 3 NHTSA reports, the most commonly cited components are: Power Train,Engine, Electrical System,Unknown Or Other,Engine, Engine And Engine Cooling. These percentages are based on the component field in owner complaints."

Q: "Is P0420 serious on my GMC TERRAIN?"

"Of 3 NHTSA reports for this code on GMC TERRAIN, none involved a crash or fire. However, any diagnostic code should be evaluated by a qualified mechanic."

Q: "What should I do if my GMC TERRAIN shows P0420?"

"1) Check for recalls on your VIN at nhtsa.gov/recalls. 2) Get a professional diagnosis — a code alone doesn't identify the exact failed part. 3) Get 2-3 repair estimates to compare."

What P0420 Means on Your GMC TERRAIN

P0420 is a diagnostic trouble code indicating: Catalyst System Efficiency Below Threshold Bank 1. This code relates to the emission controls system. The vehicle's computer detected a condition outside normal operating parameters and stored this code.

3 GMC TERRAIN owners have reported this issue to NHTSA across model years 2010-2013. Most reports come from the 2010, 2011 model years.

Real GMC TERRAIN Owner Reports

Source: NHTSA Complaints Database

"I bought my 2010 GMC Terrain SLE at 160,000. It had a rough start up but the dealer said it was because it was an older car and been sitting for a while. The rough start up never stopped even after replacing the catalytic converter because of code P0420."

— 2010 GMC TERRAIN owner · POWER TRAIN,ENGINE · Filed 11/14/2021

"Transmission failure with no warning, torque converter replacement $3,500 around 120k miles. check engine light unable to be diagnosed or repaired succesfully since 100k miles. code p0420 with no catalytic converter issues. every o2 sensor replaced, code will not clear."

— 2011 GMC TERRAIN owner · ELECTRICAL SYSTEM,UNKNOWN OR OTHER,ENGINE · Filed 06/09/2018

"The contact owns a 2013 GMC Terrain. The contact stated that when slowing down to come to a stop, the vehicle was jumping and hesitating, causing the contact to shift into neutral(N) to stop the vehicle and prevent the vehicle from shaking. There was a fuel odor that smelled like rotten eggs."

— 2013 GMC TERRAIN owner · ENGINE AND ENGINE COOLING · Filed 12/28/2024

All reports filed by vehicle owners directly with the U.S. Department of Transportation.

📊 Complaint Trend by Model Year

1

'10

1

'11

1

'13

Common Causes on GMC TERRAIN

Based on NHTSA complaint component analysis for this vehicle.

POWER TRAIN,ENGINE 33.3%

ELECTRICAL SYSTEM,UNKNOWN or OTHER,ENGINE 33.3%

ENGINE and ENGINE COOLING 33.3%

What To Do Next

1
Check for recalls on YOUR VIN
Go to nhtsa.gov/recalls and enter your VIN. If your vehicle is covered, repairs are free.
2
Get a proper diagnosis
A code alone doesn't tell you the exact failed part. A diagnostic at a shop ($50–$150) pinpoints the root cause before you spend money on parts.
3
Compare repair quotes
Get 2–3 quotes. Dealer vs. independent shop prices often differ 30–50% for the same repair.

Frequently Asked Questions

What does P0420 mean on a GMC TERRAIN? ▾

P0420 indicates: Catalyst System Efficiency Below Threshold Bank 1. This is a emission controls code. 3 GMC TERRAIN owners have reported this to NHTSA.

What are the most common causes of P0420 on GMC TERRAIN? ▾

Based on 3 NHTSA reports, the most commonly cited components are: Power Train,Engine, Electrical System,Unknown Or Other,Engine, Engine And Engine Cooling. These percentages are based on the component field in owner complaints.

Is P0420 serious on my GMC TERRAIN? ▾

Of 3 NHTSA reports for this code on GMC TERRAIN, none involved a crash or fire. However, any diagnostic code should be evaluated by a qualified mechanic.

What should I do if my GMC TERRAIN shows P0420? ▾

1) Check for recalls on your VIN at nhtsa.gov/recalls. 2) Get a professional diagnosis — a code alone doesn't identify the exact failed part. 3) Get 2-3 repair estimates to compare.