P0420

Q: What does OBD2 code P0420 mean?

P0420 means: Catalyst System Efficiency Below Threshold Bank 1. The catalytic converter on Bank 1 is not converting exhaust gases efficiently enough. The downstream O2 sensor is switching too rapidly, similar to the upstream sensor.

Catalyst System Efficiency Below Threshold Bank 1

The catalytic converter on Bank 1 is not converting exhaust gases efficiently enough. The downstream O2 sensor is switching too rapidly, similar to the upstream sensor.

SeverityModerate

CategoryEmissions

Est. Repair Cost$500–$2000

Code TypePowertrain

DIY RepairProfessional

⚡ Common Causes

›Worn or damaged catalytic converter
›Engine burning oil contaminating catalyst
›Coolant leak into exhaust
›Rich-running condition damaging catalyst
›Failed downstream O2 sensor

⚠️ Symptoms

›Check engine light
›Sulfur smell from exhaust
›Slightly reduced fuel economy
›Failed emissions test

✅ How to Fix — Step by Step

Step 1:Rule out false triggers: check for exhaust leaks, test downstream O2 sensor
Step 2:Check for oil or coolant burning
Step 3:Fix any misfire or rich condition first
Step 4:Replace catalytic converter
Step 5:Replace downstream O2 sensor if faulty

💡 Pro Tip: Fix the root cause (misfire, oil burning, rich condition) before replacing the cat — or the new one will fail too.

Repair Cost Estimate

Full Repair Estimate

$500–$2000

DIY Possible?

PRO

Severity

Moderate

Frequently Asked Questions

What does OBD2 code P0420 mean?+

P0420 stands for Catalyst System Efficiency Below Threshold Bank 1. The catalytic converter on Bank 1 is not converting exhaust gases efficiently enough. The downstream O2 sensor is switching too rapidly, similar to the upstream sensor.

What causes code P0420?+

Worn or damaged catalytic converter; Engine burning oil contaminating catalyst; Coolant leak into exhaust; Rich-running condition damaging catalyst; Failed downstream O2 sensor

How do I fix code P0420?+

Rule out false triggers: check for exhaust leaks, test downstream O2 sensor; Check for oil or coolant burning; Fix any misfire or rich condition first; Replace catalytic converter; Replace downstream O2 sensor if faulty

How much does it cost to fix P0420?+

Estimated repair cost for P0420 is $500–$2000.

Is P0420 DIY-friendly?+

P0420 typically requires professional equipment and a qualified mechanic.