ErrorCodes
SAE Standard Definition

P0420

Catalyst System Efficiency Below Threshold (Bank 1)

The Bank 1 catalytic converter is not converting exhaust pollutants efficiently enough.

Moderate
Safe to drive advanced Fix within 14 days
Estimated Cost (USD)$200 - $2,500
BudgetMajor
Estimated Labor~2 hours

The ECM compares the switching activity of the upstream and downstream O2 sensors. A healthy catalytic converter shows a relatively steady downstream signal while the upstream switches rapidly. When the downstream starts mirroring the upstream, the converter has lost its ability to store oxygen and convert pollutants. This is one of the most common OBD-II codes.

Symptoms

Check Engine Light on
Failed emissions test
Slight sulfur/rotten egg smell
Usually drives normally
Possible slight power loss

Common Causes

Worn catalytic converter

HIGH

The catalyst substrate degrades over time (typically 100k+ miles).

Engine misfire damaging converter

MEDIUM

Unburned fuel entering the converter overheats and destroys the catalyst.

Oil or coolant contamination

MEDIUM

Internal engine leaks coat the catalyst material, reducing effectiveness.

Faulty downstream O2 sensor

LOW

A lazy downstream sensor may incorrectly indicate converter failure.

Step-by-Step Diagnosis

1

Compare O2 sensor signals

medium

Monitor Bank 1 Sensor 1 vs Sensor 2. If downstream mirrors upstream switching, converter is failing.

Tools:OBD-II scanner
2

Check for underlying issues

easy

Ensure no misfire codes, lean/rich codes, or oil consumption issues exist. Fix those first.

Tools:OBD-II scanner
3

Temperature test

easy

Measure inlet vs outlet temperature with IR thermometer. Outlet should be 50-100°F hotter. If not, converter is dead.

Tools:Infrared thermometer
4

Replace catalytic converter

hard

If confirmed failed, replace with CARB-compliant converter. Consider direct-fit for easier install.

Tools:Jack standsWrench setPenetrating oil

Commonly Seen With

P0420 is frequently found alongside these codes:

Related Codes

P0430

Frequently Asked Questions

Disclaimer

The information on this page is provided for general educational and reference purposes only. ErrorCodes is not a substitute for professional automotive diagnosis or repair. Diagnostic trouble codes indicate areas for further investigation and do not definitively identify the failed component. The same code can have different causes depending on your specific vehicle, its history, and conditions. Always consult a qualified, certified mechanic before performing repairs. ErrorCodes assumes no liability for any repairs made based on information found on this site.