P0420
Medium SeverityCatalyst System Efficiency Below Threshold (Bank 1)
Emissions System
P0420 — Instant Answer
Catalyst System Efficiency Below Threshold (Bank 1)
Severity: MEDIUM SEVERITY
Meaning: The catalytic converter on bank 1 is not converting pollutants efficiently. This is one of the most common DTCs.
Safe to drive? Usually yes with caution, but confirm root cause before long trips.
Estimated repair cost: $100 - $2,500
Most common causes:
- 1. Worn catalytic converter (35-45% of cases)
- 2. Faulty downstream O2 sensor (10-25% of cases)
- 3. Exhaust leak near sensors (10-25% of cases)
- 4. Engine running rich (damaging the cat) (10-25% of cases)
Symptoms
- ⚠️Check engine light
- ⚠️Failed emissions test
- ⚠️Slightly reduced fuel economy
- ⚠️Possible rotten egg smell
Related Symptoms
Common Causes
How to Diagnose P0420
- 1Check for O2 sensor codes first — fix those before replacing cat
- 2Compare upstream vs downstream O2 waveforms
- 3Check for exhaust leaks
- 4Monitor fuel trims for rich/lean issues
OEM Procedures for P0420
Catalyst System Efficiency Below Threshold (Bank 1) Found 98 OEM service manual procedures.
Adjustment/Replacement - Inspection
Adjustment/Replacement - Inspection
Adjustment/Replacement - Inspection
Adjustment/Replacement - Inspection
Adjustment/Replacement - Inspection
Adjustment/Replacement - Inspection
Adjustment/Replacement - Inspection
Adjustment/Replacement - Inspection
Showing 8 of 98 procedures
OEM Diagnostic Flowchart
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Related Resources
Repair guides, symptom pages, and wiring diagrams related to P0420.
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Related Symptoms
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Related Codes
The bank 1 cat is not reaching efficiency during warm-up.
Same as P0420 but on bank 2. If both P0420 and P0430 are present, the issue may be systemic (engine running rich, etc.).
General fault with the downstream (post-cat) O2 sensor on bank 1. This sensor monitors catalytic converter efficiency.
Frequently Asked Questions
- Can I drive with P0420?
- Yes, it is safe to drive. There are no driveability symptoms. However, you will fail emissions testing.
- Is P0420 always the catalytic converter?
- No — about 30% of the time it is actually a bad downstream O2 sensor giving false readings. Replace the sensor first ($25-$80) before spending $500+ on a cat.
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Author & Review
Written by AllOEMManuals Editorial Team. Reviewed by ASE-certified technicians and updated with factory-reference checks for fitment-sensitive steps.
This guide was drafted with AI assistance and reviewed for accuracy by a human editor.
Last updated: 2026-05-07