P0420
Medium SeverityCatalyst System Efficiency Below Threshold (Bank 1)
Emissions System
What Does P0420 Mean?
The catalytic converter on bank 1 is not converting pollutants efficiently. This is one of the most common DTCs.
Common Fix
Replace catalytic converter or downstream O2 sensor
Estimated Cost (DIY)
$100 - $2,500
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 Diagnostic Flowchart
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Read live data and freeze frame for P0420 to pinpoint the cause before buying parts.
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Related Resources
Repair guides, symptom pages, and wiring diagrams related to P0420.
OEM Manual Evidence
Browse →A L L Diagnostic Trouble Codes ( DTC ) — 2013 GMC Truck Yukon XL Denali AWD V8-6.2L Service Manual | Operation CHARM Operation CHARM : Car...
A L L Diagnostic Trouble Codes ( DTC ) — 2013 GMC Truck Yukon XL Denali 2WD V8-6.2L Service Manual | Operation CHARM Operation CHARM : Car...
A L L Diagnostic Trouble Codes ( DTC ) — 2013 GMC Truck Yukon XL 4WD V8-6.0L Service Manual | Operation CHARM Operation CHARM : Car repair...
A L L Diagnostic Trouble Codes ( DTC ) — 2013 GMC Truck Yukon XL 4WD V8-5.3L Service Manual | Operation CHARM Operation CHARM : Car repair...
Related Symptoms
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Open the exact vehicle hub for a repair workflow commonly associated with P0420.
Open the exact vehicle hub for a repair workflow commonly associated with P0420.
Open the exact vehicle hub for a repair workflow commonly associated with P0420.
Open the exact vehicle hub for a repair workflow commonly associated with P0420.
Exact Repair Workflows
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Open the exact repair flow most commonly associated with P0420 on this vehicle.
Open the exact repair flow most commonly associated with P0420 on this vehicle.
Open the exact repair flow most commonly associated with P0420 on this vehicle.
Open the exact repair flow most commonly associated with P0420 on this vehicle.
Open the exact repair flow most commonly associated with P0420 on this vehicle.
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.
Related Repair Guides
If your vehicle has triggered P0420, these DIY repair guides may help: