P0420 on a 2015 Chevrolet Tahoe

P0420 means the downstream oxygen sensor pattern suggests the bank 1 catalytic converter is not storing oxygen properly.

Not your vehicle? See general P0420 info.

Most common causes on the 2015 Chevrolet Tahoe

Ranked from 661 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Catalyst damage from prolonged misfire (collapsed AFM lifters)
High severity
This engine has a well-documented collapsed-lifter/camshaft failure that first presents as sputtering, misfire, and rough running (some with a blinking check-engine light); sustained misfire of this type can overheat and damage the catalyst substrate.
Reported in 4 NHTSA complaints
Parts:valve lifters camshaft catalytic converter
2
Worn or failed catalytic converter
High severity
Multiple owners had a catalytic converter diagnosed as failed or needing replacement, with dealer-confirmed diagnostic codes for the catalyst.
Reported in 3 NHTSA complaints
Parts:catalytic converter
3
Failing downstream (rear) oxygen sensor
Medium severity
A degraded rear O2 sensor can mimic reduced catalyst efficiency even when the converter itself is fine; compare upstream/downstream sensor switching rates.
Common cause
Parts:oxygen sensor
4
Exhaust leak upstream of the downstream O2 sensor
Medium severity
A leak before the rear sensor lets in outside air, skewing the efficiency calculation and triggering the code.
Common cause
5
Oil or coolant contamination of the catalyst
Medium severity
Oil or coolant reaching the exhaust (e.g., from an internal leak) coats and poisons the catalyst, reducing its efficiency.
Common cause
6
High-mileage/aged catalytic converter (normal wear)
Low severity
Catalyst substrate naturally degrades with heat cycling and mileage, gradually dropping conversion efficiency below the OBD threshold even without a specific failure event.
Common cause
Parts:catalytic converter

Part links go to Amazon search results for this vehicle. As an Amazon Associate, WrenchConnect earns from qualifying purchases — always confirm fitment for your exact trim and engine before ordering.

These causes are aggregated from NHTSA owner complaints and general repair knowledge — this is not a diagnosis. P0420 on your car still needs shop confirmation before any repair.

Common symptoms

Check engine light
Usually no drivability issue
Failed emissions test
Sulfur smell in some cases
Reduced power if converter is clogged

What to check before replacing parts

Check for misfire and fuel trim codes first

Look for: Any active or recent misfire, or lean/rich trim codes, means raw fuel or an off mixture has likely already damaged or is currently overworking the converter — fix those before touching the catalyst.

Inspect for exhaust leaks ahead of the converter

Look for: A leak before the converter (cracked manifold, loose flange, blown gasket) lets in outside air that confuses the downstream O2 sensor into reading as if the catalyst isn't working, even when it is.

Compare upstream and downstream O2 sensor activity

Look for: A healthy converter makes the downstream sensor read flat and steady while the upstream sensor swings actively. If the downstream sensor switches nearly as fast as the upstream one, the converter isn't storing oxygen properly and is likely worn out.

Check converter temperature and performance

Look for: A converter outlet noticeably cooler than expected relative to its inlet under load suggests little catalytic reaction is happening inside — supports a genuinely failed converter rather than a false reading.

Hold off on replacing the converter until root causes are ruled out

Look for: If misfires, exhaust leaks, or fuel trim issues caused this code, a new converter will fail again quickly — confirm those are clean first.

Rather have a pro handle it? Find a vetted local mechanic.

Matching symptoms

Check Engine Light