P0420 on a 2016 Hyundai Tucson
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 2016 Hyundai Tucson
Ranked from 1,613 NHTSA owner complaints filed for this vehicle.
- 1
Catalytic converter contamination/damage from chronic oil consumption (Theta II defect)
High severityOne owner had the catalytic converter replaced three times while the engine kept consuming oil and eventually overheated — oil ash from a chronic burning defect coats and poisons the catalyst.
Reported in 1 NHTSA complaintParts:catalytic converter - 2
Unresolved cylinder misfires overheating and melting the catalyst substrate
High severityThis vehicle had a documented misfire (dead cylinder, spark plugs/coils replaced); raw unburned fuel reaching the converter during a misfire can overheat and melt the substrate.
Reported in 1 NHTSA complaint - 3
Worn or aged catalytic converter
Medium severityCatalyst efficiency naturally degrades with mileage/heat cycles until it can no longer keep the downstream O2 sensor's switching in range.
Common causeParts:catalytic converter - 4
Failing downstream (post-catalyst) oxygen sensor
Medium severityA lazy or biased downstream sensor can mimic a degraded catalyst reading even when the converter itself is fine; compare upstream/downstream sensor switching.
Common causeParts:oxygen sensor - 5
Exhaust leak between the engine and the downstream O2 sensor
Low severityA leak lets outside air reach the sensor, flattening the switching pattern the PCM uses to judge catalyst efficiency.
Common cause - 6
Oil or coolant intrusion into the exhaust from internal engine wear
Medium severityBurning oil past worn rings/valve seals deposits ash on the catalyst washcoat, permanently lowering its conversion efficiency.
Common cause
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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.
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