P0420 on a 2018 Ram 2500
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 2018 Ram 2500
Ranked from 2,507 NHTSA owner complaints filed for this vehicle.
- 1
DEF quality or dosing errors reducing SCR catalyst NOx conversion
Medium severityMultiple owners report DEF-system warnings and derates, indicating dosing/quality problems that directly reduce SCR catalyst efficiency.
Reported in 2 NHTSA complaints - 2
SCR/DPF aftertreatment system faults (DEF dosing, sensors)
High severityOwner reports repeated emissions/aftertreatment failures including DEF dosing issues and SCR/DPF faults directly affecting catalyst/aftertreatment efficiency monitoring.
Reported in 1 NHTSA complaint - 3
Incomplete or failed DPF regeneration cycles
Medium severityA reported case of the truck dying repeatedly during active regeneration indicates regen faults that leave the DPF/DOC loaded with soot, hurting measured efficiency.
Reported in 1 NHTSA complaint - 4
Prior/ongoing engine misfire loading the catalyst with unburned fuel
Medium severityAn owner-reported misfire/rough-running condition can dump unburned fuel into the exhaust, fouling or overheating the catalyst and lowering measured efficiency.
Reported in 1 NHTSA complaint - 5
Failing downstream NOx/O2 sensor
Medium severityA degraded post-catalyst sensor reports inaccurate readings that the PCM interprets as reduced catalyst efficiency.
Common cause - 6
Exhaust leak upstream of the monitored sensor
Low severityA leak ahead of the sensor dilutes the sample with outside air, causing a false low-efficiency reading.
Common cause - 7
Worn/soot-loaded diesel oxidation catalyst (age, mileage)
Medium severityNormal catalyst aging or heavy soot loading over time reduces conversion efficiency below the OBD threshold.
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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