P0420 on a 2017 Ram 3500
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 2017 Ram 3500
Ranked from 302 NHTSA owner complaints filed for this vehicle.
- 1
Prior cylinder misfire/engine damage (grid-heater bolt ingestion) overheating the catalyst
High severityThis truck's engine family has a documented pattern of the intake air heater bolt melting and dropping into a cylinder, producing a misfire and raw unburned fuel/soot reaching the exhaust before the engine fails outright; this can overheat and foul the downstream catalyst. Check for prior misfire/engine-damage history before condemning the converter.
Reported in 8 NHTSA complaints - 2
Degraded SCR/DOC catalyst requiring replacement
High severityOne owner's truck was stuck in constant DPF regeneration with the dealer confirming the SCR catalytic converter itself needed replacement (on backorder); a saturated or chemically degraded catalyst on this platform directly drops conversion efficiency. Confirm with an exhaust backpressure/efficiency check and NOx/O2 sensor comparison across the catalyst.
Reported in 1 NHTSA complaint - 3
Excessive DPF soot loading / failed regeneration upsetting downstream readings
Medium severityA DPF that can't complete regeneration (confirmed on this truck with a P242F code and severe backpressure) changes exhaust flow and temperature reaching the catalyst sensors; check DPF soot load and regeneration history.
Reported in 1 NHTSA complaint - 4
Exhaust leak upstream or downstream of the catalyst monitoring sensors
Medium severityA leak changes the oxygen/NOx reading the module uses to judge catalyst efficiency; inspect exhaust joints, clamps, and the DPF/SCR housing for leaks.
Common causeParts:exhaust gasket - 5
Failing NOx/oxygen sensor
Low severityA drifting sensor can make an otherwise healthy catalyst look inefficient; compare sensor readings pre- and post-catalyst.
Common cause - 6
Oil consumption or coolant contamination fouling the catalyst
High severityInternal engine damage that introduces oil or coolant into the exhaust stream will coat and degrade the catalyst; check for abnormal fluid consumption and exhaust smoke color.
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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