P0420 on a 2018 Ram 1500
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 1500
Ranked from 644 NHTSA owner complaints filed for this vehicle.
- 1
Coolant/oil contamination of the catalyst from EGR cooler failure
High severityMultiple owners report the 3.0L EcoDiesel EGR cooler (recall VB1/19V757) leaking coolant into the intake/exhaust stream, which can wash oxygen off the catalyst washcoat and drop efficiency.
Reported in 4 NHTSA complaints - 2
Oil contamination of the catalyst from internal engine failures
Medium severityThis fleet shows a pattern of oil-pressure loss, main-bearing, and catastrophic engine failures; burning oil passing through the exhaust can foul/poison the catalyst.
Reported in 3 NHTSA complaints - 3
Exhaust manifold leak/broken bolts ahead of the catalyst
Medium severityReported broken exhaust manifold bolts create an exhaust leak that both skews sensor readings and lets the catalyst run outside its efficient window.
Reported in 2 NHTSA complaints - 4
Worn/aged catalytic converter
Medium severitySimple mileage-related degradation of the catalyst substrate is the most common root cause of a P0420 on any vehicle.
Common causeParts:catalytic converter - 5
Failing downstream (post-cat) O2 sensor
Low severityA lazy or biased downstream sensor can mimic reduced catalyst efficiency even when the converter itself is fine.
Common causeParts:oxygen sensor - 6
Long-term misfires melting the catalyst substrate
High severityRepeated unburned fuel reaching the converter overheats and melts the substrate, permanently killing 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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