P0420 on a 2017 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 2017 Ram 1500
Ranked from 859 NHTSA owner complaints filed for this vehicle.
- 1
EGR cooler failure leaking coolant into the intake/exhaust stream (3.0L EcoDiesel)
High severityCracked EGR coolers on this powertrain are a recurring recall-related failure (19V-757/VB1) that dump coolant into the charge air and exhaust path, coating and poisoning the catalyst.
Reported in 7 NHTSA complaintsParts:EGR cooler - 2
Exhaust manifold / manifold bolt leak upstream of the downstream O2 sensor (5.7L HEMI)
Medium severityBroken or loosened exhaust manifold bolts are a well-known HEMI issue; the resulting leak dilutes the exhaust sample the downstream sensor reads, mimicking low catalyst efficiency. Listen for a cold-start tick and inspect manifold bolts/gasket.
Reported in 5 NHTSA complaints - 3
Engine misfire from valvetrain wear damaging the substrate
High severityUnburned fuel from a persistent misfire overheats and can melt the catalyst brick; the cylinder-6 misfire/lifter tick documented here is a plausible upstream cause.
Reported in 1 NHTSA complaint - 4
Worn or high-mileage catalytic converter
Medium severityNormal thermal aging of the substrate reduces oxygen storage capacity; confirm with comparative upstream/downstream O2 or wideband switching-frequency test.
Common causeParts:catalytic converter - 5
Failing downstream (post-catalyst) oxygen sensor
Medium severityA lazy or biased rear O2 sensor reads a false efficiency ratio even on a healthy converter; check sensor response time against a known-good pattern before replacing the cat.
Common causeParts:oxygen sensor - 6
General exhaust system leak (gaskets, flex pipe, pipe joints) ahead of the sensor
Low severityAny secondary air leak between the engine and the downstream sensor skews the efficiency calculation; smoke-test the full exhaust path.
Common causeParts:exhaust gasket
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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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