P0420 on a 2015 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 2015 Ram 1500

Ranked from 1,427 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Broken exhaust manifold bolts or a warped/soot-clogged manifold causing a pre-cat exhaust leak
High severity
Same 5.7L Hemi exhaust-manifold failure mode seen on the 2013 model year: broken bolts and manifold warping/soot buildup are reported here too, producing leaks and smell ahead of the catalyst.
Reported in 3 NHTSA complaints
Parts:exhaust manifold bolts exhaust manifold
2
Valve-train/lifter wear ('Hemi tick') causing misfires that heat-damage the catalytic converter
High severity
Multiple complaints describe engine ticking tied to lifter/valve lubrication problems and freeway power loss -- the same oil-starvation valve-train failure documented on this engine family, which can produce misfires that overheat the cat.
Reported in 3 NHTSA complaints
Parts:lifters camshaft catalytic converter
3
Worn or aged catalytic converter
Medium severity
Normal mileage-related efficiency loss in the catalyst.
Common cause
Parts:catalytic converter
4
Failing downstream (rear) oxygen sensor
Medium severity
A degraded post-cat sensor can read too similarly to the upstream sensor and falsely indicate low catalyst efficiency.
Common cause
Parts:oxygen sensor
5
Exhaust gasket or pipe leak ahead of the sensor (general wear)
Medium severity
Ordinary gasket degradation at manifold-to-pipe joints ahead of the sensor lets in outside air and skews the reading.
Common cause
Parts:exhaust gasket
6
Chronic ignition misfire from worn spark plugs or coils
Medium severity
Repeated unburned fuel entering the exhaust from ignition-side misfires overheats and fouls the catalyst over time.
Common cause
Parts:spark plugs ignition coil

Part links go to Amazon search results for this vehicle. As an Amazon Associate, WrenchConnect earns from qualifying purchases — always confirm fitment for your exact trim and engine before ordering.

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.

Rather have a pro handle it? Find a vetted local mechanic.

Matching symptoms

Check Engine Light