P0420 on a 2016 Mercedes Benz C Class Phev

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 2016 Mercedes Benz C Class Phev

Ranked from 317 NHTSA owner complaints filed for this vehicle.

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1
Oil fouling of the catalyst from chronic PCV/crankcase pressure faults
Medium severity
Owners describe the PCV/crankcase regulator valve failure on this engine building crankcase pressure that pushes oil past seals and gaskets; oil-fouled substrate is a recognized way this platform's repeated PCV issue can drag down catalyst efficiency over time.
Reported in 1 NHTSA complaint
Parts:catalytic converter PCV valve
2
Long-term unresolved misfires (ignition coil failure) overheating the substrate
High severity
An owner reports faulty ignition coils outside the C63 coil recall; sustained misfire dumps raw unburned fuel into the exhaust and can melt/overheat the catalyst substrate.
Reported in 1 NHTSA complaint
Parts:ignition coil catalytic converter
3
Worn/degraded catalytic converter
Medium severity
Catalyst efficiency naturally declines with age and mileage; this is the default cause once upstream faults are ruled out.
Common cause
Parts:catalytic converter
4
Failing downstream (post-cat) O2 sensor
Medium severity
A lazy or contaminated downstream sensor reports the same switching pattern as the upstream sensor, falsely signaling low catalyst efficiency.
Common cause
Parts:oxygen sensor
5
Exhaust leak ahead of the downstream sensor
Medium severity
A leaking manifold, flange, or flex-pipe joint introduces outside air that confuses the catalyst monitor.
Common cause
Parts:exhaust gasket
6
Coolant contamination of the catalyst from a head/intake gasket leak
Medium severity
Coolant burned through a leaking gasket coats the catalyst substrate and drops its efficiency below threshold.
Common cause
Parts:catalytic converter intake manifold 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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Matching symptoms

Check Engine Light