P0420 on a 2016 Mazda Cx 5
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 Mazda Cx 5
Ranked from 507 NHTSA owner complaints filed for this vehicle.
- 1
Oil or coolant contamination of the catalytic converter from a cracked cylinder head
High severityMultiple owners had a cracked cylinder head diagnosed (with oil/coolant mixing and a milky substance found in the coolant), and the related oil-consumption defect burns oil through the combustion chamber — both directly coat and poison the catalyst.
Reported in 5 NHTSA complaints - 2
Worn or failed catalytic converter
High severityAn independent mechanic directly diagnosed a failed catalytic converter and replaced the oxygen sensors on this vehicle after the check engine light illuminated.
Reported in 1 NHTSA complaint - 3
Failing downstream oxygen sensor
Medium severityA slow or inaccurate downstream sensor can misreport true catalyst efficiency, a standard cause of this code.
Common causeParts:oxygen sensor - 4
Exhaust leak upstream of the downstream oxygen sensor
Medium severityA leak introduces outside air before the sensor, skewing the efficiency reading independent of true catalyst health.
Common cause - 5
Fuel delivery problems (failing fuel pump) causing a rich/lean cycling that degrades the catalyst
Medium severityThe Denso fuel pump defect on this vehicle causes fuel-pressure swings; sustained rich or lean operation over time accelerates catalyst degradation.
Common cause - 6
Long-term engine misfire degrading the catalyst substrate
High severityUnburned fuel from a sustained misfire overheats and melts catalyst media, a standard secondary cause of efficiency codes.
Common cause
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.