P0420 on a 2018 Chrysler Pacifica

Most common causes on the 2018 Chrysler Pacifica

Ranked from 823 NHTSA owner complaints filed for this vehicle.

1. 1

   Head gasket failure with coolant intrusion into the cylinders (3.6L Pentastar)

   High severity

   An unusually large and consistent cluster of owners report the 3.6L Pentastar head gasket failing and leaking coolant into the cylinders, in several cases with a confirmed cylinder misfire code (P0305) and a catalytic-converter code alongside it; coolant burned through the combustion chambers is a direct, well-established way to contaminate and damage the catalyst.

   Reported in 9 NHTSA complaints

   Parts:head gasketcatalytic converter
2. 2

   Cracked catalytic converter/exhaust manifold ("Maniverter") assembly

   High severity

   One owner's dealer confirmed a crack in the Bank 2 manifold-integrated catalytic converter ("Maniverter") assembly that let raw exhaust past the catalyst bed.

   Reported in 1 NHTSA complaint

   Parts:catalytic converterexhaust manifold
3. 3

   Worn/degraded catalytic converter (high-mileage efficiency loss)

   Medium severity

   Catalyst efficiency naturally declines with age and mileage until it can no longer keep the post-cat O2/AFR sensor signal steady relative to the pre-cat sensor; most common root cause on higher-mileage vehicles.

   Common cause

   Parts:catalytic converter
4. 4

   Failing downstream (post-catalyst) O2/air-fuel sensor

   Medium severity

   A lazy or degraded rear sensor can mimic poor catalyst efficiency even when the converter itself is fine; compare pre- and post-cat sensor switching patterns on a scope or scan tool.

   Common cause

   Parts:oxygen sensor
5. 5

   Exhaust leak upstream of the catalyst or sensors

   Medium severity

   A leak at the manifold, gasket, or flex pipe ahead of the front O2 sensor lets outside air skew the air/fuel reading and can trigger a false efficiency code.

   Common cause
6. 6

   Oil or coolant contamination of the catalyst

   High severity

   Oil (from worn rings/valve seals) or coolant (from a head gasket/intake leak) burned through the engine coats and poisons the catalyst substrate, permanently reducing its efficiency.

   Common cause

   Parts:catalytic converter
7. 7

   Ignition-related misfire (worn spark plugs, coils, or plug wires) overloading the catalyst

   Medium severity

   Any chronic misfire, not just an AFM/DOD lifter fault, sends unburned fuel into the exhaust where it combusts inside the converter and overheats/damages the substrate over time.

   Common cause

   Parts:spark plugignition coil
8. 8

   Aftermarket or damaged exhaust component altering sensor placement or backpressure

   Low severity

   A non-OEM cat, resonator delete, or damaged pipe can change exhaust flow/backpressure enough to disrupt the post-cat sensor's ability to confirm normal catalyst switching.

   Common cause

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