P0421: How to Diagnose & Fix — Warm Up Catalyst Efficiency Below Threshold (Bank 1)

The RX-8 catalytic converter clogs prematurely due to the rotary engine's naturally higher oil and fuel consumption. Unburned fuel and oil residue accumulate in the catalyst substrate, restricting exhaust flow. A clogged cat causes power loss, overheating, and can destroy the engine if backpressure becomes severe. Catalytic converter failure is often a secondary symptom of apex seal wear.

2.5L Outbacks are prone to premature catalytic converter degradation that sets a P0420 (Catalyst System Efficiency Below Threshold) check-engine light, often well before 100,000 miles. On some 2015-2016 2.5L Legacy/Outback models the front exhaust pipe assembly (which houses the front catalyst) can crack at the flexible bellows — Subaru issued voluntary recall/campaign WUJ-95 to replace the assembly. Separately, cars with the FB-series oil-consumption issue can contaminate and prematurely poison the catalyst with burnt oil, accelerating the P0420 failure. Subaru extended catalytic-converter coverage to 10 years/100,000 miles (bulletin WVI-19) for certain vehicles that can set P0420, on top of the federal 8-year/80,000-mile emissions warranty.

Foresters commonly throw a P0420 'Catalyst System Efficiency Below Threshold (Bank 1)' code and an illuminated check-engine light. While the converter itself is the part that degrades, the underlying root cause on FB25 Foresters is frequently the well-documented excessive-oil-consumption defect: burnt oil and unburned fuel coat and contaminate the catalyst, reducing its efficiency until it can no longer pass the downstream O2 sensor's efficiency check. Coolant intrusion and exhaust leaks can produce the same result. Subaru issued a service program (WVI-19/WVI-19R) extending the catalytic converter coverage to 10 years/100,000 miles on certain affected vehicles, and the related Yaeger oil-consumption settlement (2011-2014 models) extended engine warranty to 8 years/100,000 miles.

The 6G75 Endeavor runs three catalysts: a pre-cat integrated at each exhaust manifold plus an unmonitored underfloor unit. Only the two manifold pre-cats are monitored, so a check-engine light for catalyst efficiency always points at a manifold cat, never at the big underfloor converter that most quick-lube shops quote. Owners and techs confirm the mapping on this engine: the right (windshield-side) bank sets P0421 and the left (radiator-side) bank sets P0431. The pre-cats sit right at the head and cook, and they are commonly killed early by an upstream problem - a misfire or leaking injector dumping raw fuel into the converter.

The B17 and B18 Sentra with the MR20DD engine is prone to premature catalytic converter failure, often before 100,000 miles. The converter substrate breaks down, triggering P0420 efficiency codes. Contributing factors include the direct injection engine's tendency to produce more particulates, short trip driving cycles that prevent the converter from reaching full operating temperature, and catalytic converter theft (the Sentra's low ground clearance makes it an easy target).