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.

Second-generation MPVs carry three catalytic converters: a main converter plus two close-coupled 'pre-cats' bolted right at the exhaust manifolds, one on the radiator side and one on the firewall side. Sitting that close to the ports, the pre-cat substrates run extremely hot and break down. The immediate symptom is a catalyst efficiency code - P0420 on earlier vehicles, P0421 (warm-up catalyst, bank 1) on Mazda's later coding - but the real hazard is that a disintegrating pre-cat can shed ceramic substrate which, on an engine that has any intake pulsation or a leaking exhaust valve, can be drawn back toward the cylinder and score the bore. The failure is almost always downstream of something else: a persistent misfire, a leaking injector, or an oil-consumption problem cooking the catalyst. Owners commonly replace the rear O2 sensor first and get the code back within a drive cycle.

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 TA60 runs close-coupled converters bolted to the manifolds plus secondary converters downstream. Owners repeatedly report the ceramic substrate in an upstream converter breaking apart, at which point the loose material blows downstream and packs the secondary converter, choking the exhaust. Symptoms progress from a P0420/P0430 efficiency code to a rattle from the exhaust, then to a truck that will not pull, runs hot, and feels like it has lost half its power because it cannot exhale. Because the same codes are also set by lazy oxygen sensors, plenty of owners replace sensors first and are surprised when the codes come back.

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).