Fault Family System: Powertrain (P-Codes) 10 SAE Codes

Catalyst Efficiency Below Threshold

Diagnostic & Technical Explanation

Code P0420 (Catalyst System Efficiency Below Threshold Bank 1) sets when the ECM compares pre- and post-catalyst oxygen sensor switching activity and finds the catalyst (or a specific catalyst brick on multi-brick systems) is no longer storing/releasing oxygen efficiently enough to meet emissions thresholds. The dominant cause is a catalyst that has aged out or been chemically poisoned (oil consumption, coolant leak into the exhaust, or leaded/high-sulfur fuel), though a persistent rich/lean fuel condition or an exhaust leak feeding false switching data can also trigger it.

These 10 diagnostic trouble codes share one detection mechanism and one repair path, which is why MechanicDB groups them into a single authored fault family rather than repeating the same explanation 10 times.

Codes in this family (10)

Code	Definition	Repair coverage
P0420	Catalyst System Efficiency Below Threshold Bank 1	4 ranked fixes
P0421	Catalyst 1 Efficiency Below Threshold Bank 1	4 ranked fixes
P0422	Catalyst 2 Efficiency Below Threshold Bank 1	4 ranked fixes
P0423	Heated Catalyst Efficiency Below Threshold Bank 1	4 ranked fixes
P0424	Heated Catalyst Temperature Below Threshold Bank 1	4 ranked fixes
P0430	Catalyst System Efficiency Below Threshold Bank 2	4 ranked fixes
P0431	Catalyst 1 Efficiency Below Threshold Bank 2	4 ranked fixes
P0432	Catalyst 2 Efficiency Below Threshold Bank 2	4 ranked fixes
P0433	Heated Catalyst Efficiency Below Threshold Bank 2	4 ranked fixes
P0434	Heated Catalyst Temperature Below Threshold Bank 2	4 ranked fixes

What this page does not show

This page publishes the fault mechanism and the code index. The licensed dataset adds, for every one of the 15,886 codes, the full authored repair set (56,729 procedures in total) with the fields deliberately withheld here:

Procedures 2..n after the first-listed procedure
Aftermarket parts cost ranges in USD (est_parts_cost_min/max_usd)
Professional labor-hour estimates (est_labor_hours)
Step-by-step diagnostic and repair instructions
Replacement-part mappings and catalog lookups
6,637 manufacturer-specific OEM codes across 32 makes ($149 tier)

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