# P0421 — Warm Up Catalyst Efficiency Below Threshold (Bank 1)

> ClearTheCode diagnostic code reference (not OEM service data).

Source: https://clearthecode.com/codes/P0421

**System:** Powertrain · Catalyst System

**Severity:** moderate

## Description

Warm-up catalyst on Bank 1 isn't reaching efficiency. Often a failed cat; verify with sensor swap first.

## Diagnostic snapshot

- **Difficulty:** 6/10 — Intermediate diagnostics
- **Est. repair cost:** $120 – $450

## Diagnostic workflow

1. Confirm P0421 with a live scan — note pending vs stored and freeze frame data.
2. Inspect wiring/connectors and related sensors before replacing modules.
3. Clear codes and road-test; re-scan after two drive cycles if the monitor must set.