# P0420 — Catalyst System Efficiency Below Threshold (Bank 1)

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

Source: https://clearthecode.com/codes/P0420

**System:** Powertrain · Catalyst System

**Severity:** moderate

## Description

The downstream O2 sensor is reading too similar to the upstream sensor, meaning the catalytic converter on Bank 1 isn't doing its job. Most common cause: aged or failed catalytic converter. Verify with a known-good downstream O2 sensor before replacing the cat. Engine misfires, oil burning, or rich/lean conditions can prematurely damage the cat.

## Diagnostic snapshot

- **Difficulty:** 6/10 — Intermediate diagnostics
- **Est. repair cost:** $40 – $450
- **Common symptoms:** Failed emissions test, Check engine light (MIL), Poor fuel economy

## Diagnostic workflow

1. Confirm P0420 with a live scan — note pending vs stored and freeze frame data.
2. Verify reported symptoms: Failed emissions test, Check engine light (MIL), Poor fuel economy.
3. Inspect wiring/connectors and related sensors before replacing modules.
4. Most likely fixes: Replace catalytic converter(s); Replace oxygen sensor(s); Cylinder-specific misfire diagnosis (swap / compression / injector).
5. Clear codes and road-test; re-scan after two drive cycles if the monitor must set.

## Likely fixes

1. **Replace catalytic converter(s)** (moderate)
2. **Replace oxygen sensor(s)** (easy)
3. **Cylinder-specific misfire diagnosis (swap / compression / injector)** (moderate)

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