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

Similar to P0420 but testing efficiency during warm-up phase.

Severity: MediumRepair: $100-$2,500Difficulty: Medium

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Common symptoms of P0421

Common causes of P0421

Repair cost & difficulty

Typical cost to fix P0421: $100-$2,500 · Difficulty: Medium

Pro tip: Same diagnostic approach as P0420.

Vehicles commonly affected

All makes

Related trouble codes

P0420P0430

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ScanFixAI is a diagnostic aid, not a substitute for professional repair. Cost estimates are typical ranges and vary by vehicle and region.