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

P0421 means the warm-up catalytic converter isn't cleaning the exhaust well enough during the warm-up phase. Fix any misfire or fuel trim fault before condemning the converter.

Cost range: $200–$2,200. Severity: medium. Safe to drive? Yes, but you'll fail emissions and the converter may keep degrading.

Most common causes of P0421

1. Unresolved misfire or rich condition — Raw fuel poisons the converter. Fix the root cause first or you'll buy two converters. ($100–$800)
2. Degraded warm-up catalyst — Ceramic substrate breaks down with age and heat cycling. ($500–$2,200)
3. Faulty downstream O2 sensor — A lazy sensor can fake a failing catalyst. ($150–$400)
4. Exhaust leak — Skews the post-catalyst reading. ($100–$400)

Symptoms drivers report with P0421

• Failed emissions test
• Sulfur smell
• Reduced power
• Check engine light