P0421 Code: Warm-Up Catalyst (B1) — Causes & Fix
P0421 means the warm-up catalytic converter efficiency on bank 1 is below threshold — the converter is not cleaning the exhaust well enough during warm-up.
- Applies to
- All OBD-II vehicles (any make or model)
- Est. repair cost
- $100 – $1,500
- Time to fix
- 30 minutes – 2 hours
What it means
Causes include an aged catalytic converter, a lazy O2 sensor, or an exhaust leak. P0421 is a generic OBD-II powertrain code defined in the SAE J2012 standard, so it applies to virtually any 1996-or-newer petrol car or light truck regardless of make. Fix the underlying fault, then clear the code with an OBD-II scan tool — it will return if the problem isn't resolved.
Symptoms
- Check engine light on
- Slight economy drop
- Rotten-egg smell; failed emissions
How to fix it
- 1
Verify O2 sensors and leaks first
1. Rule out a lazy O2 sensor or exhaust leak before condemning the converter.
- 2
Replace the converter
1. Confirm the converter is the fault. 2. Replace it with an OEM-grade unit, then clear the code.
Frequently asked questions
Is P0421 the same as P0420?
Both are catalyst-efficiency codes. P0421 refers specifically to the warm-up (close-coupled) converter on bank 1.
Last updated July 19, 2026
Guidance only — always consult a qualified professional or the official service manual before carrying out repairs.