P0420 (generic)
Catalyst System Efficiency Below Threshold (Bank 1)
System: Powertrain · Catalyst System
Severity: moderate · Inspection: often fails
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 P0420
- Difficulty
- 6/10 — Intermediate diagnostics
- Est. repair cost
- $40 – $450 (parts + typical shop labor)
- Common symptoms
- Failed emissions test
- Check engine light (MIL)
- Poor fuel economy
Diagnostic workflow
- Confirm P0420 with a live scan — note pending vs stored and freeze frame data.
- Verify reported symptoms: Failed emissions test, Check engine light (MIL), Poor fuel economy.
- Inspect wiring/connectors and related sensors before replacing modules.
- Most likely fixes: Replace catalytic converter(s); Replace oxygen sensor(s); Cylinder-specific misfire diagnosis (swap / compression / injector).
- Clear codes and road-test; re-scan after two drive cycles if the monitor must set.
Topics
- OBD-II and scan tools (guide)
- Powertrain DTCs (P-prefix) (system)
Related symptoms
- Failed emissions test (weight 0.920)
- Check engine light (MIL) (weight 0.880)
- Poor fuel economy (weight 0.580)
Common fixes (curated)
- Replace catalytic converter(s) (rank 1)
- Replace oxygen sensor(s) (rank 2)
- Cylinder-specific misfire diagnosis (swap / compression / injector) (rank 3)
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Diagnostic bundle (JSON)
Machine-readable graph: related codes, symptoms, fixes, topics, articles, and curated sources rows. Append &external=1 for optional YouTube / Google CSE hits when API keys are configured (see web/.env.example).