P0420 – Catalyst System Efficiency Below Threshold (Bank 1)

System: Emissions - catalyst
Dashboard light: Check engine light (MIL)
Check-engine blink code: —
Applies to: Unverified for 1st-gen Passo/Boon; 1KR-FE (toyota-club.net list covers all 1KR-FE variants; 1st-gen KGC10/15 fitment not individually confirmed)
Description: Catalyst System Efficiency Below Threshold (Bank 1)
Detection / symptoms: Usually MIL only
Possible causes: Aged catalyst, lazy rear O2 sensor, exhaust leak before rear sensor, prior misfire
What to check: Check exhaust leaks, rear O2 live data, then catalyst
Confidence: low - NOT in Daihatsu M300 factory chart (B8-52..55); likely later-gen/Toyota-built 1KR only

From the factory service material. Clear the code and test-drive to confirm it returns. Search all fault codes · Common problems · Warning lights