Failed Emissions / Smog Test – What Codes Mean

Emissions failures are frequently tied to catalyst efficiency (P0420/P0430), oxygen sensor heater or response codes, and incomplete readiness monitors after a recent battery disconnect or repair.

What To Do Next

Fix stored codes first, then complete a proper drive cycle so readiness monitors set. A good OBD2 scanner shows monitor status.

Practical Tips

Do not clear codes right before a test — monitors need time to set.

P0420 does not always mean a new catalytic converter; check for exhaust leaks and upstream issues first.

Gas cap codes are cheap fixes that fail many tests.

Common Related Codes

These codes frequently appear with this symptom. Click for causes, cost estimates and repair steps.

Code	Description	Severity
P0420	Catalyst System Efficiency Below Threshold (Bank 1)	Med
P0455	Evaporative Emission System Leak Detected (Large Leak)	Low
P0442	Evaporative Emission System Leak Detected (Small Leak)	Low
P0430	Catalyst System Efficiency Below Threshold (Bank 2)	Med
P0135	O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1)	Med
P0141	O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 2)	Low

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