AppointWhat does a smog check test, and why do cars fail?
A California smog check has three parts: a visual inspection of the emissions equipment, a functional check (check engine light, and the gas cap on some older cars), and either an OBD computer scan for gasoline cars from 2000 and newer or a tailpipe emissions test for older cars. The most common reasons to fail are a lit check engine light, readiness monitors that are not complete after a battery change or code clearing, and missing or modified emissions parts.
Key facts
- A lit check engine light is an automatic fail in California.
- Aftermarket parts that affect emissions need a CARB Executive Order number to be legal.
- Clearing trouble codes or disconnecting the battery resets readiness monitors and can make the car "not ready".
Common reasons for a fail
| Reason | What to do |
|---|---|
| Check engine light on | Have a repair shop diagnose and fix the cause first |
| Monitors "not ready" | Drive normally for several days so the car completes its drive cycle (readiness monitors explained) |
| Missing or modified parts | Reinstall the original part or one with a CARB Executive Order number |
| High tailpipe emissions (older cars) | Repair at a licensed smog repair station; ask about CAP assistance |
Next step
How to pass a smog checkSmog check and repair shops by city