# What 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](https://appointauto.com/guides/check-engine-light-smog-check/)) |
| 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 check](https://appointauto.com/guides/how-to-pass-smog-check/) · [Smog check and repair shops by city](https://appointauto.com/ca/)

Official sources: [bar.ca.gov](https://www.bar.ca.gov/).

## Related questions

- [Where can I get a smog check near me?](https://appointauto.com/answers/where-to-get-a-smog-check-near-me/)
- [How long does a smog check take?](https://appointauto.com/answers/how-long-does-a-smog-check-take/)
- [Do I need an appointment for a smog check, and are stations open on weekends?](https://appointauto.com/answers/smog-check-walk-in-or-appointment/)
- [How long is a California smog certificate valid?](https://appointauto.com/answers/how-long-is-a-smog-certificate-valid/)

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Source: https://appointauto.com/answers/what-does-a-smog-check-test/ · Last updated 2026-10-26. Station details are compiled from public listings and can change; call the shop to confirm hours and prices before you go.