Check Engine Light and Readiness Monitors: Why Your Car Is "Not Ready" for Smog
A California smog check of a 1996-or-newer car reads the on-board diagnostics (OBD II) system. The car fails if the check engine light is on, or if too many readiness monitors are incomplete. Monitors reset when the battery is disconnected or fault codes are cleared, and they complete again only through normal driving, usually over a few days of mixed city and highway trips.
What the smog station reads
| Item | Pass condition |
|---|---|
| Malfunction indicator lamp (check engine light) | Works when the key is on, and turns off with the engine running |
| Stored or pending fault codes | No code that commands the light on |
| Readiness monitors | All complete, or within the small number BAR allows |
| Software and VIN data | Matches the vehicle; no unapproved reprogramming |
Why monitors show "not ready"
- The 12-volt battery was disconnected or replaced.
- Codes were cleared with a scanner, often right after a repair.
- The car has only been driven on very short trips.
- A real fault is stopping a monitor from completing.
How to complete a drive cycle
Exact steps vary by manufacturer and are in some service manuals, but this pattern works for most cars:
- Let the car sit overnight so the engine starts fully cold.
- Idle for two to three minutes, then drive in town with gentle stops and starts for about 10 minutes.
- Drive at a steady highway speed (55 to 60 mph) for 10 to 15 minutes, then coast down without braking hard.
- Repeat over two to five days with the fuel tank between one-quarter and three-quarters full.
- Check monitor status with a scanner, or ask a shop, before booking the smog check.
If a monitor never completes, there is usually an underlying fault. A repair shop can diagnose it before you spend money on repeated tests.
Find a smog station
- Smog check in San Jose — 2 listings
- Smog check in Anaheim — 1 listing
- Smog check in Los Angeles — 1 listing
- Smog check in Sacramento — 1 listing
- Smog check in San Diego — 1 listing
- Smog check in San Francisco — 1 listing
- Smog check in Santa Clara — 1 listing
- Smog check in Sunnyvale — 1 listing
Frequently asked questions
How many readiness monitors can be incomplete?
The Bureau of Automotive Repair allows only a small number. For most gasoline vehicles from model year 2000 and newer, one incomplete monitor is allowed; older OBD II vehicles are allowed slightly more. More than that is a fail until the car is driven further.
How long does it take for readiness monitors to reset?
Usually 50 to 200 miles of varied driving over several days, including steady highway speed, stop-and-go traffic and overnight cool-downs. Some monitors, such as the evaporative system, need specific conditions like a fuel tank between one-quarter and three-quarters full.
Can I pass smog with the check engine light flashing?
No. A flashing light means an active misfire that can damage the catalytic converter. Stop driving hard and get it repaired.
Will a parts store code reader tell me if I will pass?
Many inexpensive OBD II scanners show the fault codes and monitor status. If there are no codes and the monitors are complete, the OBD part of the test is likely to pass, but the station still runs its own visual and functional checks.
Related guides
- How to Pass a California Smog Check: 10 Checks Before You Go — A practical pre-test checklist: check engine light, readiness monitors, gas cap, oil change, warm engine and the mistakes that make good cars fail.
- Failed a Smog Check in California? What to Do Next — Why cars fail the California smog check, how to read the Vehicle Inspection Report, where to get repairs, how retests work and where to get help paying.
- Which Cars Are Exempt From the California Smog Check? (2026) — The 8-model-year rule, electric vehicles, motorcycles, pre-1976 cars, diesels and the smog abatement fee: who needs a smog check in California and who does not.