You finally cleared that annoying check engine light, drove around for a few days, and headed straight to the SMOG station — only to be told your car failed because the readiness monitors were not set. It is one of the most frustrating and avoidable SMOG failures in California. Understanding how readiness monitors work, and how often you should check them, can save you time, money, and a second trip to the smog station.
What Are OBD2 Readiness Monitors?
Every vehicle built after 1995 uses an On-Board Diagnostics II (OBD2) system that continuously runs self-tests on your engine and emissions components. These self-tests are called readiness monitors. They check systems like the catalytic converter, oxygen sensors, evaporative emissions (EVAP), exhaust gas recirculation (EGR), and several others depending on your vehicle's make and model.
When your battery is disconnected, when a fault code is cleared with a scan tool, or when certain repair work is performed, these monitors reset to an "incomplete" or "not ready" state. California's SMOG inspection includes a check of these monitors, and too many incomplete monitors will result in an automatic failure — even if your check engine light is off and the car runs perfectly.
How Many Incomplete Monitors Are Allowed?
California has strict limits on how many incomplete monitors are acceptable during a SMOG inspection. The rules are based on your vehicle's model year:
- 1996 to 1999 model years: Up to three incomplete monitors are allowed.
- 2000 and newer model years: Only one incomplete monitor is allowed.
The EVAP monitor is the one that takes longest to complete for most drivers, and it is also the most commonly incomplete monitor at inspection time. If your 2005 vehicle shows two incomplete monitors, it will fail the SMOG test regardless of everything else checking out fine. This is why checking your monitors before you book an appointment matters so much.
When Should You Check Your Readiness Monitors?
You should check your readiness monitors in three specific situations. First, check them after any battery replacement or disconnection. Modern vehicles lose their monitor completion status the moment battery power is cut. Second, check them after any diagnostic trouble code (DTC) has been cleared, whether by a shop, a dealership, or your own OBD2 scanner. Third, check them after major repairs to emissions-related components such as the catalytic converter, oxygen sensors, or EVAP system.
Outside of those specific situations, most vehicles that are driven regularly and have no underlying problems will keep their monitors in a "complete" or "ready" state on their own. You do not need to check them every week if nothing has changed. However, checking once or twice a year — especially a week or two before your registration renewal comes due — is a smart habit that costs nothing if you own an inexpensive Bluetooth OBD2 reader.
How to Check Your Monitors at Home
Checking your readiness monitors does not require a professional. A basic OBD2 scan tool plugged into the port under your dashboard (usually near the steering column) will display monitor status within seconds. Many free and low-cost apps pair with Bluetooth or Wi-Fi OBD2 adapters and show you which monitors are complete and which are still incomplete. Look for terms like "Ready," "Complete," or "N/A" as passing states. "Incomplete," "Not Ready," or "Not Supported" tell you more driving is needed.
For a deeper explanation of how to read the data your scanner returns, check out our guide on reading OBD2 codes before your smog check.
How to Complete Your Drive Cycle
Once monitors are reset, you need to complete a drive cycle to bring them back to a ready state. A full drive cycle typically involves a mix of city and highway driving over several days. The exact sequence varies by vehicle, but in general you should:
- Allow the engine to cold-start after sitting overnight.
- Drive at speeds between 25 and 55 mph for at least 10 to 15 minutes without extended idling.
- Include a short highway segment at 55 to 60 mph for five minutes or more.
- Allow the engine to fully warm up before stopping.
- Repeat this cycle over two to three days if monitors are still incomplete.
Some manufacturers publish specific drive cycle procedures for their vehicles. Searching for your year, make, and model alongside "drive cycle procedure" often turns up the exact sequence recommended by the manufacturer.
Check Your Monitors Before You Book a SMOG Appointment
The bottom line is simple: always verify your monitors are in a ready state before scheduling your California SMOG inspection. A quick check at home with a $20 scanner can prevent a wasted trip and an unnecessary failure fee. Give yourself at least a week of normal driving after any battery or repair work before you head in. If you want to confirm your monitors are ready before the official test, most STAR-certified stations will walk you through what they see on the OBD2 system when you arrive.
Drivers in Concord can use the $49.75 coupon at Super Station Concord — see $49.75 coupon or book your smog check in Concord.
For a complete overview of what to expect during your inspection, visit our California SMOG check guide.
Official sources
California Bureau of Automotive Repair: SMOG Check Inspections — official program rules, station types, and consumer information from BAR.
California DMV: SMOG Certification Requirements — registration renewal requirements and SMOG exemption information from the DMV.