OBD2 Not Ready Smog Check: Why It Fails and How to Fix It

Quick answer: An OBD2 not ready smog check result means one or more readiness monitors, the car’s own emissions self-tests, haven’t finished since the memory was last cleared. Clearing codes, a battery disconnect or a recent repair are the usual triggers. Drive normally for about a week, then retest. If a monitor still won’t set, a faulty part may be blocking it.

“Not ready” isn’t the same as “broken.” Usually the car simply hasn’t had a chance to check itself yet.

Sometimes, though, it is a symptom. I’ll show you how to tell the two apart, because the fix for one (patience and a few highway miles) is nothing like the fix for the other (finding the failed part that keeps the self-test from ever running).

What Readiness Monitors Are

California’s Bureau of Automotive Repair (BAR) describes readiness monitors as self-tests the OBD system runs to verify that emission controls work. Typical ones cover the catalytic converter, evaporative (EVAP) system, oxygen sensors and their heaters, and EGR on cars that have it.

Each monitor reports Ready (complete) or Not Ready (incomplete). Simple enough. Inspection equipment reads that status through the diagnostic port, and you can see it yourself on most scanners under I/M Readiness.

Why It Happens

Most likely first:

1. Codes were cleared. Erasing codes with a scanner resets every monitor to Not Ready. Our guide on whether an OBD2 scanner can clear codes covers this in detail.
2. The battery was disconnected, died or was replaced. The California Air Resources Board (CARB) lists a dead, disconnected or replaced battery as a common reason for incomplete readiness.
3. A repair was just done. BAR says monitors must rerun after work like replacing a defective emissions component.
4. Your driving doesn’t match the test conditions. BAR notes some monitors need unusual operating conditions: specific speed, temperature, pressure and engine load. Short city hops may not.
5. A faulty part is blocking the monitor. BAR is clear that a vehicle may never complete a monitor until the components it depends on are diagnosed and repaired.

My take: if you recently cleared codes or touched the battery, cause 1 or 2 is almost certainly yours. Neither? Then think hard about cause 5.

How to Fix an OBD2 Not Ready Smog Check Failure

If Codes Were Cleared or the Battery Was Disconnected

Just drive. CARB says owners can usually set the indicators to complete by driving normally for about a week.

In practice, I’d mix it up during that week: a cold start after the car has sat overnight, some steady highway cruising, normal stop-and-go. Then check I/M Readiness with a scanner before booking the retest.

Don’t clear codes again “to start fresh.” That resets the clock. Every time.

If a Monitor Still Won’t Set

Now you need a plan. Here’s mine:

• Look for a specific drive cycle. BAR says the owner’s manual occasionally includes one, but usually it’s only in factory or repair-industry service information.
• Ask a repair shop. CARB says a technician can advise you on driving patterns or run the monitors on a dynamometer.
• Check for codes, including pending ones. A fault that hasn’t triggered the light yet can still stop a monitor from running.
• Fix the underlying problem. If a component is failing, the monitor won’t complete no matter how much you drive.

When I look at the table below, the EVAP exception jumps out. California’s current rules still let that one monitor stay incomplete on 2000 and newer gas cars.

What California Allows Right Now

BAR adopted new readiness rules effective October 1, 2025. On paper, they require all monitors to be set to pass. But BAR says the current pass/fail criteria stay in place while it collects data, and that changes will come gradually with advance notice.

Under the current criteria, BAR allows:

Vehicle	Unset monitors allowed
Gasoline, 1996 to 1999	Any one
Gasoline, 2000 and newer	EVAP only
Diesel, 1998 to 2006	None
Diesel, 2007 and newer	Only the DPF and NMHC catalyst monitors

BAR’s OBD test reference adds two details I’d want to know before paying for a retest:

1. The continuous monitors (misfire, fuel system and comprehensive components) are ignored during the OBD test of gasoline vehicles.
2. On 2010 and newer vehicles, a stored permanent trouble code fails the test unless the car has completed at least 15 warm-up cycles and 200 miles since its codes were last cleared.

Outside California, your state’s inspection program sets its own rules, so check its website before you go.

When to Get Help

Get a professional involved if:

• The check engine light is on. CARB says you need the malfunctioning part repaired before the car can pass.
• A monitor won’t complete after a full week or two of mixed driving.
• The same monitor keeps failing after repairs.

In California, BAR says the Smog Check Referee can help motorists who struggle to meet the readiness requirements.

If your car battery has been weak and repeatedly dying, fix that first, because every dead-battery event can reset readiness. Our explainer on whether a car battery is 12 volts shows how to check its state of charge. More guides are in Diagnostics & Engine.

Frequently Asked Questions

How Long Does It Take for Readiness Monitors to Reset?

CARB says normal driving for about a week usually does it. Some monitors need specific conditions, per BAR, so they can take longer.

Can I Pass Smog With an EVAP Monitor Not Ready?

In California, a 2000 or newer gasoline car can currently pass with only the EVAP monitor incomplete, per BAR. Older 1996 to 1999 gas cars can have any one monitor incomplete.

Does Disconnecting the Battery Reset Readiness Monitors?

Yes. CARB says a dead, disconnected or replaced battery can leave readiness indicators incomplete, which is why a fresh battery right before a test can backfire.

Will a Smog Shop Pass My Car If Monitors Aren’t Ready?

Not if more monitors are incomplete than the rules allow for your model year. Stations must follow BAR’s criteria. The test software applies them.

Last updated: November 10, 2026. Sources: BAR: New OBD readiness monitor regulations explained (Fall 2025), BAR: On-Board Diagnostic Test Reference (PDF) and CARB OBD II fact sheet.