Sometimes—but not everywhere. The allowed number and type of incomplete OBD readiness monitors depend on the vehicle and the current rules where it is tested.
Connect it to ScanWrench and see what your car actually returned.
The short answer changes by program
EPA inspection guidance gives programs a framework for deciding how many supported monitors may remain unset, but the inspection authority applies the current local rule. New York and Illinois, for example, publish allowances based on model year. California publishes a table that also distinguishes fuel type and which monitor is incomplete.
That is why a national yes-or-no answer would be misleading. One incomplete EVAP monitor on one gasoline vehicle may be treated differently from an incomplete catalyst monitor, a diesel particulate-filter monitor, or the same result in another state.
Check the current official inspection page for the exact place where the vehicle will be tested. Rules, exceptions, and implementation dates can change; an old forum post or a result from another state is not controlling evidence.
What “not ready” actually means
A readiness monitor is an onboard self-test. “Ready” means the supported monitor completed since the last reset or memory loss. “Not ready” means it has not completed under the required conditions. The emissions status for that monitor is still undetermined.
An incomplete monitor does not prove a failed part. It also does not prove the system is clean, repaired, or safe. The cause may be recent code clearing, battery power loss, recent repair, too little normal operation, enabling conditions that have not occurred, or a fault that prevents completion.
Keep unsupported separate from incomplete. A monitor the vehicle does not support should not be counted or described as a failed self-test. Record the exact status returned rather than turning every blank field into “not ready.”
- Observed fact: which monitors the vehicle reports as supported, complete, or incomplete.
- Plausible causes: recent reset, power loss, unmet enabling conditions, repair work, or a fault blocking completion.
- Still unknown: the emissions condition for an incomplete monitor and the official inspection outcome.
- Next proving test: verify the current local rule, then let the exact vehicle complete its manufacturer-defined self-test conditions safely.
Examples show why the exact monitor matters
New York currently says model-year 2001 and newer vehicles may pass with one monitor not ready, and it notes that vehicles older than 25 years do not receive emissions inspections there. Illinois still publishes rejection thresholds of more than two incomplete non-continuous monitors for 1996–2000 vehicles and more than one for 2001-and-newer vehicles; it also requires the catalyst monitor to be ready on a retest after a catalyst-code failure.
California's current OBD test reference is different: it lists one allowed incomplete monitor for 1996–1999 gasoline vehicles, only the EVAP monitor for 2000-and-newer gasoline vehicles, no incomplete monitors for 1998–2006 diesel vehicles, and specific DPF and NMHC allowances for 2007-and-newer diesel vehicles.
California also adopted updated readiness regulations effective October 1, 2025, while stating that current pass/fail criteria remain unchanged during an initial data-collection phase and that future changes will be announced. Use the live official table rather than trying to memorize a rule that may be phased later.
Do not clear codes to manufacture a clean screen
Clearing diagnostic memory or disconnecting the battery can reset readiness and remove useful freeze-frame and fault context. The dashboard may look quieter while the vehicle has completed fewer self-tests, leaving you with less evidence and a likely inspection rejection if too many monitors remain incomplete.
There is no trustworthy universal mileage, number of starts, or internet drive cycle that completes every monitor. Required conditions vary by vehicle and monitor and can include temperature, fuel level, speed, load, time, and cold-soak conditions.
Use the owner's manual or current manufacturer service information for the exact vehicle. Never attempt a drive pattern that conflicts with traffic law, weather, vehicle condition, warning-light guidance, or safe driving.
A five-step pre-inspection check
Scan before clearing anything. Record supported readiness monitors, which ones are complete, stored/pending/permanent emissions-code channels, commanded check-engine-light status, and whether communication completed normally.
Name the incomplete monitor. Confirm the exact vehicle's model year, fuel type, and current registration or inspection jurisdiction. Then read the official program's live requirement and any vehicle-specific exception.
If the monitor remains incomplete after appropriate normal operation, use current vehicle-specific diagnostic information to find out why. A qualified inspection or repair professional may be the next step; repeated guessing does not turn an unknown result into proof.
- 1. Save the scan without clearing.
- 2. Identify the exact supported monitor that is incomplete.
- 3. Check the current official rule for the exact vehicle and location.
- 4. Follow only safe, vehicle-specific completion guidance.
- 5. Recheck the monitor and fault record before entering the lane.
Can one monitor be not ready and still pass emissions?
Sometimes. Several programs allow one incomplete monitor on certain vehicles, but the allowance can change by model year, fuel type, monitor, jurisdiction, and current rule. Check the official program for the exact vehicle and location.
Does not ready mean the monitor failed?
No. It means the supported self-test has not completed. The emissions status for that monitor is undetermined until it runs; the status does not prove a failed part or a clean system.
Which readiness monitor can be incomplete?
There is no universal answer. Some programs count only the number of incomplete monitors; others name specific allowed monitors. California, for example, distinguishes gasoline and diesel vehicles and the monitor identity. Use the current official local table.
How long does it take a monitor to become ready?
There is no universal mileage or number of trips. Completion depends on the exact vehicle, monitor, temperature, fuel level, load, speed, time, recent reset, and whether a fault prevents the test from finishing.
Should I clear codes before an emissions test?
No. Clearing can erase useful evidence and reset readiness without repairing the cause. Save the record first, complete the diagnosis or repair, and then verify readiness and current local requirements.
THE TAKEAWAYOne incomplete readiness monitor may be accepted, but the exact monitor, vehicle, fuel type, location, and current rule control the inspection. “Not ready” means untested—not passed, failed, or safe. Save the evidence, verify the official rule, and do not clear and guess.
Know what finished—and what did not.
With a compatible adapter, ScanWrench can record the standardized readiness status the vehicle returns. The official inspection program—not the app—decides whether that exact vehicle passes.
Check readiness before the inspectionStandard OBD readiness · No pass guaranteeScanWrench Editorial created this issue from the platform's evidence model and current primary sources. It separates observed facts, plausible paths, decisive tests, safety limits, and remaining unknowns. Its purpose is to improve vehicle decisions; it does not replace exact manufacturer procedures or qualified professional judgment.
See how this page earns trust.
These links are primary-source starting points for the page's scope. They do not replace the current manufacturer procedure, wiring, specification, or service information for a specific vehicle.
Publisher
ScanWrench Editorial is the working byline for this library. Joshua Black, founder and publisher, is responsible for publication decisions, disclosures, and corrections.
A reviewed date means an editorial scope and safety check. It is not an ASE credential, OEM authorization, or vehicle-specific repair approval.
Primary sources
- Vehicle Emissions Inspection and Maintenance (I/M): Policy and Technical GuidanceU.S. Environmental Protection Agency · Current EPA hub for vehicle inspection and maintenance policy, OBD inspection guidance, readiness exceptions, and program implementation materials.
- Performing Onboard Diagnostic System Checks as Part of a Vehicle Inspection and Maintenance ProgramU.S. Environmental Protection Agency · Official separation of physical lamp state, commanded MIL state, diagnostic codes, readiness, communication, and inspection outcomes.
- What do you mean my car's not ready?New York State Department of Motor Vehicles · Current New York owner guidance for readiness-monitor allowances, vehicle-specific drive cycles, and readiness-only inspection rejection.
- Illinois Air Team frequently asked questionsIllinois Environmental Protection Agency · Current Illinois program guidance for readiness rejection thresholds, catalyst-monitor retests, and the effect of clearing controller memory.
- On-Board Diagnostic Test ReferenceCalifornia Bureau of Automotive Repair · Official readiness, permanent-code, communication, and inspection-test context for California.
- New OBD readiness monitor regulations explainedCalifornia Bureau of Automotive Repair · Current California notice explaining that readiness-rule changes are phased, that present pass/fail criteria remain in place while data is collected, and that future changes will be announced.
Revision history
- Initial publication and review: claim boundaries, safety language, internal links, and cited source scope checked.
No material correction is hidden. Material additions and corrections are dated above when they occur.
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