Short answer: Emissions readiness monitors are self-tests your vehicle runs to verify pollution control systems are working. Most states require monitors to show “Ready” or “Complete” before you can pass a smog check. Go to the Emissions tab in OBDAI to see your monitor status.
What Are Readiness Monitors?
Your vehicle’s on-board diagnostic (OBD-II) system continuously monitors emissions-related components to ensure they’re functioning properly. These self-tests are called “readiness monitors” or “I/M monitors” (Inspection/Maintenance monitors).
When a monitor completes its test successfully and finds no problems, it sets to “Ready” or “Complete.” If the test hasn’t run yet, the monitor shows “Not Ready” or “Incomplete.”
Emissions testing programs use monitor status to determine if your vehicle has been properly evaluated. A vehicle with several “Not Ready” monitors hasn’t proven its emissions systems are working — the testing station can’t verify compliance.
How to Check Your Monitor Status
Connect to your vehicle. Ensure OBDAI is connected and communicating.
Go to the Emissions tab. Bottom navigation bar.
View your monitor status. OBDAI displays each monitor with its current status:
- PASSED — Monitor completed, no faults found
- NOT READY — Monitor hasn’t completed its test yet
- NOT SUPPORTED — Your vehicle doesn’t have this monitor
The Emissions tab also shows:
- Number of DTCs — How many trouble codes are stored
- MIL Status — Whether the check engine light is commanded ON or OFF
- Engine Type — Gasoline or Diesel (determines which monitors apply)
How Many Monitors Must Be Ready?
This is where it gets important. Requirements vary by state, model year, and fuel type. Here’s what you need to know:
Federal EPA Guidelines
The EPA has established baseline requirements that most states follow:
| Model Year | Fuel Type | Incomplete Monitors Allowed |
|---|---|---|
| 1996-2000 | Gasoline | Up to 2 monitors can be Not Ready |
| 2001 and newer | Gasoline | Up to 1 monitor can be Not Ready |
| 1998 and newer | Diesel | Varies by state |
California (Strictest Standards)
California has stricter requirements and recently tightened them further:
| Model Year | Fuel Type | Current Rule (As of Oct 2025) |
|---|---|---|
| 1996-1999 | Gasoline | 1 monitor can be incomplete |
| 2000 and newer | Gasoline | ONLY EVAP can be incomplete; all others must be Ready |
| 1998-2006 | Diesel | Zero incomplete monitors allowed |
| 2007 and newer | Diesel | PM Filter and NMHC Catalyst can be incomplete |
Important October 2025 California Change: New regulations effective October 1, 2025 require that ALL readiness monitors must be set for a vehicle to pass inspection. The California Bureau of Automotive Repair (BAR) is implementing this gradually and will grant exceptions for vehicles with known design issues. The current criteria shown in the table above will remain in effect while BAR collects data — updates will only be made where data shows monitors can reasonably be expected to complete.
Other States
Most states follow EPA guidelines, but requirements can vary. Check with your state’s emissions testing program for specific rules. Some states:
- Allow 2 incomplete monitors for 1996-1999 vehicles
- Allow 1 incomplete monitor for 2001+ vehicles
- May have specific rules about which monitors can be incomplete
Key Point: If your check engine light is ON, you will fail regardless of monitor status. The MIL must be OFF to pass.
The Monitors Explained
Different vehicles have different monitors depending on the engine type and emissions equipment installed. There are two categories: continuous monitors that run all the time, and non-continuous monitors that require specific driving conditions.
Continuous Monitors (All Vehicles)
These run constantly during vehicle operation:
Misfire Monitor
- Tests for engine misfires — when a cylinder fails to fire properly
- Runs almost continuously during normal driving
- Misfires waste fuel and increase emissions
Fuel System Monitor
- Tests fuel control — whether the engine is running rich or lean
- Runs during closed-loop fuel control after the engine warms up
- Problems cause increased emissions and poor fuel economy
Comprehensive Component Monitor (CCM)
- Tests sensors and actuators for electrical faults
- Includes throttle position, MAP sensor, and many emissions-related components
- Runs continuously during operation
Non-Continuous Monitors — Gasoline Vehicles
These monitors only run when specific driving conditions are met:
Catalyst Monitor (CAT)
- Tests catalytic converter efficiency
- Compares upstream and downstream oxygen sensor signals
- Requires extended steady-speed driving (55-65 mph for 5-10+ minutes)
- Often the hardest monitor to set
Heated Catalyst Monitor (HCAT)
- Tests heated catalytic converters (if equipped)
- Not present on all vehicles
Evaporative System Monitor (EVAP)
- Tests the fuel vapor recovery system for leaks
- Checks gas cap seal, charcoal canister, purge valve, and vent valve
- One of the most finicky monitors — requires:
- Fuel tank between 15% and 85% full
- May need vehicle to sit overnight
- Specific temperature range (40-100°F)
Secondary Air System Monitor (AIR)
- Tests air injection system (if equipped)
- Note: Most modern vehicles don’t have this system. It was common in early 2000s through mid-2010s on some Toyota, GM, and VW/Audi vehicles. If your vehicle shows “NOT SUPPORTED,” that’s normal.
Oxygen Sensor Monitor (O2S)
- Tests oxygen sensors for proper operation
- Verifies response time and switching frequency
Oxygen Sensor Heater Monitor (HTR)
- Tests oxygen sensor heating elements
- Runs within first few minutes of driving
EGR/VVT System Monitor (EGR)
- Tests Exhaust Gas Recirculation valve and flow
- Requires specific driving conditions with sustained moderate load
Non-Continuous Monitors — Diesel Vehicles
Diesel vehicles have different monitors for their unique emissions equipment:
NMHC Catalyst Monitor
- Tests the diesel oxidation catalyst
NOx/SCR Monitor
- Tests the Selective Catalytic Reduction system
- Monitors diesel exhaust fluid (DEF) injection
Boost Pressure Monitor
- Tests turbocharger boost pressure control
Exhaust Gas Sensor Monitor
- Tests exhaust gas temperature and pressure sensors
PM Filter (DPF) Monitor
- Tests Diesel Particulate Filter for soot loading and regeneration
EGR/VVT System Monitor
- Tests exhaust gas recirculation on diesels
Why Monitors Reset to “Not Ready”
Monitors reset to “Not Ready” when:
- You clear trouble codes — The clear command resets all monitors
- The battery is disconnected — Loss of power erases monitor status
- An ECU reset occurs — Reprogramming or certain repairs reset monitors
After any reset, you must drive the vehicle through specific conditions to run each monitor and set it back to “Ready.”
How to Get Monitors to Set
The best way: Ask ARIA. Tap the “Emissions Test” chip or ask ARIA to help you get ready for your smog check. ARIA knows your specific vehicle and can figure out the exact drive cycle pattern based on your year, make, and model. She’ll walk you through the steps — no guesswork.
If you want to try it yourself, here are general guidelines:
General Drive Cycle Guidelines
A “drive cycle” enables monitors to run. While every manufacturer has different requirements, here’s a general approach:
Before you start:
- Fuel tank between 1/4 and 3/4 full (especially for EVAP)
- No trouble codes stored
- Vehicle cold (sat overnight if possible)
The drive cycle:
Cold start — Start the engine cold, idle for 2–3 minutes
Accelerate to 55 mph — Moderate acceleration, not wide-open throttle
Steady cruise at 55–60 mph — Maintain for 5–10 minutes without touching brakes. Critical for catalyst monitor.
Coast down — Take foot off gas, decelerate without braking
City driving — Stop-and-go traffic for 15–20 minutes with several starts from complete stops
Highway driving — Return to 55–65 mph steady cruise for 5 minutes
Idle for 3 minutes — Park and let engine idle
Turn off and sit — For EVAP, vehicle may need to sit several hours
Tips:
- Keep RPM between 1,500–3,000 during steady cruise
- Avoid heavy acceleration or hard braking
- Don’t use cruise control — slight throttle variations help some monitors
Stubborn Monitors
Catalyst Monitor:
- Extended steady-speed driving (55–65 mph for 10+ minutes)
- Avoid hills and traffic interrupting steady speed
EVAP Monitor:
- Fuel level 15–85%
- May require overnight cold soak
- Gas cap must be tight
- Temperature-sensitive (40–100°F)
EGR Monitor:
- Sustained moderate load
- Highway merging and hill climbing help
What OBDAI Shows You
OBDAI’s Emissions tab provides:
Summary Section:
- Number of DTCs stored
- MIL Status (check engine light ON or OFF)
- Engine Type (Gasoline or Diesel)
Readiness Status Section: Lists each monitor your vehicle supports with current status:
- PASSED — Ready, test completed with no faults
- NOT READY — Monitor hasn’t completed yet
- NOT SUPPORTED — Your vehicle doesn’t have this monitor
Before Your Smog Test
Checklist:
- No check engine light (MIL OFF)
- Required monitors show PASSED or NOT SUPPORTED
- No pending or confirmed trouble codes
- Vehicle has been driven recently (not just started)
If you recently had repairs or cleared codes, drive for several days through varied conditions before the emissions test. Don’t wait until your appointment day to check monitor status — give yourself at least a week.
Permanent Codes and Smog Testing
Some states check for permanent codes (Mode $0A) in addition to regular trouble codes. Permanent codes cannot be cleared with a scan tool — they only clear after the vehicle verifies the problem is fixed.
If you have permanent codes:
- The underlying problem must be repaired
- The vehicle must complete drive cycles after repair
- The PCM must verify the system is working correctly
- Only then will the permanent code automatically erase
This can take several drive cycles even after successful repair.
Why “Secondary Air System” Shows “Not Supported”
If OBDAI shows the Secondary Air System monitor as “NOT SUPPORTED,” that’s completely normal for most modern vehicles.
Secondary air injection was introduced in 1966 to inject fresh air into the exhaust stream, helping the catalytic converter reach operating temperature faster during cold starts. It was most common on:
- Toyota trucks and SUVs (2005-2015: Tacoma, Tundra, 4Runner, Sequoia)
- GM trucks and SUVs (2004-2010: Silverado, Tahoe, TrailBlazer)
- Some VW/Audi vehicles
- Certain BMW and Mercedes models
Advances in fuel injection, catalyst design, and engine management made this system unnecessary on most modern vehicles. If your vehicle shows “NOT SUPPORTED,” it simply means your vehicle doesn’t have this system — and that’s expected.
Hardware Required
Checking emissions readiness requires an OBD-II adapter connected to your vehicle. OBDAI hardware provides reliable Bluetooth connections across all platforms.
Related Articles
- Getting Started with OBDAI
- How to Read and Clear Trouble Codes
- What Does Each Dashboard Gauge Mean?
- What OBDAI Can and Cannot Diagnose
Questions? support@obdai.app