What readiness monitors are and why your car tracks them

Readiness monitors are tests your car's engine computer runs continuously to check whether emissions systems are working correctly. Your vehicle has eight main monitors — oxygen sensor, catalyst, evaporative system, secondary air system, EGR system, heated catalyst, comprehensive component, and fuel system — and each one reports pass or fail to the onboard diagnostic system.

When you take your car for an emissions test, the testing station checks whether these monitors show "ready" or "not ready." A monitor marked "not ready" means the computer hasn't finished its test cycle yet, usually because you haven't driven the car enough since the last time the diagnostic trouble codes were cleared. Some states require all monitors to be ready before you can pass inspection; others allow one or two to remain not ready.

The Foxwell NT520 is a mid-range diagnostic scanner that reads these monitor statuses directly from your vehicle's computer. It costs between $200 and $300 and connects to your car's OBD-II port — a standardized connector usually located under the steering wheel or dashboard on the driver's side.

Key Takeaways

  • The Foxwell NT520 connects to your car's OBD-II port and displays which readiness monitors are ready and which are not ready in real time.
  • You navigate to the vehicle's make and model, then select the emissions or readiness monitor function to see the current status of all eight monitors.
  • A monitor showing "not ready" typically means your car needs more driving time to complete that particular emissions test, not that something is broken.
  • Different states have different rules about how many monitors can be not ready during an emissions inspection, so check your local requirements before scheduling a test.

Locating the OBD-II port and connecting the scanner

The OBD-II port is a 16-pin rectangular connector, usually black or gray, located beneath the steering wheel on the driver's side. On some vehicles it may be mounted on the passenger side or even under the hood near the engine. Check your owner's manual if you cannot find it when ready — the manual will have a diagram showing the exact location for your model year.

Turn off the engine completely before plugging in the Foxwell NT520. Insert the scanner's connector cable into the OBD-II port until you hear or feel a click. The scanner will power on automatically once it detects the connection. Do not force the connector; if it does not slide in smoothly, check the angle and try again.

Once connected, the NT520 screen will display a welcome menu. The scanner draws power from the vehicle's battery through the OBD-II port, so you do not need to start the engine to use it, though some functions work better with the engine running.

Navigating to the readiness monitor screen

From the main menu on the NT520 screen, select Vehicle Selection or Select Vehicle. The scanner will prompt you to choose your vehicle's make (Ford, Honda, Toyota, etc.), then model year, then specific model. This step is necessary because different manufacturers organize diagnostic data differently, and the scanner needs to know which format to expect.

Once you have selected your vehicle, the main diagnostic menu will appear. Look for an option labeled Emissions, Readiness Status, Monitor Status, or I/M Readiness — different Foxwell firmware versions use slightly different names, but all refer to the same function. Select this option.

The scanner will then display a list of all eight monitors with their current status next to each one. Each monitor will show either Ready, Not Ready, or occasionally Incomplete. Some scanners also show a percentage of completion for monitors that are partway through their test cycle.

Understanding what each monitor status means

A monitor showing Ready means the computer has completed its test for that system and found no faults. This is the status you want to see before taking your car for an emissions inspection. Ready monitors stay ready until the diagnostic trouble codes are cleared — either by a technician, by disconnecting the battery for several minutes, or by the scanner itself if it has a clear codes function.

Not Ready means the computer has not yet run its test for that system, or the test is still in progress. This is normal after a code has been cleared or after a battery has been disconnected. The computer will run these tests automatically as you drive, but different monitors have different requirements. The oxygen sensor monitor typically completes within 50 to 100 miles of mixed city and highway driving. The catalyst monitor may take 200 miles or more. The evaporative system monitor sometimes requires a full fuel tank and specific driving conditions.

If a monitor shows Not Ready weeks after you cleared codes or replaced a battery, and you have driven the car normally, the system may have a genuine fault that prevents the test from running. In that case, the scanner should also show a pending or stored diagnostic trouble code that explains what is wrong.

What to do if monitors show not ready before an emissions test

Check your state's emissions testing rules first. Some states require all monitors to be ready; others allow one or two to remain not ready. Your state's Department of Environmental Quality or Department of Motor Vehicles website will list the specific requirement. If your state allows not-ready monitors, you may be able to schedule your test when ready.

If your state requires all monitors to be ready, you will need to drive the car to complete the remaining test cycles. Drive a mix of city streets and highway at varying speeds and engine loads. Avoid short trips; the computer needs sustained driving time to run its tests. Plan for at least 100 to 300 miles of normal driving, depending on which monitors are not ready. Check the monitor status again with the NT520 after a few days of driving to see whether they have moved to ready.

If monitors remain not ready after 300 miles of normal driving, or if the scanner shows a diagnostic trouble code, the vehicle likely has a fault that needs repair before the emissions test. A code like P0420 (catalyst system efficiency below threshold) or P0133 (oxygen sensor slow response) means the system has detected a real problem, not just an incomplete test cycle.

Common reasons monitors stay not ready

The most common reason is insufficient driving time since the codes were last cleared. Each monitor has its own test schedule, and some require specific conditions. The secondary air system monitor, for example, only runs during cold starts and warm-up, so it may take several days of morning drives to complete. The EGR monitor requires highway driving at steady speeds.

A disconnected or recently replaced battery will set all monitors to not ready. The computer needs to relearn the vehicle's baseline emissions data before it can run its tests. This process is called a drive cycle and typically takes 100 to 500 miles depending on the vehicle and which monitors need to complete.

A loose or missing gas cap will trigger the evaporative system monitor to fail or stay not ready. Check that your fuel door closes properly and the cap seals tightly. A faulty oxygen sensor, catalytic converter, or EGR valve will prevent its corresponding monitor from reaching ready status. In these cases, the scanner will show a diagnostic trouble code alongside the not-ready status.

Frequently Asked Questions

Can I clear the codes myself with the NT520 to reset the monitors?

Yes, the Foxwell NT520 has a clear codes function in its diagnostic menu. Clearing codes will set all monitors back to not ready, and you will need to drive the vehicle again to complete the test cycles. Only clear codes if you are intentionally starting a fresh drive cycle — for example, after replacing a faulty oxygen sensor or fixing a gas cap leak.

What if only one or two monitors show not ready?

Continue driving normally for another 50 to 200 miles and check again. Most remaining monitors complete within a few days of regular driving. If they still show not ready after a week of normal use, note which specific monitors are stuck and research that system's test requirements — some monitors need highway driving, others need cold starts, and a few require specific fuel tank levels.

Do I need to start the engine to check monitor status?

No, the scanner can read monitor status with the engine off because it draws power from the battery through the OBD-II port. However, some vehicles display more detailed information or allow the scanner to run live data when the engine is running, so turning it on does not hurt.

Will the NT520 tell me if a monitor will never reach ready?

Not directly, but if a monitor stays not ready for more than 300 miles of normal driving, the scanner should show a diagnostic trouble code that explains why. That code tells you what system has a fault. If no code appears but a monitor remains stuck, the vehicle may need professional diagnosis to determine whether the system is faulty or straightforward requires a specific driving pattern you have not yet completed.

Can I use the NT520 on someone else's car?

Yes, as long as you have permission. The scanner works on any vehicle with an OBD-II port, which includes all cars sold in the United States since 1996. You will need to select the correct make and model year when you connect, but the process is the same regardless of whose vehicle you are checking.