Check Engine Light

What Should You Do When OBD2 Monitors Not Ready To Test?

Mike Cross
Updated Sep 2, 2021
What Should You Do When OBD2 Monitors Not Ready To Test

If you live in an area that requires emission checks, then you may have encountered a reading that says test not ready or something along those lines.

This could have caused you some headaches when you went to the DMV or state testing area. This is because you have to go back at a different time and have the test done and you may not even know what this issue really is.

Well, after reading this article you will understand what a not ready indicator means, and some solutions on how to fix it to make sure it doesn’t happen again when you go back.

What Should You Do When OBD2 Monitors Not Ready To Test
What Should You Do When OBD2 Monitors Not Ready To Test

OBD2 System Explained

The onboard diagnostic system has two different categories of tests that it runs to make sure it is running smoothly. The different tests that it does to make sure emission levels fall into a safe zone fall into two categories. Those are the continuous test which is performed all the time while the vehicle is running and non-continuous monitors that only occur at specific times after certain requirements are met.

Continuous Monitors

The checks that happen every time you drive your vehicle are misfire monitoring, fuel system monitoring, and comprehensive component monitoring. No matter how long you drive your vehicle or at what speed, these checks occur.

With misfire monitoring the computer checks to make sure the engine is firing correctly. It checks to make sure the ignition and fuel mixture is correct. This because if it is not than emissions may be increased and this can cause damage to the catalytic converter.

The fuel system check is very similar. This only looks at the fuel mixture and makes sure that the right amount of air and gas ratio is being met. This again makes sure that emissions are at the proper level.

Lastly, the comprehensive component check makes sure that all the major sensors are working. This ensures that all the other things are being monitored right and that no computer related issues are going on. Also, some sensors affect how much air and fuel end up in the gas mixture, so it makes sure those are working right.

Non-continuous Monitors

A lot of the checks being done are non-continuous however and if these checks haven’t all performed, then that is when you get a not ready reading when you go to have your emissions checked.

This is because if the computer hasn’t run or passed all the checks, then it doesn’t know if it is running to the state standards or not. The non-continuous checks are the EVAP monitor which is evaporative emission control system, the HEGO monitor which is the heated exhaust gas oxygen sensor, and the catalyst efficiency monitor.

The EVAP monitor checks to make sure that there are little to no fuel vapor leaks, which can include a loss or missing gas cap. It does this test by applying a vacuum to the gas tank system when certain conditions are met.

While the system is under vacuum or pressure, if it does not detect airflow while the canister purge valve is open, then it passes. Otherwise, it will trigger a code if the airflow reads to high.

The HEGO monitor checks to make sure that the oxygen sensors are running properly and within there normal range. This check runs when the engine gets to operating temperature and a specific speed is met.  It will not run however if there are any faults in the heating circuit, if the coolant or speed sensors aren’t functioning right, or if there are any oxygen sensor codes that have not been cleared. Also, if this check doesn’t run, then the catalyst efficiency check will not run causing another not ready signal.

The catalyst efficiency monitor makes sure that the catalytic converter is running at high enough efficiency to keep exhaust in an appropriate range for the amount of emission the vehicle is putting out. All vehicles put out some emissions, but the test is to make sure that it is not putting out more than it is supposed too. The way the test works is it compares the signals from the upstream and downstream oxygen sensors. If it has a reading of less than 80 percent on most vehicles for any pollutant, then it will trigger a code. If the vehicle is a low emission vehicle then if the rating drops as low as 90 percent, then it might trigger a code.

When are non-continuous test ran?

So you may be wondering when these non-continuous tests are run. Well, it is hard to give a straight answer. Each manufacturer determines when they are running so it all depends on what kind of car you have. You will have to look up the make and possibly the model of your vehicle to figure out when the test runs and then drive your car to make sure the right conditions are met.

Common causes for Not Ready reading

Some common causes for getting a not ready reading when going to have your emissions checked is that you just haven’t driven enough. This is true especially if you have done a few different things. If you have disconnected your battery recently, then that is a good cause for the test not running because when the battery is disconnected it resets the system. Also, if you have erased any stored codes, then that might cause it to trigger a not ready result because it needs the codes to know if the system is working right or not. Also, if you disconnect sensors than this resets things so you will have to drive enough for the test to rerun after reconnecting them. Lastly removing the catalytic converter will cause things not to run and will cause the car not to pass. This is technically illegal in a lot of places, but if you don’t have emission test now in your area and want to risk it, it is up to you, but you just have to be aware that that car will never pass if they start emission testing.


So now you know what a not ready result means and some common causes of it. This guide mainly only covers gas vehicles, however. If you have a diesel getting a not ready reading it is for the same reason of some test has not been running and the common causes are the same for diesel vehicles. The test however that runs on them is a bit different. Now you know though that the best way to fix the issue is to just drive more and if you know what specific test hasn’t run you can look up the conditions to cause that test to trigger based on your cars make.

Mike Cross
Life is too short to drive with stock audio

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