If you cleared your codes and the test station then told you the car was “not ready,” nothing is broken. Clearing codes also resets your car’s self-test monitors, and until the car has re-run those tests it cannot report that its emissions systems have been checked. A station will generally not test a vehicle with too many incomplete monitors. The fix is driving, not repair, and it usually takes days rather than minutes.
What a readiness monitor is
Your car continuously and periodically tests its own emissions-related systems. Each of those self-tests is a monitor, and each monitor reports one of two states: complete, meaning it has run and reached a verdict, or incomplete, usually displayed as “not ready.”
An inspection that reads your car’s OBD2 data is asking two questions, not one. It asks whether there are emissions faults stored, and it asks whether the car has actually finished testing itself. A car with no faults but no completed self-tests has not demonstrated anything, which is why “not ready” is treated as an unusable result rather than as a pass.
Monitors reset to incomplete whenever stored codes are cleared, and also after a battery disconnection on many vehicles. That is the causal chain most people arrive here having just experienced, and it is covered from the other direction in should you clear OBD2 codes.
Continuous versus non-continuous monitors
The distinction matters because the rules that decide whether you pass only apply to one of the two groups.
Continuous monitors run whenever the conditions allow, which in practice is most of the time. There are three: misfire, fuel system, and comprehensive components. These generally report ready very quickly and are not usually the reason a car is rejected.
Non-continuous monitors run only when a specific set of conditions is met, and each has its own recipe. Catalyst, evaporative system, oxygen sensor, oxygen sensor heater, EGR and, where fitted, secondary air injection all sit in this group. Which ones your vehicle has depends on how it is built; a car without a given system does not run a monitor for it and reports it as unsupported rather than incomplete.
The evaporative system monitor is the one that most often refuses to complete, because its conditions tend to be the fussiest, frequently involving a particular fuel level and a stable temperature range.
How many are allowed to be incomplete
This is where the answer stops being universal and starts depending on where you live and how old your car is.
The pattern used across much of the United States, described by state programs including the Texas Department of Public Safety and the Arizona Department of Environmental Quality, allows some non-continuous monitors to be incomplete: commonly two for model years 1996 to 2000, and one for model year 2001 and later. Exceed the allowance and the vehicle is rejected for readiness rather than failed on emissions, which is a different outcome and normally means you can return once the monitors have set.
California has moved further. Under regulatory text adopted by its Bureau of Automotive Repair, a requirement took effect on 1 October 2025 that all readiness monitors be set for a vehicle to pass inspection.
Because these thresholds are set jurisdiction by jurisdiction and do change, treat the figures above as a description of the general pattern and check your own testing authority’s current published rules before planning around them. We are describing what the rules are, not advising you on your obligations under them.
Setting monitors again: the drive cycle
A drive cycle is a pattern of driving that meets the conditions each monitor needs. Manufacturers publish specific procedures for their own vehicles, and they differ, which is why we are not printing a universal recipe here.
What is generally true:
- Ordinary mixed driving sets most monitors eventually. A combination of cold starts, town driving and a steady run at higher speed covers a lot of the conditions monitors look for.
- The car needs cold starts, plural. Several monitors will only attempt to run after the engine has been off long enough to cool properly, so repeated short trips in one day do less than the same mileage spread across several mornings.
- Some monitors need particular states, the evaporative monitor’s fuel-level and temperature conditions being the usual culprit. If one monitor is holding out while the rest have completed, that is normally why.
- Your manufacturer’s published drive cycle is the reliable route if ordinary driving is not getting there. It is in the service documentation for your vehicle.
If a permanent code is also present, the same driving is doing double duty, since a permanent code clears only when its monitor re-runs successfully. That mechanism is explained in why your scanner refuses to erase permanent codes.
How to check before you drive to the test station
Any OBD2 scanner can read monitor status, including inexpensive ones, and doing so before you set off saves a wasted trip and often a fee. On most tools it is a menu item labelled readiness, I/M readiness or monitor status, and it lists each monitor as ready, not ready, or not supported. If you are new to the tool, our first plug-in walkthrough covers finding your way around the menus.
Read it, note which monitors are outstanding, and drive accordingly. Using a scanner to pre-check the whole emissions picture rather than just readiness is covered in pre-checking your own emissions test.
FAQ
Why does my car say not ready after clearing codes?
Clearing stored codes resets the readiness monitors along with them. The car has to re-run each self-test before it can report a result, and until it has, it reports not ready.
How long does it take to set readiness monitors?
There is no single answer. Mixed driving with several separate cold starts sets most monitors over a few days for many vehicles, but individual monitors have their own conditions and one stubborn monitor can take considerably longer. Your manufacturer’s published drive cycle is the reliable route.
How many monitors can be not ready and still pass?
It depends on your jurisdiction and model year. A common US pattern allows two incomplete non-continuous monitors for 1996 to 2000 vehicles and one for 2001 and later. California requires all monitors set as of 1 October 2025. Check your own testing authority.
Which monitor takes longest to set?
The evaporative system monitor is the one most often reported as stubborn, because its running conditions typically include a particular fuel level and a stable temperature range that ordinary driving does not always deliver.
Does disconnecting the battery reset readiness monitors?
On many vehicles yes, with the same consequence as clearing codes. If an inspection is imminent, a battery disconnection is worth avoiding for the same reason clearing codes is.
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