A computer monitor is a program or series of programs inside the car's main engine control computer that will run individual tests and performance checks of specific systems such as the catalytic converter or EVAP systems. When these monitors are in transit, or test mode without completion the monitor is not "set" or satisfied therefore not ready, this condition will not allow the car to have an emission test.
Each monitor program will vary from one manufacturer to the next so there is no set way to close a monitor, but as a general rule of thumb this is how it is done:
Some scanners or code readers can tell you if the monitors have been reset which is called a "monitor readiness status". This feature can help you know exactly when the monitors are ready and the car can be emissions tested.
Occasionally a monitor will not reset, this is because the computer could have a P1000 code which means there is something disabling the monitor's function. A popular cause for this is a bad thermostat which is stuck open, this will exceed the engine warm up time which will shut the monitoring off until repaired. Anytime there is a trouble code the monitors will not run.
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