What are Readiness Monitors in the OBD-II system, and how do they affect emissions testing?

"Readiness Monitors" are diagnostic self-tests performed by the OBD-II system to assess the proper functioning of specific emissions control systems within the vehicle. The OBD-II system continuously monitors various components, such as the catalytic converter, oxygen sensors, EGR system, and EVAP system, to ensure they are operating within specified parameters. The results of these self-tests are stored in the vehicle's onboard computer.

When a vehicle undergoes an emissions test or smog check, the OBD-II system's "Readiness Monitors" are queried to determine if the emissions control systems have completed their self-tests. If the "Readiness Monitors" indicate that the required tests have not been run, the vehicle may not pass the emissions test.

There are two types of "Readiness Monitors":

Continuous Monitors: These are ongoing tests that continuously monitor specific emissions-related systems while the vehicle is running. For example, the oxygen sensor monitor continuously checks the sensors' functionality and response times. Continuous monitors typically run whenever the vehicle is operational, and they are relatively quick to complete their self-tests.

Non-Continuous Monitors: These are non-continuous tests that only run under specific driving conditions or predetermined criteria. For example, the EVAP system monitor may require the vehicle to be driven at specific speeds and temperatures to complete its self-test. Non-continuous monitors may take several driving cycles or specific driving conditions to complete.

The number of completed "Readiness Monitors" required to pass an emissions test varies depending on the state or region's regulations. In most cases, a specific number of monitors must be in the "Ready" state before the vehicle can pass the emissions test. If any of the "Readiness Monitors" are in the "Not Ready" state, the vehicle will typically fail the emissions test.

To ensure all "Readiness Monitors" are in the "Ready" state before an emissions test, follow these steps:
Make sure the vehicle is in good operating condition and there are no active "Check Engine" codes present.
Ensure the fuel tank is between 1/4 and 3/4 full. A full tank or very low fuel level may prevent certain "Readiness Monitors" from running.

Perform a series of normal driving cycles to allow the OBD-II system to run the necessary self-tests. This may involve highway driving, stop-and-go traffic, and various speed ranges.

Consult the vehicle's service manual or contact a qualified mechanic for information on specific driving conditions required for completing non-continuous monitors.

Use an OBD-II scanner to check the "Readiness Monitors" status before taking the vehicle for emissions testing.

Ensuring that all "Readiness Monitors" are in the "Ready" state is essential for a successful emissions test. If any "Readiness Monitors" are not ready, consult a qualified mechanic to diagnose and address the underlying issues that prevent the completion of the self-tests.