Understanding OBD2 Readiness Monitors: Ensuring Your Vehicle is Inspection Ready

Readiness Monitors, often referred to as Emissions Monitors, are integral self-diagnostic checks within your vehicle’s On-Board Diagnostics system (OBD2). These routines offer valuable insights into your car’s emission control systems, playing a crucial role in maintaining vehicle health and ensuring compliance with emission standards. This guide provides a comprehensive overview of OBD2 readiness monitors, their function, types, and how to interpret their status.

The primary function of readiness monitors is to perform self-tests on your vehicle’s emission control systems. As their name suggests, they continuously or periodically monitor the performance of emission-related components and systems.

Modern vehicles can conduct up to 11 system tests, each designated as a readiness monitor. The results of these monitors indicate whether your car’s computer has successfully completed these tests. Understanding these results is key to diagnosing potential issues and preparing your vehicle for emission inspections.

Types of Readiness Monitors: Continuous and Non-Continuous

OBD2 readiness monitors are broadly categorized into two types: continuous and non-continuous. This distinction lies in how and when these monitors perform their self-checks.

Continuous monitors operate constantly while the engine is running. They are designed to detect malfunctions in real-time, ensuring immediate identification of critical emission control issues.

Non-continuous monitors, conversely, require specific conditions to be met before a test can be initiated and completed. These conditions can vary significantly depending on the specific monitor and the vehicle’s make and model. Some monitors might need a predefined drive cycle – a specific sequence of driving conditions – to run, while others may require multiple drive cycles with cool-down and warm-up periods in between. Each non-continuous emission monitor can have unique requirements for these conditions.

Historically, the OBD2 standard (SAE J1979) classified each monitor definitively as either continuous or non-continuous. However, the latest iterations of the standard have moved away from this rigid categorization for some monitors. Consequently, modern diagnostic tools like OBD Auto Doctor often do not strictly adhere to this older classification.

Monitors: Continuous or Non-continuous (Manufacturer Dependent)

The following monitors can be implemented as either continuous or non-continuous, depending on the vehicle manufacturer’s design choices:

Misfire Monitor: Detects engine misfires, which can significantly increase emissions.
Fuel System Monitor: Checks the fuel delivery system to ensure proper fuel metering and efficiency.
Comprehensive Component Monitor: Monitors various electrical and electronic components for proper function.

Non-Continuous Monitors: System Specific Tests

Non-continuous monitors are further differentiated between spark ignition (gasoline) and compression ignition (diesel) engines, reflecting the different emission control technologies employed in these engine types.

Non-Continuous Monitors for Spark Ignition (Gas) Vehicles:

Catalyst (CAT) Monitor: Evaluates the efficiency of the catalytic converter in reducing harmful emissions.
Heated Catalyst Monitor: Checks the performance of heated catalytic converters, which reach optimal operating temperature faster.
Evaporative (EVAP) System Monitor: Tests the evaporative emission control system to prevent fuel vapor release into the atmosphere.
Secondary Air System Monitor: Monitors the secondary air injection system, which helps reduce emissions during cold starts.
Oxygen (O2) Sensor Monitor: Assesses the functionality of oxygen sensors, crucial for air-fuel mixture control.
Oxygen Sensor Heater Monitor: Checks the heaters for oxygen sensors, ensuring they reach operating temperature quickly for accurate readings.
EGR (Exhaust Gas Recirculation) and/or VVT System Monitor: Monitors the Exhaust Gas Recirculation system and/or Variable Valve Timing system, which reduce NOx emissions and improve engine efficiency.

Non-Continuous Monitors for Compression Ignition (Diesel) Vehicles:

NMHC Catalyst Monitor: Evaluates the Non-Methane Hydrocarbon catalyst used in diesel emission control.
NOx/SCR Aftertreatment Monitor: Monitors the NOx reduction system, often using Selective Catalytic Reduction (SCR) technology.
Boost Pressure Monitor: Checks the boost pressure control system in turbocharged diesel engines.
Exhaust Gas Sensor Monitor: Monitors various exhaust gas sensors specific to diesel engines.
PM Filter Monitor: Assesses the performance of the Particulate Matter filter (diesel particulate filter – DPF) in reducing soot emissions.
EGR and/or VVT System Monitor: Similar to gasoline engines, monitors EGR and/or VVT systems for emission control and efficiency.

Monitoring Cycles: Current and Since Reset

OBD2 systems track readiness monitor status in different monitoring cycles. Understanding these cycles is important for interpreting monitor status correctly.

Traditionally, the primary monitor status reported was the status since the diagnostic trouble codes were cleared. This status is mandatory for all OBD2 compliant vehicles and reflects the long-term readiness of the emission systems after any reset events, such as clearing the check engine light or DTCs.

More advanced, newer vehicles also report emission monitor status for the current driving cycle. These monitors initiate a new test cycle every time a new driving cycle begins (typically when the engine is started). It’s important to note that older vehicles may not support this “current driving cycle” status. In such cases, diagnostic tools like OBD Auto Doctor will indicate this as “NA” or “Not Available”.

Understanding Monitor Status: Complete, Incomplete, Disabled

Each readiness monitor reports its test result as a specific status. The possible completion statuses are:

Complete or Ready: This status, often indicated by a green check mark in diagnostic software, signifies that the monitor has successfully completed its self-test and the emission control system has passed. This is the desired status for all monitors, especially before a vehicle inspection.
Incomplete or Not Ready: Indicated by a red exclamation mark, this status means the monitor has not yet completed its test. This could be because the OBD2 system has not yet run the routine, or the test has run and failed. An incomplete status doesn’t necessarily indicate a failure, but rather that the system needs more time or specific driving conditions to complete the test.
Disabled: This status indicates that the monitor test has been intentionally disabled for the current monitoring cycle. Monitors might be disabled under certain conditions where it’s impractical for the driver to operate the vehicle in a way that allows the monitor to run – for example, if the ambient air temperature is outside the monitor’s operating range.
Not Available (NA): As mentioned earlier, “NA” means the vehicle does not support a particular monitor. It doesn’t indicate a problem, but simply that the vehicle’s system is not equipped with that specific monitoring function.

It’s important to remember that vehicles are not required to support all defined monitors. Diagnostic tools like OBD Auto Doctor list all possible monitors, but the actual status reported will only be for the monitors supported by your specific vehicle.

Reasons for Incomplete or “Not Ready” Monitor Status

A common reason for monitors to be incomplete or “not ready” is after clearing diagnostic trouble codes (DTCs) and resetting the Check Engine Light. Clearing DTCs also resets the readiness monitor statuses. This often happens during or after vehicle repairs.

Monitor statuses are also reset in the event of a power failure, such as when the car battery is disconnected. This is why disconnecting the battery unnecessarily is generally discouraged, particularly before an emissions test. If battery disconnection is unavoidable (e.g., for battery replacement), understanding how to reset the monitors afterward is crucial (discussed below).

For the current monitoring cycle, it’s normal for monitors to initially show an “incomplete” status when the engine is first started. This is simply because the monitors have not yet had the opportunity to run their self-tests in the new driving cycle.

Crucially, depending on local regulations, an OBD2 vehicle might fail an annual inspection if the required monitors since reset are not in a “complete” status. For instance, in the US, EPA guidelines allow for up to two “not ready” monitors for 1996-2000 model year vehicles, and only one “not ready” monitor for 2001 and newer vehicles. Therefore, ensuring monitor readiness is vital for passing emissions tests.

Achieving “Complete” or “Ready” Monitor Status

Since readiness monitors are self-check routines, the most effective way to get them to a “ready” status is to drive the vehicle. However, simply driving in a monotonous manner is unlikely to fulfill all the necessary conditions for all monitors to run. This is where the concept of an OBD drive cycle becomes relevant.

Before delving into drive cycles, consider these preliminary steps:

Check for Check Engine Light and DTCs: Ensure the Malfunction Indicator Light (MIL), or Check Engine Light, is not illuminated. Active stored or pending diagnostic trouble codes can prevent monitors from running to completion. Address any underlying issues causing DTCs first.
Fuel Level: Verify adequate fuel in the tank. Some monitors, particularly the EVAP monitor, require the fuel level to be between 35% and 85% to initiate testing.
Normal Driving: For many vehicles, approximately one week of combined city and highway driving under normal conditions is often sufficient to allow most monitors to reach a “complete” status.

OBD Drive Cycle: A Structured Approach

The purpose of an OBD2 drive cycle is to provide the specific operating conditions needed for your car’s on-board diagnostics to run, enabling the readiness monitors to operate and detect any potential malfunctions in the emission system.

The precise drive cycle required for your vehicle can vary significantly based on the car model, manufacturer, and the specific monitor in question. Modern vehicle owner’s manuals often include manufacturer-specific drive cycle procedures.

If a specific drive cycle isn’t available, a generic OBD-II drive cycle can be used as a guideline to assist in resetting monitors. However, it’s important to understand that this generic cycle may not be effective for all vehicles and all monitors.

Generic OBD-II Drive Cycle (Use with Caution and in a Safe, Legal Area):

Important Safety Note: This drive cycle involves specific driving maneuvers that should be performed in a safe and legal environment, away from public roads and traffic if possible. Always prioritize safety and obey traffic laws.

Cold Start: Begin with a cold start. The coolant temperature should be below 50°C (122°F), and the coolant and air temperatures should be within 11°C (20°F) of each other. Allowing the vehicle to sit overnight typically achieves this.
Ignition Key Position: Do not leave the ignition key in the “ON” position before the cold start. This can interfere with the heated oxygen sensor diagnostic.
Idling: Start the engine and idle in drive (or neutral for manual transmissions with parking brake engaged) for two and a half minutes. Turn on the A/C and rear defroster if equipped to increase engine load.
Moderate Acceleration and Steady Speed: Turn off the A/C and rear defroster. Accelerate moderately and steadily to 90 km/h (55 mph). Maintain this steady speed for three minutes.
Deceleration: Decelerate to 30 km/h (20 mph) without braking. For manual transmissions, do not depress the clutch to allow engine braking.
Moderate to Heavy Acceleration and Steady Speed: Accelerate again to 90-100 km/h (55-60 mph) using approximately 3/4 throttle. Hold this steady speed for five minutes.
Stop: Decelerate to a complete stop without braking.

This drive cycle can be challenging to execute precisely under normal driving conditions. It’s crucial to perform it in a controlled and safe environment.

Preparing for Vehicle Inspection

To proactively prepare for an annual vehicle inspection and avoid potential rejection due to emission monitor status, you can take the following steps:

Read Readiness Monitors: Use an OBD2 diagnostic tool to check the status of your readiness monitors. Ensure they are all, or at least the required number, in a “ready” or “complete” status.
Check Diagnostic Trouble Codes (DTCs): Read and clear any diagnostic trouble codes. A pending or active DTC can also lead to inspection failure. You can use OBD Auto Doctor or similar software to read diagnostic trouble codes.

By proactively checking your readiness monitors and DTCs, you can significantly increase your chances of passing your vehicle inspection and avoid unnecessary delays and potential repair costs. Consider using a diagnostic tool like OBD Auto Doctor – even the free version allows you to check monitor statuses and DTCs. Try the software now and ensure your vehicle is ready for inspection.

Addressing potential emission issues promptly, rather than waiting for an inspection, can save you time, prevent future repairs, and even improve fuel efficiency.

Editor’s note: This article is based on information updated as of March 2020 and reflects general OBD2 practices. Always consult your vehicle’s service manual and local emission testing regulations for the most accurate and up-to-date information.