Understanding OBDII Readiness Monitors: Ensuring Your Vehicle is Ready for Inspection

Obdii Readiness Monitors are essential self-diagnostic checks within your vehicle’s onboard computer system. As a car owner or automotive enthusiast, understanding these monitors is crucial for maintaining your vehicle’s health and ensuring it passes emission inspections. This guide will provide a comprehensive overview of OBDII readiness monitors, explaining their purpose, types, status, and how to ensure they are “ready” for your next vehicle inspection.

What are OBDII Readiness Monitors?

Readiness monitors, also known as emissions monitors, are diagnostic routines designed to test your car’s emission control systems. Think of them as internal health checks for components that directly impact your vehicle’s environmental performance. These monitors run in the background, constantly evaluating the effectiveness of systems like your catalytic converter, oxygen sensors, and evaporative emissions control (EVAP) system.

Modern vehicles can perform up to eleven different system tests, each represented by a readiness monitor. The outcome of these tests provides valuable insight into whether your car’s computer has successfully completed its self-diagnostic checks. A “ready” status for these monitors is often a prerequisite for passing vehicle emissions inspections, ensuring that your car is operating cleanly and efficiently.

Types of OBDII Readiness Monitors: Continuous and Non-Continuous

OBDII readiness monitors are broadly categorized into two types: continuous and non-continuous. Understanding the difference is key to interpreting their status and ensuring they are set correctly.

Continuous Monitors

Continuous monitors operate constantly while your engine is running. They are designed to detect malfunctions in real-time as they occur. The three main continuous monitors are:

Misfire Monitor: Detects engine misfires, which can significantly increase emissions and damage the catalytic converter.
Fuel System Monitor: Checks the fuel delivery system to ensure the correct air-fuel mixture for optimal combustion and emission control.
Comprehensive Components Monitor: Oversees a wide range of engine management system components for proper function.

Because these monitors run continuously, they are usually “ready” whenever the engine is running and no faults are detected in these systems.

Non-Continuous Monitors

Non-continuous monitors, in contrast, require specific operating conditions to be met before they can run their diagnostic tests. These conditions, often referred to as “enable criteria” or “drive cycles,” vary depending on the monitor and vehicle manufacturer. These monitors typically assess systems that don’t operate constantly or require specific conditions to evaluate effectively.

For gasoline (spark ignition) vehicles, common non-continuous monitors include:

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 operating temperature faster for improved cold-start emission control.
Evaporative (EVAP) System Monitor: Tests the fuel vapor recovery system to prevent fuel vapors from escaping into the atmosphere.
Secondary Air System Monitor: (If equipped) Verifies the operation of the secondary air injection system, which helps reduce emissions during cold starts.
Oxygen (O2) Sensor Monitor: Assesses the accuracy and response time of the oxygen sensors, crucial for fuel control and catalyst efficiency.
Oxygen Sensor Heater Monitor: Checks the functionality of the oxygen sensor heaters, ensuring they reach operating temperature quickly.
EGR (Exhaust Gas Recirculation) and/or VVT System Monitor: Evaluates the performance of the Exhaust Gas Recirculation (EGR) system and Variable Valve Timing (VVT) system, both used to reduce NOx emissions.

For diesel (compression ignition) vehicles, non-continuous monitors typically include:

NMHC Catalyst Monitor: Monitors the Non-Methane Hydrocarbon (NMHC) catalyst used in diesel emission control.
NOx/SCR Aftertreatment Monitor: Checks the Selective Catalytic Reduction (SCR) system and NOx aftertreatment systems used to reduce nitrogen oxide emissions in diesel engines.
Boost Pressure Monitor: Evaluates the boost pressure control system in turbocharged diesel engines.
Exhaust Gas Sensor Monitor: Monitors various exhaust gas sensors specific to diesel emission systems.
PM Filter Monitor: Checks the Diesel Particulate Filter (DPF) for proper operation and soot accumulation.
EGR and/or VVT System Monitor: Similar to gasoline engines, diesel engines also use EGR and VVT systems for emission control.

OBDII readiness monitor status as displayed in Windows-based OBD Auto Doctor software, showing a mix of completed (green checkmarks) and incomplete (red exclamation marks) monitors.

Understanding OBDII Monitor Status: Complete, Incomplete, and Disabled

Each OBDII readiness monitor reports a status indicating whether its self-diagnostic test has been completed and passed. The possible statuses are:

Complete (Ready): This status, often indicated by a green check mark in diagnostic tools, signifies that the monitor has run its test and the emission control system has passed. This is the desired status for all monitors when preparing for a vehicle inspection.
Incomplete (Not Ready): Indicated by a red exclamation mark or similar symbol, this status means the monitor has not yet run its diagnostic test or the test has failed. A monitor can be incomplete for various reasons, including recently cleared diagnostic trouble codes (DTCs) or interrupted drive cycles.
Disabled: A disabled monitor means the test for that particular system has been temporarily deactivated by the vehicle’s computer. This can occur if certain conditions are not met, such as ambient temperature being outside of the acceptable range for the test to run reliably.
Not Available (N/A): This indicates that the vehicle does not support a particular monitor. Not all vehicles are required to support every possible monitor defined in the OBDII standard.

It’s important to note that clearing DTCs (Diagnostic Trouble Codes) using a scan tool will reset the readiness monitor statuses to “incomplete.” This is because clearing codes erases the history of completed tests, and the monitors need to run again to confirm system readiness. Similarly, disconnecting your car battery can also reset monitor statuses.

Why are OBDII Monitors Incomplete or “Not Ready”?

As mentioned, resetting DTCs and disconnecting the battery are common reasons for monitors to become incomplete. This often happens during or after vehicle repairs when codes are cleared. For the current driving cycle, monitors will also initially show as “incomplete” when you start the engine. This is normal, as the monitors need time and specific driving conditions to run their tests.

The issue of incomplete monitors becomes important when it comes to vehicle inspections. Most jurisdictions have limits on the number of “not ready” monitors allowed for a vehicle to pass inspection. For example, in the United States, EPA guidelines typically allow up to two “not ready” monitors for 1996-2000 model year vehicles and only one “not ready” monitor for 2001 and newer vehicles. Exceeding these limits will result in inspection failure.

Getting OBDII Monitors “Ready” for Inspection: The Drive Cycle

The most effective way to set OBDII readiness monitors to “complete” is to drive your vehicle under conditions that allow each monitor to run its diagnostic test. This often involves performing what’s known as an OBDII drive cycle.

Before initiating a drive cycle, ensure the following:

No Check Engine Light (MIL): Make sure the Malfunction Indicator Light (Check Engine Light) is not illuminated. Active or pending DTCs can prevent monitors from running to completion. Address any underlying issues and clear DTCs before proceeding.
Sufficient Fuel Level: Some monitors, particularly the EVAP monitor, require the fuel tank to be between 35% and 85% full to initiate testing.
Understand the Drive Cycle: While a generic drive cycle exists, the most effective drive cycle for your specific vehicle is often outlined in your owner’s manual or available from the vehicle manufacturer. Manufacturer-specific drive cycles are tailored to the enable criteria of each monitor for that particular vehicle model.

Generic OBDII Drive Cycle

If a specific drive cycle is unavailable, the following generic OBDII drive cycle can be used as a guideline. However, it may not work for all vehicles and monitors, and manufacturer-specific procedures are always recommended when available. Always perform drive cycles in a safe and legal environment, away from public roads if possible.

Cold Start: Begin with a cold start. The coolant temperature should be below 122°F (50°C), and the ambient air temperature and coolant temperature should be within 11°°F (6°C) of each other. Allow the vehicle to sit overnight to ensure a cold start.
Avoid Key ON Before Start: Do not turn the ignition key to the “ON” position before starting the engine, as this can interfere with the heated oxygen sensor monitor.
Idle: Start the engine and idle in drive for two and a half minutes. Turn on the A/C and rear defroster (if equipped) during this idle period to increase engine load.
Moderate Acceleration and Steady Speed: Turn off the A/C and rear defroster. Accelerate moderately to 55 mph (90 km/h) and maintain a steady speed for three minutes.
Deceleration: Slow down to 20 mph (30 km/h) without braking. If you have a manual transmission, do not depress the clutch during deceleration.
Moderate to Heavy Acceleration and Steady Speed: Accelerate again to 55-60 mph (90-100 km/h) using approximately 3/4 throttle. Maintain this steady speed for five minutes.
Stop: Slow down and come to a complete stop without braking.

After completing the drive cycle, use an OBDII scan tool to check the readiness monitor status. You may need to repeat the drive cycle or drive for a few days under normal city and highway conditions to allow all monitors to set to “ready.”

Preparing for Vehicle Inspection: Check Your Readiness Monitors

Before heading to your annual vehicle inspection, it’s highly recommended to check your OBDII readiness monitors yourself. Using an OBDII scan tool, even a basic and affordable one, or diagnostic software like OBD Auto Doctor, you can easily read the monitor statuses. Ensuring all required monitors are “ready” will significantly increase your chances of passing the inspection and avoid potential rejection and re-inspection fees.

Furthermore, it’s always a good practice to read and address any Diagnostic Trouble Codes (DTCs) before an inspection. You can use the same OBDII tools to check for and clear DTCs. Proactive maintenance and addressing issues promptly not only ensure inspection readiness but also contribute to your vehicle’s longevity, fuel efficiency, and overall performance. Don’t wait until the last minute; taking care of your vehicle’s emission systems is an ongoing responsibility for every car owner.