Understanding I/M Readiness Monitors: Your Guide to Passing Emissions Tests

Ensuring your vehicle is environmentally friendly and compliant with emission standards is a crucial part of car ownership. A key component in this process is understanding I/m Readiness Monitors. But what exactly are they, and why are they so important for your car’s health and passing emissions inspections? This guide, brought to you by the automotive experts at obd-de.com, will explain everything you need to know about I/M readiness, making complex diagnostics simple and understandable.

Decoding OBD2 Readiness Monitors for Emission Compliance

Modern vehicles are equipped with sophisticated onboard diagnostic systems, commonly known as OBD2 (On-Board Diagnostics II). A core function of this system is to continuously monitor your car’s emission control systems. This is where readiness monitors come into play. These monitors, also referred to as emission monitors, are essentially self-check routines that your car’s computer performs to ensure all emission-related systems are functioning correctly.

Think of them as internal health checks for your car’s emissions. There can be up to 11 different system tests, and the results of these readiness monitors provide valuable insight into whether your vehicle is meeting emission standards. Crucially, they tell you if your car’s computer has successfully completed these tests.

Types of Readiness Monitors: Continuous vs. Non-Continuous

Readiness monitors are broadly categorized into two types based on how they operate:

Continuous Monitors: These monitors are constantly active whenever your engine is running. They are designed to continuously evaluate critical emission systems in real-time.
Non-Continuous Monitors: Unlike their continuous counterparts, non-continuous monitors require specific conditions to be met before they can run their tests. These conditions can vary depending on the monitor and the vehicle. Some might need a specific driving pattern, known as a drive cycle, to activate. Others might require multiple drive cycles due to cool-down and warm-up periods between tests. Each non-continuous emission monitor can have its own unique set of requirements.

Historically, the OBD2 standard (SAE J1979) clearly defined each monitor as either continuous or non-continuous. However, the latest revisions of the standard have become less rigid in this categorization for some monitors. Therefore, modern diagnostic tools like OBD Auto Doctor often don’t strictly adhere to this older categorization.

Continuous or Non-Continuous? It Depends on the Manufacturer

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

Misfire Monitor: Detects engine misfires, which can significantly increase emissions.
Fuel System Monitor: Checks the fuel delivery system for proper operation and fuel mixture control.
Comprehensive Components Monitor: Monitors a wide array of other emission-related components and sensors for malfunctions.

Non-Continuous Monitors: Gasoline vs. Diesel

Non-continuous monitors are further differentiated based on the type of engine: spark ignition (gasoline) and compression ignition (diesel).

Spark Ignition Vehicles (Gas)

Catalyst (CAT) Monitor: Evaluates the efficiency of the catalytic converter in reducing harmful emissions.
Heated Catalyst Monitor: Specifically checks the performance of heated catalytic converters, which reach operating temperature faster.
Evaporative (EVAP) System Monitor: Tests the evaporative emissions 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: Checks the functionality of oxygen sensors, crucial for air-fuel ratio control.
Oxygen Sensor Heater Monitor: Verifies the operation of oxygen sensor heaters, ensuring sensors reach optimal temperature quickly.
EGR (Exhaust Gas Recirculation) and/or VVT System Monitor: Monitors the exhaust gas recirculation system and/or variable valve timing system, both used to reduce NOx emissions.

Compression Ignition Vehicles (Diesel)

NMHC Catalyst Monitor: Evaluates the Non-Methane Hydrocarbon Catalyst used in diesel engines.
NOx/SCR Aftertreatment Monitor: Checks the NOx reduction system, often using Selective Catalytic Reduction (SCR) technology in diesel vehicles.
Boost Pressure Monitor: Monitors the boost pressure system in turbocharged diesel engines, impacting emission control.
Exhaust Gas Sensor Monitor: Verifies the functionality of various exhaust gas sensors specific to diesel engines.
PM Filter Monitor: Evaluates the performance of the Particulate Matter (PM) filter or 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 NOx control in diesels.

Alt text: OBD2 software interface on Windows displaying I/M Readiness Monitor statuses for vehicle emission systems.

Understanding Monitoring Cycles: “Since DTC Reset” and “This Drive Cycle”

Traditionally, OBD2 systems primarily reported monitor status since the last time diagnostic trouble codes (DTCs) were cleared. This “since DTC reset” status is mandatory for all OBD2 compliant vehicles. It provides a long-term view of monitor status after the check engine light has been reset and DTCs have been cleared.

However, newer vehicles offer enhanced reporting capabilities and can also provide emission monitor status for the current driving cycle. These “this drive cycle” monitors reset at the beginning of each new driving cycle (when you start your car). While older cars may not support this feature, modern diagnostic tools like OBD Auto Doctor will indicate “NA” or “Not Available” if your vehicle doesn’t report “this drive cycle” monitor status.

Interpreting Monitor Status: Complete, Incomplete, and Disabled

The result of each readiness monitor test is a status indication. The possible statuses are:

Complete (Ready): This indicates that the monitor test has been successfully completed, and the emission control system has passed the test. Diagnostic tools typically represent this with a green checkmark.
Incomplete (Not Ready): This means the monitor test has not yet been completed. This could be because the OBD2 system hasn’t had the opportunity to run the routine, or the test has failed. Often displayed with a red exclamation mark.
Disabled: In some situations, a monitor might be temporarily disabled for the current monitoring cycle. This can happen when conditions are not suitable for the monitor to run, for example, if the ambient air temperature is outside the required range.

Diagnostic software like OBD Auto Doctor lists all defined monitors, but it will only report the status for the monitors that your specific vehicle supports. It’s important to remember that not all vehicles are required to support every single monitor. If a monitor is listed as “NA” or “Not Available,” it simply means your car doesn’t have that particular monitor and therefore, it cannot be tested.

Alt text: Screenshot of an OBD2 Android app showing I/M Readiness Monitor status, indicating “Not Available” for ‘This Drive Cycle’ monitors, suggesting limited support on the vehicle.

Why is a Monitor Incomplete or “Not Ready”?

Seeing “incomplete” or “not ready” statuses for your readiness monitors can be concerning. Here are the most common reasons:

Recent DTC Clearing: Clearing diagnostic trouble codes (DTCs) and resetting the Check Engine Light will also reset all readiness monitor statuses to “incomplete.” This often occurs after vehicle repairs or during troubleshooting.
Battery Disconnection: Power loss, such as disconnecting the car battery, will also reset monitor statuses. This is why disconnecting the battery is generally not recommended unless necessary. If you do disconnect it for maintenance like battery replacement, understanding how to reset the monitors is essential.
New Monitoring Cycle: For “this drive cycle” monitors, it’s normal for them to initially show as “incomplete” when you start your engine, as the tests haven’t run yet in the current driving session.

The status of your readiness monitors is directly linked to your vehicle’s ability to pass emissions inspections. Depending on your location, regulations dictate how many monitors can be “not ready” for a vehicle to still pass inspection. For example, in the United States, EPA guidelines typically allow up to two monitors to be “not ready” for 1996-2000 model year vehicles, and only one for 2001 and newer models. Exceeding these limits will result in inspection failure.

How to Get Readiness Monitors “Complete” or “Ready”

Since readiness monitors are self-check routines, the most effective way to get them to complete is by driving your car. However, simply driving without a plan may not trigger all the necessary tests. This is where the concept of an OBD drive cycle comes in.

Before diving into the drive cycle, consider these initial steps:

Check for the Check Engine Light (MIL): 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: Make sure you have adequate fuel in your tank. Some monitors, like the EVAP monitor, require the fuel level to be between 35% and 85% to initiate their diagnostic testing.
Complete an OBD Drive Cycle: A general drive cycle, or a manufacturer-specific drive cycle, is designed to create the conditions necessary for all monitors to run. About a week of combined city and highway driving under normal conditions is often sufficient for monitors to become ready.

The OBD Drive Cycle: A Step-by-Step Guide

The purpose of an OBD2 drive cycle is to provide your car with the specific operating conditions needed to run its onboard diagnostics, allowing readiness monitors to execute and detect any potential emission system malfunctions. The ideal drive cycle for your vehicle can vary depending on the make, model, and even the specific monitor you’re trying to set.

Many vehicle manufacturers now include recommended drive cycles in the owner’s manual. If a specific drive cycle isn’t available, the following generic OBD-II drive cycle can be used as a guideline. However, remember that it may not work for all vehicles and monitors. It’s crucial to perform this drive cycle in a safe, controlled environment, away from public roads if possible, as some steps require specific speeds and accelerations.

Cold Start: Begin with a cold start. The coolant temperature should be below 122°F (50°C), and the coolant and air temperatures should be within 11 degrees of each other. Allowing your car to sit overnight typically achieves these conditions.
Ignition ON (Engine OFF): Do not leave the ignition key in the “ON” position before starting the engine for a cold start. This can interfere with the heated oxygen sensor diagnostic.
Idle in Drive (2.5 minutes): Start the engine and let it idle in drive for two and a half minutes. If equipped, turn on the A/C and rear defroster to increase engine load.
Accelerate and Cruise (3 minutes): Turn off the A/C and rear defroster. Accelerate smoothly and moderately to 55 mph (90 km/h). Maintain a steady speed of 55 mph for three minutes.
Decelerate (No Brakes): Slow down to 20 mph (30 km/h) without using the brakes. If you have a manual transmission, do not depress the clutch during deceleration.
Accelerate and Cruise (5 minutes): Accelerate again to 55-60 mph (90-100 km/h) at approximately 3/4 throttle. Hold this steady speed for five minutes.
Decelerate to Stop (No Brakes): Slow down to a complete stop without applying the brakes.

Get Ready for Your Emissions Inspection: Check Your I/M Readiness Status

To avoid failing your annual emissions inspection, proactive preparation is key. Before heading to the inspection station, you should always check your vehicle’s readiness monitor status. Ensuring all required monitors are “ready” will prevent a near-certain rejection.

Furthermore, it’s highly recommended to read your diagnostic trouble codes and confirm that no DTCs are present. You can easily perform both of these checks using OBD Auto Doctor diagnostic software. Even the free version allows you to read monitor statuses and diagnostic trouble codes. Why not try the software right now and get peace of mind?

Remember, addressing potential emission issues promptly, rather than waiting for your annual inspection, can save you significant time, prevent future repair costs, and even improve your vehicle’s fuel efficiency.

Editor’s note: This guide was updated in October 2024 for accuracy and the latest information on I/M Readiness Monitors.