Understanding OBD2 I/M Readiness: Your Guide to Passing Emissions Tests

As a car owner, you’ve likely encountered the term OBD2, especially when it comes to vehicle maintenance or inspections. But have you heard of “I/M Readiness”? If you’re concerned about passing your next emissions test, understanding what is I/M readiness with OBD2 is crucial. This guide, brought to you by the experts at techcarusa.com, will break down everything you need to know about OBD2 readiness monitors, ensuring you’re well-prepared and informed.

Decoding OBD2 Readiness Monitors

OBD2 readiness monitors are essentially your car’s built-in self-check system for its emission control components. Think of them as routine health checks that your vehicle performs on itself. These monitors, also known as emissions monitors, constantly observe and evaluate the performance of your car’s emission-related systems while you drive.

Modern vehicles can conduct up to 11 different system tests, and these are what we call readiness monitors. The results of these monitors provide valuable insights into whether your car’s computer has successfully completed these tests. A “ready” or “complete” status indicates that the system is functioning as expected, which is vital for both your car’s health and passing emissions inspections.

Alt text: OBD2 readiness monitor status displayed on a Windows laptop using diagnostic software, showing “Complete” and “Incomplete” statuses for various monitors.

Continuous vs. Non-Continuous Monitors: Understanding the Difference

Readiness monitors are categorized into two main types: continuous monitors and non-continuous monitors. The key difference lies in how and when these tests are conducted.

Continuous Monitors: These monitors are constantly active and evaluated whenever your engine is running. They are designed to detect issues in real-time. The three main continuous monitors are:

Misfire Monitor: Detects engine misfires, which can significantly increase emissions.
Fuel System Monitor: Checks the fuel delivery system to ensure proper fuel mixture and efficiency.
Comprehensive Component Monitor: Oversees various other engine components and sensors for proper function.

Non-Continuous Monitors: Unlike continuous monitors, non-continuous monitors 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 manufacturer. Some monitors might need a predefined drive cycle, which is a specific pattern of driving conditions, to run their diagnostics. Some even require multiple drive cycles with cool-down and warm-up periods in between.

Historically, the OBD2 standard (SAE J1979) clearly classified each monitor as either continuous or non-continuous. However, the latest OBD2 standards are less rigid in this categorization. Therefore, modern OBD2 diagnostic tools like OBD Auto Doctor often don’t strictly adhere to this older classification. It’s more important to understand that some monitors run constantly, while others need specific driving patterns to activate.

Non-Continuous Monitors: Gasoline vs. Diesel

The specific non-continuous monitors present in your vehicle can also depend on whether you have a gasoline (spark ignition) or diesel (compression ignition) engine.

For Spark Ignition Vehicles (Gasoline):

Catalyst (CAT) Monitor: Checks the efficiency of the catalytic converter in reducing harmful emissions.
Heated Catalyst Monitor: Evaluates the performance of the heated catalytic converter, which warms up faster for better cold-start emission control.
Evaporative (EVAP) System Monitor: Tests the evaporative emissions control system, which prevents fuel vapors from escaping into the atmosphere.
Secondary Air System Monitor: Monitors the secondary air injection system, used to reduce emissions during cold starts.
Oxygen (O2) Sensor Monitor: Checks the functionality of the oxygen sensors, crucial for proper fuel mixture and catalytic converter efficiency.
Oxygen Sensor Heater Monitor: Evaluates the heaters for the oxygen sensors, ensuring they reach operating temperature quickly.
EGR (Exhaust Gas Recirculation) and/or VVT System Monitor: Monitors the Exhaust Gas Recirculation system, which reduces NOx emissions, and Variable Valve Timing (VVT) systems, which improve engine efficiency and reduce emissions.

For Compression Ignition Vehicles (Diesel):

NMHC Catalyst Monitor: Checks the Non-Methane Hydrocarbon Catalyst, used to reduce hydrocarbon emissions in diesel engines.
NOx/SCR Aftertreatment Monitor: Monitors the NOx reduction system, often using Selective Catalytic Reduction (SCR) with diesel exhaust fluid (DEF), to reduce nitrogen oxide emissions.
Boost Pressure Monitor: Checks the boost pressure control system in turbocharged diesel engines, which affects engine efficiency and emissions.
Exhaust Gas Sensor Monitor: Monitors various exhaust gas sensors specific to diesel engines, such as NOx sensors or particulate matter sensors.
PM Filter Monitor: Evaluates the Diesel Particulate Filter (DPF), which traps and burns soot particles to reduce particulate matter emissions.
EGR and/or VVT System Monitor: Similar to gasoline engines, diesel engines also utilize EGR and VVT systems for emission control and efficiency.

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

OBD2 systems have evolved over time, and so have the ways they report monitor status. Traditionally, the primary monitor status reported was the status since the diagnostic trouble codes (DTCs) were last cleared. This “since DTC reset” status is mandatory for all OBD2 compliant vehicles and reflects the long-term readiness of the emission systems after any resets, such as after repairs or clearing the “check engine light”.

Newer vehicles often offer an additional feature: reporting emission monitor status for the current driving cycle, also known as “this drive cycle”. These monitors reset and start anew every time a new driving cycle begins (typically when you start the car). This feature provides a more immediate snapshot of monitor status. Older cars may not support “this drive cycle” monitoring. In such cases, OBD2 diagnostic tools will often display “NA” or “Not Available” for this status.

Understanding Monitor Status: Complete, Incomplete, Disabled, and N/A

Each readiness monitor will report one of several possible statuses, indicating the outcome of its self-test:

Complete (Ready): This status, often indicated by a green checkmark in diagnostic tools like OBD Auto Doctor, signifies that the monitor has successfully run its self-test and the emission control system has passed. This is the desired status for passing emissions inspections.
Incomplete (Not Ready): Indicated by a red exclamation mark, this status means that the monitor has not yet completed its self-test. This could be because the necessary driving conditions haven’t been met, or there might be an underlying issue preventing the test from running or passing.
Disabled: A monitor can be temporarily disabled for the current monitoring cycle under certain conditions. For example, extreme ambient temperatures might prevent certain monitors from running reliably. This is less common but can occur.
Not Available (N/A): This status indicates that the vehicle does not support a particular monitor. Not all vehicles are required to support every single monitor defined in the OBD2 standard. If a monitor is listed as N/A, it simply means that specific test is not applicable to your car.

Alt text: OBD2 readiness monitor status displayed on an Android phone app, showing “Not Available” for “current drive cycle” monitors, indicating the vehicle may not support this feature.

Why Are Monitors Incomplete or “Not Ready”?

There are several common reasons why your OBD2 readiness monitors might show an “incomplete” or “not ready” status:

Recent DTC Clearing: The most frequent cause is clearing diagnostic trouble codes (DTCs) and resetting the check engine light. When DTCs are cleared, all readiness monitor statuses are also reset to “incomplete”. This is a standard procedure, and the monitors need to rerun their tests after a reset. This often happens after vehicle repairs.
Battery Disconnection: Disconnecting your car battery, even for a short time, can also reset the readiness monitor statuses. Power loss can erase the stored monitor data, forcing them to restart their tests.
Normal Start of Drive Cycle: For the “this drive cycle” monitors, it’s perfectly normal to see an “incomplete” status immediately after starting your engine. These monitors begin their tests at the start of each new drive cycle, and it takes time and driving for them to complete.
Driving Conditions Not Met: As mentioned earlier, non-continuous monitors require specific driving conditions to run. If your recent driving hasn’t included the necessary conditions (speed, acceleration, temperature, etc.), these monitors might remain incomplete.

Implications for Vehicle Inspections: It’s crucial to understand that in many regions, your vehicle may fail its annual emissions inspection if the required readiness monitors are not in the “complete” or “ready” status. Regulations vary, but the US EPA guidelines, for example, allow for a certain number of “not ready” monitors depending on the vehicle model year. Generally, newer vehicles have stricter requirements, often allowing only one or even zero monitors to be incomplete.

Getting Your Monitors “Ready”: The OBD2 Drive Cycle

The best way to get your OBD2 readiness monitors to a “complete” status is simply to drive your car. However, just any type of driving might not be sufficient. Monotonous driving or only short trips might not trigger all the necessary conditions for all monitors to run. This is where the concept of an OBD2 drive cycle comes in.

An OBD2 drive cycle is a specific, pre-defined driving pattern designed to provide the conditions necessary for all (or most) of your car’s readiness monitors to run their self-tests. The exact drive cycle for your vehicle can vary depending on the make, model, and year. Some vehicle manufacturers include specific drive cycle instructions in the owner’s manual.

If you don’t have a specific drive cycle for your car, a generic OBD2 drive cycle can often be used as a guideline. Keep in mind that a generic drive cycle might not work for all vehicles or all monitors, but it’s a good starting point. It’s also crucial to perform any drive cycle in a safe and legal area, away from public roads if possible, as some steps may involve specific speeds and accelerations.

Before attempting a drive cycle, ensure the following:

No Check Engine Light (MIL): Make sure the Malfunction Indicator Light (MIL), or check engine light, is not illuminated. Active or pending DTCs can prevent monitors from running to completion.
Sufficient Fuel: Some monitors, like the EVAP monitor, require a certain fuel level (often between 35% and 85%) to initiate their tests.

A Common Generic OBD2 Drive Cycle:

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°°F (6°C) of each other. This usually means letting the car sit overnight.
Ignition ON, Engine OFF (Briefly): Turn the ignition key to the “ON” position for a few seconds before starting the engine. This can help the heated oxygen sensor diagnostic to run properly.
Idle in Drive (2.5 minutes): Start the engine and let it idle in “Drive” (for automatic transmissions) or neutral with the clutch engaged (manual transmissions) for two and a half minutes. Turn on the A/C and rear defroster if equipped to add load to the engine.
Moderate Acceleration and Steady Speed (3 minutes at 55 mph): Turn off the A/C and rear defroster. Accelerate moderately and smoothly to 55 mph (90 km/h) and maintain a steady speed for three minutes.
Deceleration (No Braking): Slow down to 20 mph (30 km/h) without using the brakes. For manual transmissions, do not depress the clutch during deceleration.
Moderate to Heavy Acceleration and Steady Speed (5 minutes at 55-60 mph): Accelerate again, this time using about 3/4 throttle, to 55-60 mph (90-100 km/h) and maintain a steady speed for five minutes.
Stop (No Braking): Slow down to a complete stop without using the brakes.

After completing a drive cycle, you can use an OBD2 scanner or diagnostic software to check your readiness monitor status. You may need to repeat the drive cycle or portions of it if some monitors are still incomplete. It can sometimes take several drive cycles and a week or so of normal combined city and highway driving for all monitors to reach a “ready” status.

Get Prepared for Your Emissions Inspection

Don’t wait until your annual vehicle inspection to discover your car isn’t ready! Proactive preparation is key. Before your inspection, use an OBD2 scanner or software like OBD Auto Doctor to:

Check Readiness Monitors: Verify that all required monitors are in the “complete” status.
Check for DTCs: Read and clear any diagnostic trouble codes. Address any underlying issues that are causing DTCs to set.

By taking these steps, you can significantly increase your chances of passing your emissions inspection and avoid potential rejection and costly re-inspections. Remember, addressing emission issues promptly not only helps you pass inspections but also contributes to cleaner air and can improve your car’s fuel efficiency and longevity. Why not try checking your readiness monitors today with a free version of OBD Auto Doctor and be prepared?

Editor’s note: This article was updated in 2023 for accuracy and to include the latest OBD2 information.