Pass Emissions with Ease: OBD Scanners & Readiness Monitors Explained

Ensuring your vehicle is environmentally compliant and passes emissions testing is a crucial part of car ownership. A key aspect of this is understanding OBD2 readiness monitors. These monitors are your car’s built-in self-check system for emissions control. This guide will delve into what readiness monitors are, why they matter, and how an Obd Scanner With Readiness check can help you navigate emissions testing with confidence.

Decoding OBD2 Readiness Monitors

Readiness monitors, also known as Emissions Monitors, are diagnostic routines within your car’s On-Board Diagnostics (OBD2) system. Their primary function is to continuously evaluate the performance of your vehicle’s emission control systems. Think of them as silent guardians, constantly checking to ensure your car is running cleanly and efficiently.

Modern vehicles can perform up to 11 different system tests, each represented by a readiness monitor. The result of these monitor checks indicates whether your car’s computer has successfully completed the self-tests. This information is vital for both vehicle maintenance and emissions compliance.

Continuous vs. Non-Continuous Monitors: Understanding the Difference

Readiness monitors are broadly classified into two categories: continuous and non-continuous. This distinction lies in how and when these monitors perform their checks.

Continuous Monitors: These monitors operate constantly whenever the engine is running. They are designed for systems that require ongoing evaluation. 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 reduced emissions.
Comprehensive Components Monitor: Oversees a wide array of engine and emission-related components for proper function.

Non-Continuous Monitors: Unlike their continuous counterparts, non-continuous monitors require specific conditions to be met before they can run their diagnostic tests. These conditions, often referred to as “enabling criteria,” vary depending on the monitor and the vehicle. Some monitors might require a specific driving pattern, known as a drive cycle, while others may need temperature thresholds to be reached.

The categorization of monitors as strictly continuous or non-continuous has become less rigid in the latest OBD2 standards (SAE J1979). Manufacturers have flexibility in implementation. Therefore, while the general concepts remain, the specific categorization may not always be definitively applied.

Non-Continuous Monitors: Gasoline vs. Diesel Vehicles

The suite of non-continuous monitors differs between spark ignition (gasoline) and compression ignition (diesel) engines, reflecting the distinct emission control technologies employed in each.

Spark Ignition Vehicles (Gasoline):

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

Compression Ignition Vehicles (Diesel):

NMHC Catalyst Monitor: Evaluates the Non-Methane Hydrocarbon (NMHC) catalytic converter used in diesel engines.
NOx/SCR Aftertreatment Monitor: Monitors the Selective Catalytic Reduction (SCR) system, crucial for reducing Nitrogen Oxides (NOx) emissions from diesel engines.
Boost Pressure Monitor: Checks the boost pressure control system in turbocharged diesel engines, impacting engine efficiency and emissions.
Exhaust Gas Sensor Monitor: Assesses various exhaust gas sensors specific to diesel engines, ensuring proper emission control.
PM Filter Monitor: Monitors the Particulate Matter (PM) filter (diesel particulate filter – DPF), which traps soot particles from diesel exhaust.
EGR and/or VVT System Monitor: Similar to gasoline engines, checks EGR and VVT systems for emission control and engine optimization.

Alt text: OBD scanner software interface displaying readiness monitor status on a Windows computer, showing both complete (green check marks) and incomplete (red exclamation marks) statuses for various monitors.

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

OBD2 systems track readiness monitor status in different ways, reflecting the evolution of OBD standards. Understanding these monitoring cycles is key to interpreting readiness information correctly.

Monitor Status Since Diagnostic Trouble Code (DTC) Reset: This is the traditional and mandatory monitor status for all OBD2 compliant vehicles. It represents the long-term status of the monitors since the last time diagnostic trouble codes (DTCs) were cleared and the check engine light was reset. This status is crucial for emissions inspections.

Monitor Status for the Current Driving Cycle (“This Drive Cycle”): Newer vehicles often offer an additional layer of readiness information: the monitor status for the current driving cycle. These monitors initiate their tests at the beginning of each new driving cycle (typically when the engine is started). This real-time status provides immediate feedback. Older vehicles may not support this “this drive cycle” feature, in which case an OBD scanner with readiness check will typically indicate “N/A” (Not Available).

Decoding Monitor Status: Complete, Incomplete, Disabled, Not Available

Each readiness monitor reports a specific status, providing a clear picture of the emission control system’s self-test results. Common monitor statuses you’ll encounter when using an OBD scanner with readiness check include:

Complete (Ready): Indicates that the monitor has run its diagnostic test and the emission control system has passed. This is the desired status for passing emissions inspections. OBD scanning tools often represent this with a green check mark.
Incomplete (Not Ready): Signifies that the monitor has not yet completed its diagnostic test. This doesn’t necessarily mean there’s a problem, but rather that the required conditions for the test haven’t been met. OBD scanners often display this with a red exclamation mark.
Disabled: Means the monitor has been temporarily disabled for the current monitoring cycle. This can occur when certain conditions prevent the monitor from running, such as extreme ambient temperatures that are outside the operational range for a specific test.
Not Available (N/A): Indicates that the vehicle does not support a particular monitor. Not all vehicles are required to support every possible monitor defined in the OBD2 standard.

It’s important to note that an OBD scanner with readiness function will only display statuses for the monitors that your specific vehicle supports.

Alt text: Screenshot of an OBD2 Android app interface displaying readiness monitor status, highlighting the “This Drive Cycle” section and indicating “Vehicle doesn’t support readiness monitors for this drive cycle,” with N/A statuses shown.

Why Are Monitors Incomplete or “Not Ready”?

Seeing “Incomplete” or “Not Ready” statuses on your OBD scanner with readiness check can be concerning, but it’s often a normal occurrence, especially in certain situations:

Recent DTC Clearing or Check Engine Light Reset: Clearing diagnostic trouble codes (DTCs) or resetting the check engine light will always reset all readiness monitor statuses to “Incomplete.” This is because clearing codes erases the history of completed tests.
Battery Disconnection: Power loss, such as disconnecting the car battery, can also reset monitor statuses to “Incomplete.” This is why disconnecting the battery unnecessarily is generally discouraged, particularly before an emissions test.
New Monitoring Cycle: For “this drive cycle” monitors, the status will initially be “Incomplete” at the start of each new driving cycle (engine start). This is expected behavior as the monitors begin their self-testing process.

However, persistent “Incomplete” statuses, especially for monitors required for emissions testing in your region, can prevent your vehicle from passing inspection. Regulations vary, but in many areas, a certain number of monitors (typically one or two, depending on vehicle model year) can be “Not Ready” and still pass. Exceeding this limit will result in failure.

Getting Monitors “Complete” or “Ready”: The Drive Cycle and More

If your OBD scanner with readiness check reveals incomplete monitors, especially before an emissions test, you’ll need to get them to a “Complete” status. The most effective way to achieve this is by driving your car. However, simply driving around randomly may not be sufficient.

Here are key steps to get your monitors ready:

Address Check Engine Light and DTCs: Ensure the Malfunction Indicator Light (MIL), or check engine light, is not illuminated. Active or pending diagnostic trouble codes can prevent monitors from running to completion. Use your OBD scanner to check for and address any DTCs.
Fuel Level: Certain monitors, such as the EVAP monitor, have fuel level requirements. Typically, maintaining a fuel level between 35% and 85% of tank capacity is recommended for optimal monitor operation.
Complete an OBD Drive Cycle: The most structured approach to set readiness monitors is to perform an OBD drive cycle. This is a specific driving pattern designed to fulfill the enabling criteria for various non-continuous monitors.

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

The precise drive cycle for your vehicle can vary depending on the make, model, and the specific monitor you’re trying to set. Vehicle owner’s manuals sometimes include drive cycle information. However, a generic OBD-II drive cycle can often be effective, especially when a vehicle-specific cycle isn’t available.

Important Safety Note: Drive cycles often involve specific speeds and maneuvers that may be difficult or unsafe to perform in normal traffic. It’s strongly recommended to perform drive cycles in a restricted area where it’s safe to execute the required driving patterns.

Here’s a commonly used generic OBD-II drive cycle guideline:

Cold Start: Begin with a cold start. 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. Allowing the car to sit overnight typically achieves these conditions.
Ignition Key Position: Do not leave the ignition key in the “ON” position before the cold start, as this can interfere with the heated oxygen sensor diagnostic.
Idling: 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 idling period.
Moderate Acceleration & Steady Speed (Part 1): Turn off the A/C and rear defroster. Accelerate moderately and steadily to 55 mph (90 km/h). Maintain this speed for three minutes.
Deceleration: Slow down to 20 mph (30 km/h) without using the brakes. If driving a manual transmission vehicle, do not depress the clutch during deceleration.
Moderate Acceleration & Steady Speed (Part 2): Accelerate again to 55-60 mph (90-100 km/h) at approximately 3/4 throttle. Maintain this speed for five minutes.
Stop: Decelerate to a complete stop without using the brakes.

Repeating this generic drive cycle, combined with a mix of normal city and highway driving over a few days to a week, is often sufficient to set most readiness monitors. Use your OBD scanner with readiness check after performing drive cycles and normal driving to monitor the status of your monitors.

Prepare for Emissions Inspection: Your OBD Scanner is Key

To avoid failing your vehicle emissions inspection, proactive preparation is essential. Using an OBD scanner with readiness check is a simple yet powerful step you can take.

Before heading to the inspection station, use your OBD scanner to:

Check Readiness Monitor Status: Verify that all required monitors for your vehicle and region are “Complete” or “Ready.”
Scan for Diagnostic Trouble Codes (DTCs): Ensure there are no stored or pending DTCs. Even if the check engine light isn’t on, DTCs can still cause an inspection failure.

By performing these checks with an OBD scanner, you can identify potential issues before the official inspection, saving you time, potential rejection, and the hassle of re-testing.

Investing in an OBD scanner with readiness capability is a smart move for any car owner. It empowers you to understand your vehicle’s emission system status, address potential problems proactively, and approach emissions testing with confidence. Explore the range of OBD scanners available and take control of your vehicle’s health and environmental compliance today.