Understanding OBD2 Readiness Monitors: Your Guide to Emission Testing

Readiness Monitors, often referred to as Emissions Monitors, are crucial self-diagnostic checks within your car’s On-Board Diagnostics system (OBD2). As an essential part of your vehicle’s emission control system, these monitors perform self-tests to ensure that your car is running cleanly and efficiently. This article, brought to you by the automotive experts at obd-de.com, will delve into the world of OBD2 readiness monitors, explaining what they are, why they matter, and how to ensure they are “ready” for your next vehicle inspection.

The primary function of readiness monitors is to evaluate the effectiveness of your vehicle’s emission control systems. Think of them as diligent inspectors, constantly observing the performance of emission-related components. Your car might conduct up to 11 different system tests, each known as a readiness monitor. The result of these monitors provides valuable information about whether your car’s computer has successfully completed these tests. A “ready” status generally indicates that the emission system is functioning as expected.

Types of Readiness Monitors: Continuous and Non-Continuous

Readiness monitors are broadly categorized into two types: continuous and non-continuous. This distinction lies in how and when these tests are performed.

Continuous monitors operate constantly while your engine is running. They are designed to detect issues in real-time. Conversely, non-continuous monitors require specific operating conditions to be met before a test can be initiated and completed.

These conditions for non-continuous tests can vary significantly. Some monitors need the vehicle to undergo a specific drive cycle – a predefined driving pattern designed to activate the monitor. Certain monitors might even require two drive cycles to complete, incorporating cool-down and warm-up phases. Each emission monitor can have unique requirements for these conditions, depending on what system it is testing.

Historically, the OBD2 standard (SAE J1979) explicitly classified each monitor as either continuous or non-continuous. However, the latest revisions of the standard have moved away from this strict categorization for some monitors. Modern OBD2 diagnostic tools, like OBD Auto Doctor, also reflect this change and may not always adhere to the old classifications.

Examples of Monitor Types

It’s important to note that the classification of some monitors can be at the discretion of the vehicle manufacturer.

Monitors that can be either Continuous or Non-continuous:

Misfire Monitor: Detects engine misfires that can increase emissions.
Fuel System Monitor: Checks the fuel delivery and control system for proper operation.
Comprehensive Component Monitor: Monitors various electrical components and sensors for malfunctions.

Non-Continuous Monitors:

The specific non-continuous monitors differ based on whether your vehicle has a spark ignition engine (gasoline) or a compression ignition engine (diesel).

For Spark Ignition Vehicles (Gas):

Catalyst (CAT) Monitor: Evaluates the efficiency of the catalytic converter in reducing emissions.
Heated Catalyst Monitor: Tests the performance of heated catalytic converters, which reach operating temperature faster.
Evaporative (EVAP) System Monitor: Checks for leaks in the fuel vapor recovery system.
Secondary Air System Monitor: Monitors the secondary air injection system, which helps reduce emissions during cold starts.
Oxygen (O2) Sensor Monitor: Tests the functionality of oxygen sensors, crucial for fuel mixture control.
Oxygen Sensor Heater Monitor: Checks the heaters for the oxygen sensors, ensuring they reach operating temperature quickly.
EGR (Exhaust Gas Recirculation) and/or VVT System Monitor: Evaluates the exhaust gas recirculation system and variable valve timing system, both aimed at reducing NOx emissions.

For Compression Ignition Vehicles (Diesel):

NMHC Catalyst Monitor: Monitors the Non-Methane Hydrocarbon catalyst used in diesel engines.
NOx/SCR Aftertreatment Monitor: Checks the Selective Catalytic Reduction (SCR) system and NOx reduction technologies.
Boost Pressure Monitor: Evaluates the boost pressure system in turbocharged diesel engines.
Exhaust Gas Sensor Monitor: Tests various exhaust gas sensors specific to diesel engines.
PM Filter Monitor: Monitors the Particulate Matter filter (diesel particulate filter – DPF) for proper function.
EGR and/or VVT System Monitor: Similar to gasoline engines, this monitors EGR and VVT systems for diesel applications.

Image: Example of OBD readiness monitors displayed on a Windows computer using diagnostic software, showing monitor status.

Understanding Monitoring Cycles: Since Reset and This Drive Cycle

OBD2 systems track monitor status in different ways. Traditionally, the primary monitor status reported was the status since the diagnostic trouble codes (DTCs) were last cleared. This “since reset” status is mandatory for all OBD2 compliant vehicles. It reflects the long-term status of the monitors after events like clearing the check engine light or resetting DTCs.

Modern vehicles often offer an additional monitor status: the status for the current driving cycle. These “this drive cycle” monitors reset and begin testing every time a new driving cycle starts (typically when you start your car). This provides a more immediate snapshot of monitor status. Older vehicles may not support “this drive cycle” monitoring. In such cases, OBD Auto Doctor and similar tools will typically indicate the status as “NA” or “Not Available”.

Interpreting Monitor Status: Complete, Incomplete, and Disabled

The outcome of a readiness monitor test is its status. Each monitor reports its individual status, which can be one of the following:

Complete (Ready): This indicates that the monitor test has been successfully completed, and the emission control system has passed the test. OBD Auto Doctor typically uses a green check mark to signify a “complete” status.
Incomplete (Not Ready): This means the monitor test has not yet been finished. This could be because the OBD2 system hasn’t had the opportunity to run the test, or the test has failed. OBD Auto Doctor often uses a red exclamation mark to indicate an “incomplete” status.
Disabled: This status signifies that the monitor test has been intentionally disabled for the current monitoring cycle. Monitors might be disabled under certain conditions where it’s not feasible 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 of the monitor’s operating range.
Not Available (NA): This status, as mentioned earlier, means the vehicle does not support a particular monitor. It’s not mandatory for every vehicle to support all defined monitors, so “NA” simply indicates that the specific monitor is not applicable to that vehicle.

Image: Readiness monitors as seen on an Android app interface, illustrating a scenario where “readiness monitors for this drive cycle” are not supported by the vehicle.

Reasons for an Incomplete or “Not Ready” Monitor Status

Seeing an “incomplete” or “not ready” status can be concerning, especially when preparing for a vehicle inspection. Here are common reasons why monitors might be incomplete:

Recent DTC Clearing: Clearing diagnostic trouble codes (DTCs) and turning off the Check Engine Light will always reset all readiness monitor statuses to “incomplete.” This is a standard procedure and often occurs after vehicle repairs.
Battery Disconnection: Power loss, such as disconnecting the car battery, can also reset monitor statuses. This is why disconnecting the battery unnecessarily is generally discouraged. If battery disconnection is required (e.g., for replacement), you’ll need to allow the monitors to run again to achieve a “complete” status.
New Monitoring Cycle: For “this drive cycle” monitors, it’s normal for them to be “incomplete” immediately after starting the engine, as the tests haven’t run yet in the current driving session.

It’s crucial to understand the implications of “not ready” monitors for vehicle inspections. Depending on your location and regulations, your vehicle might fail an emissions test if too many monitors are “not ready.” For instance, in the United States, EPA guidelines allow for up to two “not ready” monitors for 1996-2000 model year vehicles, and only one for 2001 and newer models.

Getting Monitors to “Complete” or “Ready” Status

The most effective way to set readiness monitors to “complete” is by driving your car. Since these are self-check routines, normal vehicle operation allows the car to run the necessary tests. However, simply driving without a plan might not activate all monitors. This is where the concept of an OBD drive cycle becomes relevant.

Before delving into drive cycles, consider these preliminary 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 DTCs first.
Fuel Level: Make sure you have adequate fuel. Some monitors, especially the EVAP monitor, require the fuel tank to be between 35% and 85% full to initiate testing.
Complete a Drive Cycle: For many monitors, a period of combined city and highway driving – approximately one week of normal use – is often sufficient to allow them to reach a “complete” status. A more structured approach is following a specific OBD drive cycle.

The OBD Drive Cycle: A Structured Approach

An OBD2 drive cycle is a specific driving pattern designed to trigger the on-board diagnostics and enable readiness monitors to run their tests. The precise drive cycle for your vehicle can vary depending on the make, model, and even the specific monitor in question.

Many vehicle manufacturers now include the recommended drive cycle in the owner’s manual. If a specific drive cycle isn’t available, a generic drive cycle can be used as a guideline. Keep in mind that generic cycles might not work for all vehicles or monitors.

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

It’s important to perform drive cycles in a safe and legal environment, as some steps might involve specific speeds and accelerations that are not suitable for normal traffic.

Cold Start: Begin with a cold start. The engine coolant temperature should be below 50°C (122°F), and the ambient air temperature and coolant temperature should be within 11°C (20°F) of each other. Allowing the car to sit overnight typically achieves this.
Ignition Timing: Do not leave the ignition key in the “ON” position before the cold start. This could prevent the heated oxygen sensor diagnostic from running correctly.
Idle (2.5 minutes): Start the engine and let it idle in drive (or in gear for manual transmissions) for two and a half minutes. Turn on the air conditioning (A/C) and rear defroster if equipped to increase engine load.
Moderate Acceleration and Steady Speed (3 minutes): Turn off the A/C and rear defroster. Accelerate moderately and steadily to 90 km/h (55 mph). Maintain this constant speed for three minutes.
Deceleration (No Braking): Slow down to 30 km/h (20 mph) without using the brakes. If you have a manual transmission, do not depress the clutch during deceleration.
Moderate to Heavy Acceleration and Steady Speed (5 minutes): Accelerate again to 90-100 km/h (55-60 mph) using approximately 3/4 throttle. Hold this steady speed for five minutes.
Stop (No Braking): Decelerate to a complete stop without using the brakes.

Preparing for Your Vehicle Inspection

To avoid failing your annual vehicle inspection due to emission readiness issues, proactive preparation is key. Before your inspection, use an OBD2 scanner or diagnostic software like OBD Auto Doctor to:

Read Readiness Monitors: Check the status of all readiness monitors. Ensure they are mostly “complete.”
Read Diagnostic Trouble Codes (DTCs): Check for and resolve any diagnostic trouble codes. Even pending codes can cause issues.

Taking these steps beforehand can prevent a failed inspection and save you time and potential repair costs. Remember, addressing emission issues promptly can also improve fuel economy and reduce future repair expenses. Try OBD Auto Doctor today to check your monitor status and DTCs – even the free version offers these essential features. Don’t wait until inspection time to address potential problems!

Editor’s note: This article was updated in March 2020 to ensure accuracy and reflect the latest information on OBD2 readiness monitors.