Understanding OBD2 Readiness Monitors: Your Guide to Vehicle Emission Testing

Ensuring your vehicle is environmentally compliant and passes emission tests is a crucial part of car ownership. A key component in this process is understanding OBD2 readiness monitors. These self-diagnostic checks are built into your car’s computer system to monitor the emission control systems. This guide, brought to you by the automotive experts at carparteu.com, will delve into the intricacies of readiness monitors, explaining what they are, why they matter, and how to ensure they are “ready” for your next vehicle inspection.

What are OBD2 Readiness Monitors?

Readiness monitors, also known as emissions monitors, are essentially self-tests performed by your car’s onboard computer. Their primary purpose is to evaluate the performance of your vehicle’s emission control systems. Think of them as routine health checks for your car’s environmental systems. Modern vehicles can conduct up to 11 different system tests, each designed to scrutinize a specific part of the emission control apparatus. The outcome of these readiness monitor tests provides valuable information about whether your car has successfully completed these self-assessments.

Types of Readiness Monitors: Continuous and Non-Continuous

OBD2 readiness monitors are categorized into two main types: continuous monitors and non-continuous monitors. The fundamental difference lies in how they are tested.

Continuous Monitors: These monitors operate constantly while your engine is running. They are designed for systems that require ongoing evaluation.
Non-Continuous Monitors: These monitors, conversely, require specific conditions to be met before they can initiate and complete their tests. These conditions can vary significantly depending on the specific monitor and the vehicle manufacturer.

The conditions needed to trigger non-continuous monitor tests can range from following a specific drive cycle to ensuring the car undergoes cool-down and warm-up periods. Historically, the OBD2 standard (SAE J1979) explicitly classified each monitor as either continuous or non-continuous. However, the latest OBD2 standards have moved away from this rigid categorization for some monitors, allowing manufacturers more flexibility in implementation. Therefore, modern OBD diagnostic tools like OBD Auto Doctor often reflect this updated approach and may not strictly adhere to the old categorization.

Continuous or Non-Continuous: Manufacturer’s Choice

Interestingly, some monitors can be implemented as either continuous or non-continuous, depending on the vehicle manufacturer’s design choices. These monitors include:

Misfire Monitor: Detects engine misfires that can increase emissions.
Fuel System Monitor: Checks the fuel delivery and metering system for proper operation.
Comprehensive Component Monitor: Monitors a wide array of engine and emission-related components for malfunctions.

Non-Continuous Monitors: Gasoline vs. Diesel Engines

Non-continuous monitors differ depending on whether your vehicle has a spark ignition engine (gasoline) or a compression ignition engine (diesel).

Spark Ignition Vehicles (Gasoline Engines)

Catalyst (CAT) Monitor: Evaluates the efficiency of the catalytic converter in reducing emissions.
Heated Catalyst Monitor: Checks the performance of heated catalytic converters, which reach optimal operating temperature faster.
Evaporative (EVAP) System Monitor: Tests the evaporative emissions control system to prevent fuel vapor release.
Secondary Air System Monitor: Monitors the secondary air injection system, which helps reduce emissions during cold starts.
Oxygen (O2) Sensor Monitor: Assesses the functionality of oxygen sensors that measure oxygen levels in the exhaust gas.
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: Monitors the exhaust gas recirculation system and/or variable valve timing system, which reduce NOx emissions.

Compression Ignition Vehicles (Diesel Engines)

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

Alt text: A screenshot of OBD Auto Doctor software on a Windows laptop, displaying readiness monitor statuses with green checkmarks for ‘Complete’ monitors and red exclamation marks for ‘Incomplete’ monitors.

Monitoring Cycles: Since Reset and Current Drive Cycle

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

However, more advanced vehicles now also report emission monitor status for the current driving cycle. These monitors initiate a new test cycle every time a new driving cycle begins (typically when you start your car). Older vehicles may not support this feature. In such cases, OBD Auto Doctor and similar tools will indicate the status as “NA” or “Not Available.”

Understanding Monitor Status: Complete, Incomplete, Disabled

Each readiness monitor test culminates in a monitor status, indicating the outcome of the self-test. The possible status outcomes are:

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 represents this status with a green checkmark.
Incomplete (Not Ready): This status signifies that the monitor test has not yet been completed. This could be because the OBD2 system hasn’t had the opportunity to run the test or because the test failed. OBD Auto Doctor usually displays this with a red exclamation mark.
Disabled: In some situations, a monitor might be temporarily disabled for the current monitoring cycle. This can happen when specific conditions prevent the monitor from running, such as extreme ambient air temperatures that are outside the operational range for the test.

It’s important to note that while OBD Auto Doctor lists all defined monitors, your specific vehicle may not support all of them. A status of “NA” or “Not Available” simply means that the car doesn’t have that particular monitor, and therefore it is not tested or reported.

Alt text: An Android smartphone screen showing the OBD Auto Doctor app displaying readiness monitor statuses. The “This Drive Cycle” tab is selected, and all monitors are marked as “NA” (Not Available), indicating the vehicle does not support current drive cycle readiness monitor reporting.

Why is a Monitor Incomplete or “Not Ready”?

There are several reasons why a readiness monitor might show an “Incomplete” or “Not Ready” status:

Recent DTC Clearing: Clearing diagnostic trouble codes (DTCs) and resetting the Check Engine Light will also reset all readiness monitor statuses to “Incomplete.” This commonly occurs after vehicle repairs or during diagnostic procedures.
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 battery replacement), you’ll need to perform a drive cycle to reset the monitors.
Current Monitoring Cycle: For monitors tracking the “current drive cycle,” the status will initially be “Incomplete” when you start the engine and begin a new driving cycle. This is normal, as the monitors need time and specific driving conditions to complete their tests.

For vehicle inspections, especially in regions like the US, OBDII regulations often stipulate that vehicles must have a certain number of “Complete” monitors to pass. For example, in the US, EPA guidelines permit up to two “Not Ready” monitors for 1996-2000 model year vehicles, and only one “Not Ready” monitor for 2001 and newer vehicles. Failing to meet these readiness monitor requirements can result in inspection failure.

Getting Monitors to “Complete” or “Ready”: The Drive Cycle

Since readiness monitors are self-check routines, the most effective way to get them to a “Complete” status is to drive your car. However, simply driving without a specific pattern may not fulfill the conditions required for all monitors to run. This is where the concept of an OBD drive cycle comes into play.

Before initiating a drive cycle, consider these preliminary steps:

Check for Check Engine Light and DTCs: Ensure the Malfunction Indicator Light (MIL), or Check Engine Light, is not illuminated. Active stored or pending diagnostic trouble codes can prevent monitors from completing. Address any DTCs before proceeding.
Fuel Level: Verify you have sufficient fuel. Some monitors, particularly the EVAP monitor, require the fuel tank to be between 35% and 85% full to initiate testing.
Normal Driving: For many vehicles, a week of typical combined city and highway driving is often sufficient to allow all monitors to reach a “Complete” status.

If normal driving isn’t enough, or if you need to expedite the process, you can perform a specific OBD drive cycle.

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

The purpose of an OBD2 drive cycle is to create the precise conditions needed for your car’s on-board diagnostics to run, allowing the readiness monitors to operate and detect any potential emission system malfunctions. The exact drive cycle for your vehicle can vary depending on the make, model, and the specific monitor in question.

Many vehicle manufacturers now include drive cycle information in the owner’s manual. If a specific drive cycle for your car is unavailable, the following generic OBD-II drive cycle can serve as a guideline. However, it’s crucial to understand that this generic cycle may not work for all vehicles and all monitors. It’s also important to perform this drive cycle in a safe, restricted area due to the driving maneuvers involved.

Generic OBD-II Drive Cycle:

Cold Start: Begin with a cold start. The coolant temperature should be below 50°C (122°F), and the coolant and air temperatures should be within 11°C (20°F) 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 may prevent the heated oxygen sensor diagnostic from running.
Idle in Drive (2.5 minutes): Start the engine and idle in drive for two and a half minutes. Turn on the 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 steady 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 Acceleration and Steady Speed (5 minutes): Accelerate again to 90-100 km/h (55-60 mph) at approximately 3/4 throttle. Hold this steady speed for five minutes.
Stop (No Braking): Slow down 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, it’s highly recommended to proactively check your readiness monitors beforehand. Using an OBD2 diagnostic tool like OBD Auto Doctor, you can easily read your readiness monitor status and confirm they are “ready.” This simple step can prevent a near-certain rejection at the inspection station.

Furthermore, it’s wise to also read diagnostic trouble codes before your inspection to ensure there are no active DTCs that could cause a failure. OBD Auto Doctor, even the free version, allows you to read both monitor statuses and DTCs. Why not try the software right now to assess your vehicle’s readiness?

Remember, addressing potential emission issues promptly, rather than waiting until your inspection, can save you significant time, prevent future repairs, and even improve fuel efficiency.

Editor’s note: This guide was updated in October 2024 to ensure accuracy and reflect the latest OBD2 information.