Understanding OBD2 Tests: Readiness Monitors Explained for Car Diagnostics

OBD2 (On-Board Diagnostics II) systems are integral to modern vehicles, acting as a self-diagnostic powerhouse that keeps your car running efficiently and cleanly. A crucial part of this system is the OBD2 readiness monitors. These simple yet powerful self-check routines provide valuable insight into your car’s emission control systems. Often referred to as emission monitors, they essentially perform a series of Obd2 Tests to ensure everything is functioning as it should. This guide will delve into the details of readiness monitors, explaining what they are, how they work, and why understanding them is beneficial for every car owner.

What are OBD2 Readiness Monitors?

The primary purpose of readiness monitors is to conduct self-tests on your vehicle’s emission control systems. Think of them as vigilant observers, constantly monitoring the performance of emission-related components. Your car might perform up to 11 different system tests, each known as a readiness monitor. The outcome of these OBD2 tests reveals whether your car’s computer has successfully completed these diagnostic checks. In essence, readiness monitors give you a snapshot of your car’s emission system health through a series of automated OBD2 tests.

Types of OBD2 Monitors: Continuous vs. Non-Continuous

Readiness monitors are broadly categorized into two types: continuous and non-continuous. These categories differ significantly in their testing approach. Continuous monitors are designed for ongoing assessment. They are constantly running and evaluating system performance whenever the engine is operating. On the other hand, non-continuous monitors are more conditional. They require specific driving conditions to be met before a test can be initiated and completed.

The precise conditions needed to trigger non-continuous self-diagnostic tests vary depending on the specific monitor and the vehicle manufacturer. Some monitors necessitate following a predefined drive cycle – a specific sequence of driving maneuvers. Some may even require two drive cycles due to the need for cool-down and warm-up periods between tests. Each emission monitor can have unique requirements for these conditions to ensure accurate OBD2 tests.

Historically, the OBD2 standard (SAE J1979) distinctly classified each monitor as either continuous or non-continuous. However, the latest standard edition has moved away from this rigid categorization for some monitors. Consequently, modern OBD2 diagnostic tools, like OBD Auto Doctor, often do not strictly adhere to this older classification.

Continuous OBD2 Monitors

These monitors are designed for constant operation during engine runtime. While manufacturers have the flexibility to categorize them, common examples of continuous monitors include:

Misfire Monitor: Detects engine misfires, which can significantly increase emissions.
Fuel System Monitor: Checks the fuel delivery system for proper operation and fuel trim accuracy.
Comprehensive Component Monitor (CCM): Oversees a wide array of engine and emission-related components for electrical circuit integrity, rationality, and performance.

Non-Continuous OBD2 Monitors

Non-continuous monitors are tailored differently for spark ignition (gasoline) and compression ignition (diesel) engines due to the inherent differences in their emission control systems. These OBD2 tests are run when specific criteria are met during driving.

Non-Continuous Monitors for Spark Ignition (Gas) Vehicles

Catalyst (CAT) Monitor: Evaluates the efficiency of the catalytic converter in reducing harmful pollutants.
Heated Catalyst Monitor: Checks the performance of heated catalytic converters, which reach optimal operating temperature faster.
Evaporative (EVAP) System Monitor: Tests the fuel evaporative system for leaks, preventing fuel vapor release into the atmosphere.
Secondary Air System Monitor: Verifies the functionality of the secondary air injection system, which helps reduce emissions during cold starts.
Oxygen (O2) Sensor Monitor: Assesses the response and accuracy of oxygen sensors, crucial for air-fuel mixture control.
Oxygen Sensor Heater Monitor: Checks the heaters in oxygen sensors, ensuring they reach operating temperature quickly for accurate readings.
EGR (Exhaust Gas Recirculation) and/or VVT System Monitor: Tests the Exhaust Gas Recirculation system (EGR), which reduces NOx emissions, and Variable Valve Timing (VVT) systems that optimize engine performance and efficiency.

Non-Continuous Monitors for Compression Ignition (Diesel) Vehicles

NMHC Catalyst Monitor: Evaluates the Non-Methane Hydrocarbon Catalyst used in diesel engines to reduce hydrocarbon emissions.
NOx/SCR Aftertreatment Monitor: Checks the Selective Catalytic Reduction (SCR) system and NOx catalysts, critical for reducing nitrogen oxide emissions in diesel engines.
Boost Pressure Monitor: Monitors the boost pressure in turbocharged diesel engines, affecting engine efficiency and emissions.
Exhaust Gas Sensor Monitor: Assesses various exhaust gas sensors specific to diesel engines, ensuring accurate emission control.
PM Filter Monitor: Evaluates the Diesel Particulate Filter (DPF) for proper function in trapping and oxidizing particulate matter.
EGR and/or VVT System Monitor: Similar to gasoline engines, these monitors check the EGR and VVT systems relevant to diesel emission control.

OBD2 readiness monitors displayed on a Windows-based diagnostic tool interface.

Monitoring Cycles: Status Since Reset & Current Cycle

Historically, OBD2 systems primarily reported monitor status based on the time since diagnostic trouble codes were last cleared. This “status since reset” is a mandatory feature for all OBD2 compliant vehicles. It reflects the long-term status of the monitors after the check engine light has been reset and DTCs have been erased. This provides an overall picture of emission system health since the last intervention.

As OBD2 technology has advanced, newer vehicles now offer the capability to report emission monitor status for the “current driving cycle” as well. These monitors initiate a fresh test cycle every time a new driving cycle begins. This provides a more immediate and up-to-date assessment of system status. Older vehicles might not support this feature. In such cases, OBD Auto Doctor, and similar diagnostic tools, will typically indicate this as “NA” or “Not Available,” signifying that current cycle OBD2 tests are not supported.

Understanding OBD2 Monitor Status: Complete, Incomplete, Disabled

The result of each readiness monitor test yields a specific monitor status. Each monitor will have its own independent status output. The possible completion statuses are:

Complete (or Ready): This status indicates that the OBD2 test for this specific emission control system has been successfully completed and passed. OBD Auto Doctor, and many scan tools, typically represent this with a green check mark, signifying a passing OBD2 test.
Incomplete (or Not Ready): This status means that the OBD2 test for this monitor has not yet been completed. This could be because the necessary driving conditions haven’t been met, or the test may have been initiated but failed. OBD Auto Doctor often uses a red exclamation mark to indicate an incomplete or failed OBD2 test.
Disabled: A “disabled” status signifies that the OBD2 test for a particular monitor has been temporarily deactivated for the current monitoring cycle. Monitors might be disabled under certain conditions, such as extreme ambient temperatures, where it’s impractical for the driver to operate the vehicle in a way that allows the monitor to run effectively.
Not Available (NA): If a monitor is listed as “NA,” it simply means that the vehicle is not equipped with that particular emission control system or monitor. Therefore, that specific OBD2 test is not applicable to the vehicle.

OBD Auto Doctor and similar software will list all defined monitors, but the actual status can only be reported for the monitors that the car’s computer supports. It’s important to note that not all vehicles are required to support every possible monitor.

Why are OBD2 Monitors Incomplete or “Not Ready”?

Having incomplete or “not ready” monitors is a common occurrence, and understanding the reasons behind it is helpful.

Recent DTC Clearing: Clearing diagnostic trouble codes (DTCs) and resetting the Check Engine Light will invariably reset the status of all readiness monitors to “incomplete.” This typically happens during or after vehicle repairs when technicians clear codes after addressing issues.
Power Loss: Monitor statuses are also reset in the event of a power failure, most commonly when the car battery is disconnected or has completely died. This is why disconnecting the battery unnecessarily is generally discouraged, especially if you need to get your car inspected soon.
New Monitoring Cycle Start: For the “current monitoring cycle” status, it’s normal for monitors to initially show as “incomplete” when the engine is first started. They need to run their self-tests during the driving cycle.

The readiness monitor status has significant implications for vehicle inspections. Depending on your location and regulations, an OBD2-equipped vehicle might fail its annual inspection if the required “since reset” monitors are not in a “complete” state. For instance, in the United States, EPA guidelines permit a limited number of “not ready” monitors for passing inspection. Vehicles from model year 1996 through 2000 can have up to two monitors “not ready,” while for 2001 and newer models, only one monitor can be “incomplete” to still pass. Therefore, ensuring your OBD2 tests are complete is crucial for inspection success.

How to Get OBD2 Monitors to “Complete” or “Ready” for Testing

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 in a monotonous manner is unlikely to fulfill all the necessary conditions for all monitors to run their OBD2 tests. This is where the concept of an OBD drive cycle comes into play. Before diving into the drive cycle, consider these preliminary steps:

Check for Check Engine Light and DTCs: First and foremost, ensure that the Malfunction Indicator Light (MIL), or Check Engine Light, is not illuminated. The presence of stored or even pending diagnostic trouble codes can prevent monitors from running to completion. Address any underlying issues causing DTCs before attempting to complete monitors.
Maintain Adequate Fuel Level: Some monitors, particularly the EVAP monitor, have fuel level requirements. For example, the EVAP monitor often requires the fuel tank to be between 35% and 85% full to initiate its diagnostic testing. Ensure you have sufficient fuel according to your vehicle’s specifications.
Perform an OBD Drive Cycle: Completing a designated “drive cycle” is often necessary to trigger the non-continuous monitors to run their OBD2 tests. About one week of combined city and highway driving under normal conditions is often sufficient for most monitors to reach a “complete” status. The drive cycle is explained in more detail below.

The OBD2 Drive Cycle: A Step-by-Step Guide

The purpose of the OBD2 drive cycle is to create the specific operating conditions that allow your car’s on-board diagnostics to run comprehensively. This, in turn, enables the readiness monitors to operate and perform their OBD2 tests, detecting any potential malfunctions within your car’s emission system. It’s important to understand that the correct drive cycle for your car can vary significantly depending on the vehicle model, manufacturer, and even the specific monitor in question.

Many vehicle manufacturers now include specific drive cycle procedures in the vehicle owner’s manual. Often, a few days of normal, varied driving, incorporating both city and highway conditions, will be enough to make the monitors “ready.” If a specific drive cycle for your vehicle is not readily available, the following generic OBD-II drive cycle can be used as a guideline. It can assist in resetting monitors when a car-specific drive cycle is unknown. However, keep in mind that it may not work for all vehicles and all monitors.

Caution: The generic drive cycle can be challenging to execute precisely under normal driving conditions and public roads. It is advisable to perform it in a controlled or restricted area where safe and legal to do so.

Cold Start: The universal OBD-II drive cycle begins with a cold start. Ensure the engine coolant temperature is below 50°C (122°F), and the ambient air temperature and coolant temperature are within 11 degrees Celsius (20 degrees Fahrenheit) of each other. This condition is typically achieved by allowing the car to sit overnight for several hours.
Ignition Key Position: Crucially, avoid leaving the ignition key in the “ON” position before the cold start. Doing so might prevent the heated oxygen sensor diagnostic test from running correctly.
Idle in Drive (with Accessories): Start the engine and allow it to idle in “Drive” (or equivalent gear for automatic transmissions) for two and a half minutes. Engage the air conditioning (A/C) and rear defroster, if your vehicle is equipped with them, to increase engine load.
Moderate Acceleration and Steady Speed: Turn off the A/C and rear defroster. Accelerate smoothly and moderately to 90 km/h (55 mph), maintaining constant acceleration. Hold this steady speed for three minutes.
Deceleration without Braking: Slow down to 30 km/h (20 mph) without applying the brakes. If you have a manual transmission, do not depress the clutch during deceleration to allow engine braking.
Moderate to Heavy Acceleration and Steady Speed: Accelerate again to 90-100 km/h (55-60 mph) using approximately 3/4 throttle. Maintain this steady speed for five minutes.
Smooth Stop: Finally, decelerate and come to a complete stop without using the brakes, if possible.

Preparing for Vehicle Inspection: Checking OBD2 Tests (Readiness Monitors)

To avoid potential rejection during your annual vehicle inspection, proactively prepare your car by checking the readiness monitors yourself. At a minimum, read the readiness monitors and confirm they are in a “ready” or “complete” state before taking your car for inspection. This simple step can save you from a near-certain rejection and the inconvenience of a re-inspection. Performing these OBD2 tests beforehand gives you peace of mind.

In addition to readiness monitors, it’s also prudent to read the diagnostic trouble codes and verify that no DTCs are present. You can easily accomplish both of these checks using OBD Auto Doctor diagnostic software or similar OBD2 scan tools. Even the free version of OBD Auto Doctor allows you to read monitor statuses and diagnostic trouble codes. So, consider taking a moment to try the software right now and assess your vehicle’s readiness.

Remember, addressing potential issues promptly is always better than waiting until the annual inspection. Taking immediate action can save you considerable time, prevent future repair expenses, and even improve your car’s fuel efficiency. By understanding and utilizing OBD2 tests and readiness monitors, you can stay informed about your vehicle’s health and ensure a smoother inspection process.