Understanding OBD2 Tests: Readiness Monitors Explained

As a car owner, you might have heard about OBD2 systems and tests, especially when dealing with vehicle inspections or engine issues. But what exactly are Obd2 Tests, and how do they relate to your car’s performance and emissions? This article delves into the world of OBD2 readiness monitors, explaining these crucial self-check routines and how they provide valuable insights into your vehicle’s health.

OBD2 readiness monitors, also known as emission monitors, are essentially self-diagnostic tests that your car’s computer runs on its emission control systems. Think of them as internal health checks for your car’s pollution control equipment. These monitors are designed to ensure that all emission-related systems are functioning correctly, helping to reduce harmful pollutants released into the environment. Up to 11 different system tests, or readiness monitors, can be performed by your car. The results of these obd2 tests indicate whether your car’s computer has successfully completed each test. This is vital information for both vehicle maintenance and passing emission inspections.

Decoding Readiness Monitor Types

Readiness monitors are categorized into two main types: continuous and non-continuous. The distinction lies in how and when these obd2 tests are conducted.

Continuous monitors are constantly active while your engine is running. They are designed for systems that require ongoing evaluation. In contrast, non-continuous monitors require specific driving conditions to be met before they can initiate and complete their tests.

These necessary conditions for non-continuous obd2 tests vary 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. Others may even require two drive cycles, incorporating cool-down and warm-up periods. Each emission monitor can have its own unique set of requirements for these conditions to be fulfilled.

Historically, the OBD2 standard (SAE J1979) clearly classified each monitor as either continuous or non-continuous. However, the latest version of the standard has moved away from this rigid categorization for all monitors. Therefore, modern OBD2 diagnostic tools like OBD Auto Doctor may not strictly adhere to this older classification.

Continuous or Non-continuous Monitors: Manufacturer’s Choice

The following monitors can be implemented as either continuous or non-continuous, depending on the vehicle manufacturer’s design:

Misfire Monitor: Detects engine misfires that can increase emissions and damage catalytic converters.
Fuel System Monitor: Checks the fuel delivery system to ensure proper air-fuel mixture for efficient combustion and emission control.
Comprehensive Component Monitor: Oversees various electronic components and sensors within the emission control system for proper function.

Non-Continuous Monitors: Varied for Gas and Diesel

Non-continuous monitors differ between spark ignition (gasoline) and compression ignition (diesel) engines, reflecting the different emission control technologies used in these engine types.

Spark Ignition Vehicles (Gas Engines)

Catalyst (CAT) Monitor: Evaluates the efficiency of the catalytic converter in reducing harmful exhaust gases.
Heated Catalyst Monitor: Specifically tests the performance of heated catalytic converters, which reach optimal operating temperature faster.
Evaporative (EVAP) System Monitor: Checks for leaks in the evaporative emission control system, which prevents fuel vapor release into the atmosphere.
Secondary Air System Monitor: Verifies the functionality of the secondary air injection system, used to reduce emissions during cold starts.
Oxygen (O2) Sensor Monitor: Tests the oxygen sensors that provide feedback to the engine control unit for air-fuel mixture adjustments.
Oxygen Sensor Heater Monitor: Checks the heaters in oxygen sensors to ensure they reach operating temperature quickly for accurate readings.
EGR (Exhaust Gas Recirculation) and/or VVT System Monitor: Evaluates the Exhaust Gas Recirculation system, which reduces NOx emissions, and Variable Valve Timing systems that can also affect emissions.

Compression Ignition Vehicles (Diesel Engines)

NMHC Catalyst Monitor: Monitors the Non-Methane Hydrocarbon Catalyst used in diesel engines to reduce hydrocarbon emissions.
NOx/SCR Aftertreatment Monitor: Tests the Selective Catalytic Reduction (SCR) system and NOx catalysts, crucial for reducing nitrogen oxide emissions from diesel engines.
Boost Pressure Monitor: Checks the boost pressure control system in turbocharged diesel engines for optimal performance and emission control.
Exhaust Gas Sensor Monitor: Monitors various exhaust gas sensors specific to diesel engines, ensuring accurate readings for emission control.
PM Filter Monitor: Evaluates the Diesel Particulate Filter (DPF) system, which traps and burns soot particles from diesel exhaust.
EGR and/or VVT System Monitor: Similar to gasoline engines, this monitors the EGR and Variable Valve Timing systems in diesel applications for emission reduction.

OBD2 tests for readiness monitors as displayed on Windows software.

Monitoring Cycles: Long-Term and Current Status

Traditionally, OBD2 systems primarily reported monitor status since the last time diagnostic trouble codes were cleared. This “since reset” status is mandatory for all OBD2 compliant vehicles. It provides a long-term view of monitor performance after events like check engine light resets or DTC clearing.

However, modern vehicles have evolved to offer more detailed information. Newer cars can now also report emission monitor status for the current driving cycle. These “this drive cycle” monitors reset and begin testing every time a new driving cycle starts (typically when the engine is started). While older cars might not support this feature, in such cases, OBD Auto Doctor and similar tools will indicate the status as “NA” or Not Available.

Understanding OBD2 Monitor Status Outputs

The result of each obd2 test or readiness monitor is a status output. This status indicates the completion and outcome of the test. The possible completion statuses are:

Complete (Ready): This positive status, often indicated by a green check mark in diagnostic tools, signifies that the obd2 test for the emission control system has been successfully completed and passed. It means the system is functioning as expected.
Incomplete (Not Ready): An incomplete or “not ready” status, often shown as a red exclamation mark, means that the obd2 test has not yet been completed. This could be because the OBD2 system hasn’t had the opportunity to run the test due to unmet driving conditions, or because the test has run and failed.
Disabled: A “disabled” status indicates that the obd2 test for a particular monitor has been temporarily disabled for the current monitoring cycle. This might occur when environmental conditions, such as extreme ambient air temperatures, prevent the monitor from running reliably.

OBD2 diagnostic software like OBD Auto Doctor will list all defined monitors. However, the reported status will only be available for the monitors that your specific vehicle actually supports. It’s important to note that not all cars are required to support every possible monitor. A status of “NA” or “not available” simply means that the vehicle does not have that particular monitor and, therefore, it cannot be tested.

Reasons for Incomplete or “Not Ready” OBD2 Tests

A common reason for monitors to be incomplete or “not ready” is the clearing of diagnostic trouble codes (DTCs) and the Check Engine Light. This action resets not only the DTCs but also the statuses of all readiness monitors. This often happens during or after vehicle repairs when technicians clear codes.

Monitor statuses are also reset in the event of a power failure, such as when the car battery is disconnected. This is why disconnecting the battery unnecessarily is generally not recommended. If battery disconnection is required, for example, during battery replacement, understanding how to reset the monitors afterward becomes crucial.

For “this drive cycle” monitors, it’s normal for the status to initially be “incomplete” when starting a new driving cycle (upon engine start). These monitors need time and appropriate driving conditions to run their tests.

Crucially, depending on your location, an OBD2-equipped vehicle may 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 up to two monitors to be “not ready” for 1996-2000 model year vehicles. For 2001 and newer vehicles, only a single monitor can be incomplete for the vehicle to pass inspection.

How to Get OBD2 Tests “Complete” or “Ready”

Since readiness monitors are self-check routines, the most effective way to get them ready is by driving your car. However, simply driving without variation is unlikely to fulfill the specific conditions needed for all monitors to run. This is where the concept of an “OBD drive cycle” comes into play. But before diving into drive cycles, consider these initial 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 their obd2 tests. Address any underlying issues causing DTCs first.
Fuel Level: Make sure you have sufficient fuel in your car’s tank. Some monitors, particularly the EVAP monitor, may require the fuel level to be between 35% and 85% to initiate their diagnostic testing.
Complete an OBD Drive Cycle: About a week of combined city and highway driving under normal conditions is often sufficient for most monitors to reach a “complete” status. For a quicker and more targeted approach, you can perform a specific OBD drive cycle.

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

The purpose of an OBD2 drive cycle is to provide your car with the specific operating conditions needed to run its on-board diagnostics and allow the readiness monitors to perform their obd2 tests. The precise drive cycle for your vehicle can vary significantly depending on the car model, manufacturer, 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, the following generic OBD-II drive cycle can serve as a guideline. However, it may not work for all vehicles and monitors. Always prioritize a manufacturer-specific drive cycle if available.

Important Safety Note: Drive cycles often involve specific speeds and accelerations. It is crucial to perform these steps in a safe and controlled environment, ideally in a restricted area where traffic and speed limits are not a concern.

Here are the steps for a universal 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 degrees of each other. Achieve this by letting the car sit overnight.
Ignition Key Position: 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 in Drive (2.5 minutes): Start the engine and idle in “Drive” (or equivalent for your transmission type) for two and a half minutes. Turn on the A/C and rear defroster if your car is equipped with them to increase engine load.
Moderate Acceleration and Steady Speed (3 minutes): Turn off the A/C and rear defroster. Accelerate smoothly and moderately to 90 km/h (55 mph) and maintain a steady speed for three minutes.
Deceleration Without 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 back to 90-100 km/h (55-60 mph) using approximately 3/4 throttle. Maintain a steady speed for five minutes.
Stop Without Braking: Gradually slow down and come to a complete stop without using the brakes if possible.

Preparing for Your Vehicle Inspection Using OBD2 Tests

To avoid failing your annual vehicle inspection, proactively check your car’s obd2 tests and readiness monitors beforehand. Using an OBD2 diagnostic tool, read the readiness monitor statuses to confirm they are “complete” and ready for inspection. This simple step can prevent a likely rejection.

Additionally, it’s wise to read the diagnostic trouble codes and ensure there are no present DTCs before your inspection. OBD Auto Doctor and similar diagnostic software allow you to check both monitor statuses and DTCs, even with free versions. Consider trying the software now to assess your vehicle’s readiness.

Remember, addressing potential issues identified by obd2 tests promptly is always better than waiting for an inspection failure. Proactive maintenance can save you time, prevent future costly repairs, and even improve fuel efficiency.

Editor’s note: This tutorial has been updated in October 2024 for accuracy and the latest information.