OBD2 readiness monitors are essential self-diagnostic checks within your vehicle’s onboard computer. They act as a window into your car’s emission control systems, providing valuable insights into its health and ability to meet emission standards. This guide will delve into the details of Obd2 Monitors, explaining their function, types, and importance for vehicle owners.
The primary purpose of readiness monitors, also known as emission monitors, is to rigorously test your car’s emission control systems. As their name suggests, these monitors constantly observe the performance of emission-related components and systems while you drive.
Modern vehicles can perform up to eleven different system tests, each represented by a readiness monitor. The results of these monitors indicate whether your car’s computer has successfully completed these self-tests. Understanding these results is crucial for ensuring your vehicle is running efficiently and environmentally responsibly.
Types of OBD2 Readiness Monitors: Continuous and Non-Continuous
There are two main categories of readiness monitors: continuous and non-continuous. The fundamental difference lies in how they are tested. Continuous monitors operate constantly while the engine is running, providing real-time feedback. Non-continuous monitors, on the other hand, require specific operating conditions to be met before a test can be initiated and completed.
The precise conditions necessary for non-continuous monitors to run vary depending on the monitor and the vehicle manufacturer. Some monitors require the vehicle to complete a predefined drive cycle, a specific sequence of driving conditions designed to activate the monitor. Some may even require two drive cycles to allow for cool-down and warm-up periods between tests. Each emission monitor can have unique requirements for these conditions.
In older OBD2 standards (SAE J1979), monitors were strictly categorized as either continuous or non-continuous. However, the latest standards have removed this rigid classification for some monitors. Therefore, modern diagnostic tools like OBD Auto Doctor often don’t adhere to this outdated categorization.
Examples of Continuous and Non-Continuous Monitors
The following monitors can be implemented as either continuous or non-continuous, depending on the vehicle manufacturer’s design choices:
- Misfire Monitor: Detects engine misfires, which can significantly increase emissions.
- Fuel System Monitor: Checks the fuel delivery system for proper operation and fuel mixture control.
- Comprehensive Component Monitor: Monitors a wide range of engine and emission-related components for malfunctions.
Non-Continuous Monitors: Gasoline vs. Diesel Engines
Non-continuous monitors differ based on whether your vehicle has a spark ignition engine (gasoline) or a compression ignition engine (diesel).
Non-Continuous Monitors for Spark Ignition (Gas) Vehicles:
- Catalyst (CAT) Monitor: Evaluates the efficiency of the catalytic converter in reducing harmful 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 evaporative emissions system, preventing fuel vapor release.
- 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 air-fuel ratio control.
- Oxygen Sensor Heater Monitor: Checks the heaters for oxygen sensors, ensuring they reach operating temperature quickly.
- EGR (Exhaust Gas Recirculation) and/or VVT System Monitor: Monitors the exhaust gas recirculation system and variable valve timing system, both important for emission control and engine efficiency.
Non-Continuous Monitors for Compression Ignition (Diesel) Vehicles:
- NMHC Catalyst Monitor: Evaluates the catalyst for Non-Methane Hydrocarbons, reducing these pollutants in diesel exhaust.
- NOx/SCR Aftertreatment Monitor: Monitors the NOx reduction system, often using Selective Catalytic Reduction (SCR) to minimize nitrogen oxides.
- Boost Pressure Monitor: Checks the boost pressure system in turbocharged diesel engines, impacting combustion efficiency and emissions.
- Exhaust Gas Sensor Monitor: Tests various exhaust gas sensors specific to diesel engines, ensuring accurate readings for emission control.
- PM Filter Monitor: Monitors the particulate matter filter (diesel particulate filter – DPF), trapping soot particles from diesel exhaust.
- EGR and/or VVT System Monitor: Similar to gasoline engines, monitors EGR and VVT systems for emission and efficiency control in diesel applications.
Image alt text: OBD2 readiness monitor status displayed on a Windows computer using diagnostic software, showing “Complete” with green checkmarks and “Incomplete” with red exclamation points for various monitors.
Understanding OBD2 Monitor Cycles: Since Reset and This Drive Cycle
Historically, 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 completion after the check engine light has been reset and DTCs cleared.
More advanced, newer vehicles now also offer emission monitor status for the “current driving cycle,” or “this drive cycle.” These monitors reset and begin testing anew at the start of each driving cycle (typically when the engine is started). Older vehicles might not support this “this drive cycle” feature. In such cases, diagnostic tools like OBD Auto Doctor will indicate the status as “NA” or “Not Available.”
Interpreting OBD2 Monitor Status: Complete, Incomplete, and Disabled
Each readiness monitor provides a status indicating the outcome of its self-test. The possible status results are:
- Complete (or Ready): Signifies that the monitor has successfully completed its test and the emission control system has passed. Diagnostic software typically indicates this status with a green check mark.
- Incomplete (or Not Ready): Indicates that the monitor test has not yet been completed. This could be because the necessary driving conditions haven’t been met, or the test has failed. Diagnostic software usually displays this status with a red exclamation mark.
- Disabled: Means the monitor test has been temporarily disabled for the current monitoring cycle. Monitors might be disabled if certain conditions are not suitable for testing, such as extreme ambient air temperatures that could prevent accurate results.
Diagnostic tools like OBD Auto Doctor list all defined monitors, but the actual status reported depends on the monitors supported by your specific vehicle. It’s not mandatory for every car to support all possible monitors. An “NA” or “Not Available” status simply means that the vehicle doesn’t have that particular monitor and therefore it is not tested.
Image alt text: OBD2 readiness monitor display on an Android app showing “Not Available for This Drive Cycle” indicating the vehicle’s limitations in reporting current drive cycle monitor status.
Reasons for an “Incomplete” or “Not Ready” Monitor Status
Clearing diagnostic trouble codes (DTCs) and turning off the Check Engine Light will invariably reset the readiness monitor statuses to “incomplete.” This commonly happens during or after vehicle repairs when codes are cleared.
Monitor statuses are also reset if the vehicle experiences a power interruption, such as when the battery is disconnected. This is why disconnecting the battery unnecessarily is generally discouraged. If battery disconnection is required, for example, for replacement, understanding how to reset the monitors afterward becomes important, as discussed below.
For the “current monitoring cycle” or “this drive cycle” monitors, an “incomplete” status is normal upon starting the engine. These monitors are designed to begin testing anew each time the engine is started.
Crucially, depending on your location’s regulations, an OBD2-compliant vehicle might fail its annual inspection if the required “since reset” monitors are not in a “complete” status. For example, in the United States, EPA guidelines permit up to two monitors to be “not ready” for 1996-2000 model year vehicles. However, for 2001 and newer models, only a single monitor can be “incomplete” or “not ready” to pass inspection.
How to Complete or “Ready” OBD2 Monitors
Since readiness monitors are self-check routines, the most effective way to get them to a “complete” status is by driving your car under conditions that allow the tests to run. However, simply driving without a plan might not fulfill all the necessary conditions. This is where the concept of an OBD drive cycle becomes important. But before delving into drive cycles, consider these initial steps:
- Address the Check Engine Light: Ensure the Malfunction Indicator Light (MIL), or Check Engine Light, is not illuminated. Active stored or even pending diagnostic trouble codes can prevent monitors from running to completion. Resolve any underlying issues causing the check engine light to come on.
- Maintain Adequate Fuel Level: Some monitors, particularly the EVAP monitor, have fuel level requirements. Typically, the fuel tank needs to be between 35% and 85% full for the EVAP monitor to initiate testing. Ensure your fuel level is within this range.
- Perform an OBD Drive Cycle: Completing a drive cycle is often necessary to set the non-continuous monitors to “ready.” Generally, about a week of mixed city and highway driving under normal conditions is sufficient for most monitors to complete. A more structured OBD drive cycle can expedite this process, as explained in detail below.
The OBD Drive Cycle Explained
The purpose of an OBD2 drive cycle is to provide the specific operating conditions needed for your car’s on-board diagnostics to run, allowing the readiness monitors to execute their tests. The precise drive cycle for your vehicle can vary significantly depending on the car model, manufacturer, and the specific monitor in question.
Many vehicle manufacturers now include the recommended drive cycles in the owner’s manual. Typically, a few days of normal driving, encompassing both city and highway conditions, will allow the monitors to become ready. If a car-specific drive cycle isn’t available, the following generic OBD-II drive cycle can serve as a guideline. It can assist in resetting monitors when specific instructions are lacking, although it may not be universally effective for all vehicles and monitors.
It’s important to note that this drive cycle can be challenging to execute precisely under normal traffic conditions. Therefore, it’s advisable to perform it in a safe, restricted area where you can control your speed and driving maneuvers.
Generic OBD-II Drive Cycle Procedure:
- 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 Celsius (20 degrees Fahrenheit) of each other. Allowing the car to sit overnight typically achieves these conditions.
- Ignition Key Position: Avoid leaving the ignition key in the “ON” position before the cold start. Doing so might prevent the heated oxygen sensor diagnostic from running.
- Idling Phase: Start the engine and idle in drive (or neutral for manual transmissions with parking brake engaged) for two and a half minutes. Turn on the A/C and rear defroster (if equipped) during this idling phase to increase engine load.
- Initial Acceleration and Steady Speed: Turn off the A/C and rear defroster. Accelerate smoothly and moderately to 90 km/h (55 mph) and maintain a constant speed for three minutes.
- Deceleration Phase: Decelerate to 30 km/h (20 mph) without using the brakes. If you have a manual transmission, do not depress the clutch during deceleration to allow engine braking.
- Second 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.
- Final Deceleration and Stop: Slow down to a complete stop without braking. Allow the vehicle to idle briefly before turning off the ignition.
Preparing for Vehicle Inspection: Check Your OBD2 Monitors
To avoid failing your annual vehicle inspection due to emission readiness issues, it’s prudent to proactively check your OBD2 monitors before going for the official test. By reading your readiness monitors beforehand and ensuring they are “ready,” you can prevent a likely rejection.
It’s also recommended to read diagnostic trouble codes and confirm that no DTCs are present. You can easily perform both of these checks using OBD Auto Doctor diagnostic software. Even the free version of the software allows you to read monitor statuses and diagnostic trouble codes. Consider trying the software today to assess your vehicle’s emission 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 economy.
Editor’s note: This guide was updated in October 2024 for accuracy and to include the latest information on OBD2 monitors.
