Understanding your car’s OBD2 readiness monitors is crucial for vehicle maintenance and ensuring you pass emission inspections. These monitors are essentially self-check routines that your car performs to evaluate its emission control systems. This guide will provide a detailed explanation of readiness monitors, helping you understand their status and what it means for your vehicle.
What are OBD2 Readiness Monitors?
Readiness monitors, also known as emissions monitors, are self-diagnostic tests that your car’s onboard computer runs to assess the functionality of its emission control systems. Think of them as internal health checks for your car’s components that are critical for reducing pollutants. These monitors continuously or periodically evaluate the performance of emission-related systems to ensure they are operating correctly. Modern vehicles can perform up to 11 different system tests, each represented by a readiness monitor. The status of these monitors indicates whether your car has successfully completed these self-tests.
Image: A screenshot of OBD readiness monitors being read using Windows-based OBD diagnostic software, illustrating how the monitor status is displayed on a computer interface.
Continuous vs. Non-Continuous Monitors: Understanding the Difference
There are two primary types of readiness monitors: continuous and non-continuous. The key difference lies in how and when these tests are conducted.
Continuous Monitors: These monitors operate constantly while your engine is running. They are designed to detect issues that occur in real-time during vehicle operation. The three main continuous monitors are:
- Misfire Monitor: Detects engine misfires, which are incomplete combustions in the cylinders that can lead to increased emissions and engine damage.
- Fuel System Monitor: Checks the fuel delivery system to ensure the correct air-fuel mixture for optimal combustion and emission control.
- Comprehensive Components Monitor: Oversees a wide range of other emission-related components and sensors to ensure they are functioning within specified parameters.
Non-Continuous Monitors: Unlike continuous monitors, non-continuous monitors require specific driving conditions to be met before they can run their diagnostic tests. These conditions, often referred to as “drive cycles,” vary depending on the monitor and the vehicle manufacturer. Some monitors may require a single drive cycle, while others need multiple cycles with cool-down and warm-up periods. The conditions are designed to simulate real-world driving scenarios that allow for a thorough evaluation of specific emission systems.
It’s important to note that while older OBD2 standards categorized monitors definitively as continuous or non-continuous, the latest standards provide more flexibility. Therefore, the exact categorization can sometimes be manufacturer-dependent, and diagnostic tools like OBD Auto Doctor reflect this by not rigidly adhering to the older classifications.
Non-Continuous Monitors: Gasoline vs. Diesel Vehicles
Non-continuous monitors differ based on whether your vehicle has a spark ignition engine (gasoline) or a compression ignition engine (diesel).
Spark Ignition Vehicles (Gasoline):
- Catalyst (CAT) Monitor: Evaluates the efficiency of the catalytic converter in reducing harmful emissions.
- Heated Catalyst Monitor: Checks the performance of the heater element in the catalytic converter, ensuring it reaches optimal operating temperature quickly.
- Evaporative (EVAP) System Monitor: Tests the evaporative emission control system to prevent fuel vapor leaks into the atmosphere.
- Secondary Air System Monitor: If equipped, this monitor verifies the functionality of the secondary air injection system, which helps reduce emissions during cold starts.
- Oxygen (O2) Sensor Monitor: Assesses the performance of the oxygen sensors, which are crucial for monitoring exhaust gas composition and adjusting the air-fuel mixture.
- Oxygen Sensor Heater Monitor: Checks the heater circuits of the oxygen sensors to ensure they heat up rapidly for accurate readings.
- EGR and/or VVT System Monitor: Monitors the Exhaust Gas Recirculation (EGR) system and/or Variable Valve Timing (VVT) system, which are used to reduce NOx emissions and improve engine efficiency.
Compression Ignition Vehicles (Diesel):
- NMHC Catalyst Monitor: Evaluates the Non-Methane Hydrocarbon (NMHC) catalyst, used to reduce hydrocarbon emissions in diesel engines.
- NOx/SCR Aftertreatment Monitor: Checks the Selective Catalytic Reduction (SCR) system, which reduces nitrogen oxide (NOx) emissions in diesel vehicles.
- Boost Pressure Monitor: Monitors the boost pressure in turbocharged diesel engines, which affects engine performance and emissions.
- Exhaust Gas Sensor Monitor: Evaluates various exhaust gas sensors specific to diesel engines.
- PM Filter Monitor: Checks the Diesel Particulate Filter (DPF) to ensure it is effectively trapping and oxidizing particulate matter.
- EGR and/or VVT System Monitor: Similar to gasoline engines, this monitors the EGR and/or VVT systems for emission control and engine performance.
Monitoring Cycles: “Since Reset” vs. “This Drive Cycle”
Historically, OBD2 systems primarily reported monitor status since the last time diagnostic trouble codes (DTCs) were cleared. This “since reset” status is mandatory for all OBD2 compliant vehicles and reflects the long-term readiness after a reset, such as after repairs or clearing the check engine light.
However, modern vehicles often offer an additional feature: monitoring the status within the current driving cycle. These “this drive cycle” monitors reset at the beginning of each new driving cycle (when the engine is started). While newer cars typically support this feature, older vehicles might not. In such cases, diagnostic tools will indicate the “this drive cycle” status as “Not Available” (NA).
Understanding Readiness Monitor Status
Each readiness monitor will report one of the following statuses, indicating the outcome of its self-test:
- Complete (Ready): A “Complete” or “Ready” status signifies that the monitor has successfully run its diagnostic test and the emission control system has passed. This is the desired status for all monitors to ensure your vehicle is emission compliant. Diagnostic tools usually indicate this with a green check mark.
- Incomplete (Not Ready): An “Incomplete” or “Not Ready” status means that the monitor has not yet completed its diagnostic test. This could be because the necessary driving conditions haven’t been met, or it could indicate a potential issue that prevented the test from running successfully. This status is often represented by a red exclamation mark in diagnostic tools.
- Disabled: A “Disabled” status indicates that the monitor has been temporarily deactivated for the current monitoring cycle. This can occur when certain conditions prevent the monitor from running, such as extreme ambient temperatures that are outside the operational range for the system being tested.
- Not Available (NA): “Not Available” means that the specific monitor is not supported by your vehicle. It’s not mandatory for all vehicles to support every possible monitor, so this status simply indicates that the monitor is not applicable to your car’s system.
It’s important to understand that seeing some monitors as “Incomplete” immediately after starting your car is normal, especially for “this drive cycle” monitors. These monitors require specific driving conditions to run and may complete as you drive.
Image: OBD readiness monitors displayed on an Android smartphone app, showing monitor statuses and highlighting the “Not Available” status for certain monitors, indicating vehicle-specific monitor support.
Why is a Monitor Incomplete or “Not Ready”?
There are several common 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 is a common occurrence after vehicle repairs or when troubleshooting engine issues.
- Battery Disconnection: Power loss, such as disconnecting the car battery, can also reset monitor statuses. This is why disconnecting the battery unnecessarily is generally not recommended. If you must disconnect the battery, for example, for replacement, you’ll need to perform a drive cycle to reset the monitors.
- New Monitoring Cycle: For “this drive cycle” monitors, the status is initially set to “Incomplete” at the beginning of each new driving cycle (engine start).
For vehicle inspections, especially in regions with emission testing requirements, it’s crucial to have the required monitors in a “Complete” or “Ready” state. Regulations vary, but in the US, for example, the EPA guidelines allow a limited number of “Not Ready” monitors to still pass inspection, depending on the vehicle’s model year. Generally, it’s best to have all applicable monitors “Ready” to avoid inspection failures.
How to Get Readiness Monitors to “Complete” or “Ready”
Since readiness monitors are self-check routines, the most effective way to get them to a “Complete” status is to drive your car under conditions that allow the monitors to run. Simply driving normally, however, may not always fulfill all the necessary conditions for each monitor. This is where the concept of an OBD drive cycle comes in.
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 and that there are no stored or pending diagnostic trouble codes. Active DTCs can prevent monitors from running to completion.
- Fuel Level: Make sure you have adequate fuel in your tank. Some monitors, particularly the EVAP monitor, require the fuel level to be between 35% and 85% to initiate their tests.
- Normal Driving: Often, simply driving your car under a mix of city and highway conditions for about a week is sufficient to allow most monitors to reach a “Complete” status.
OBD Drive Cycle: A Guided Procedure
An OBD drive cycle is a specific set of driving maneuvers designed to provide the necessary conditions for all readiness monitors to run their diagnostic tests. The exact drive cycle can vary significantly depending on your car’s make and model, and even the specific monitor in question. Some vehicle manufacturers include specific drive cycle procedures in the owner’s manual.
If a specific drive cycle for your car is unavailable, a generic OBD-II drive cycle can be used as a guideline. However, it’s important to note that this generic cycle may not work for all vehicles and all monitors. It’s also crucial to perform drive cycles in a safe and legal environment, as some steps may involve specific speeds and accelerations that are not suitable for normal traffic conditions.
Here’s a common generic OBD-II drive cycle:
- Cold Start: Begin with a cold start. The coolant temperature should be below 122°F (50°C), and the ambient air temperature and coolant temperature should be within 11°F (6°C) of each other. This condition is typically achieved after the vehicle has been sitting overnight.
- Ignition Key Position: Do not leave the ignition key in the “ON” position before the cold start, as this can prevent the heated oxygen sensor diagnostic from running.
- Idling: Start the engine and let it idle in drive for two and a half minutes. Turn on the air conditioning (A/C) and rear defroster if equipped.
- Acceleration and Steady Speed (1): Turn off the A/C and rear defroster. Accelerate smoothly and moderately to 55 mph (90 km/h). Maintain a steady speed of 55 mph for three minutes.
- Deceleration: Slow down to 20 mph (30 km/h) without braking. If you have a manual transmission, do not depress the clutch during deceleration.
- Acceleration and Steady Speed (2): Accelerate again, this time using ¾ throttle, back to 55-60 mph (90-100 km/h). Maintain a steady speed for five minutes.
- Stop: Slow down to a complete stop without braking.
Get Prepared for Your Vehicle Inspection
To avoid failing your annual vehicle inspection due to “Not Ready” monitors, it’s highly recommended to check your readiness monitor status beforehand. Using an OBD2 diagnostic tool, like OBD Auto Doctor, you can easily read the monitor statuses and confirm they are “Complete.” This proactive step can save you from the inconvenience and potential costs of inspection rejection.
Furthermore, it’s wise to check for diagnostic trouble codes and address any issues before your inspection. OBD Auto Doctor, even the free version, allows you to read both monitor statuses and DTCs. Why not try the software today and ensure your vehicle is ready?
Remember, addressing potential issues promptly, rather than waiting for an inspection, can save you time, prevent more costly repairs down the road, and even improve your fuel efficiency.
Editor’s note: This article was updated in October 2024 for accuracy and comprehensiveness.
