OBD2 Readiness Monitors are essential self-diagnostic checks that provide valuable insights into your vehicle’s emission control systems. These routines act as a window into your car’s internal health, specifically concerning systems that affect air pollution. This guide will thoroughly explain what Obd2 Monitors are, their function, and why they are important for your vehicle.
The primary purpose of readiness monitors, also known as Emissions Monitors, is to automatically test and assess the efficiency of your car’s emission control systems. As the name suggests, they continuously or periodically observe the performance of emission-related components and systems.
Modern vehicles can perform up to 11 different system tests, each known as a readiness monitor. The results of these monitors indicate whether your car’s computer has successfully completed these self-tests. A “ready” status generally means the system is functioning as expected, while a “not ready” status can signal potential issues or incomplete testing.
Types of OBD2 Monitors: Continuous and Non-Continuous
OBD2 monitors are broadly categorized into two types: continuous and non-continuous monitors. The fundamental difference lies in how and when these tests are conducted.
Continuous Monitors: These monitors operate constantly while the engine is running. They are designed for systems that require ongoing evaluation.
Non-Continuous Monitors: In contrast, non-continuous monitors require specific conditions to be met before a test can be initiated and completed. These conditions can vary significantly depending on the monitor and the vehicle manufacturer. Some monitors might need a specific driving pattern, known as a drive cycle, while others may require multiple drive cycles with cool-down and warm-up periods in between. Each non-continuous emission monitor can have its own unique set of prerequisites.
Historically, the OBD2 standard (SAE J1979) distinctly classified each monitor as either continuous or non-continuous. However, the latest revisions of the standard have removed this strict categorization for some monitors. Consequently, modern OBD2 diagnostic tools, including OBD Auto Doctor, often no longer adhere to this rigid classification.
Monitors: Continuous or Non-continuous (Manufacturer’s Choice)
The following monitors can be implemented as either continuous or non-continuous, depending on the vehicle manufacturer’s design and strategy:
- Misfire Monitor: Detects engine misfires that can lead to increased emissions and engine damage.
- 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 electrical and rationality faults.
Non-Continuous Monitors: System Specific Tests
Non-continuous monitors are further differentiated based on whether the vehicle has a spark ignition engine (gasoline) or a compression ignition engine (diesel).
Non-Continuous Monitors for Spark Ignition Vehicles (Gasoline Engines):
- Catalyst (CAT) Monitor: Evaluates the efficiency of the catalytic converter in reducing harmful exhaust emissions.
- Heated Catalyst Monitor: Checks the performance of heated catalytic converters, which reach operating temperature faster for improved cold-start emission control.
- Evaporative (EVAP) System Monitor: Tests the evaporative emission control system for leaks and proper functioning, preventing fuel vapor release into the atmosphere.
- Secondary Air System Monitor: Verifies the operation of the secondary air injection system, which helps reduce emissions during cold starts.
- Oxygen (O2) Sensor Monitor: Assesses the accuracy and response time of oxygen sensors, crucial for fuel mixture control and catalytic converter efficiency.
- Oxygen Sensor Heater Monitor: Checks the functionality of oxygen sensor heaters, ensuring sensors 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, both designed to reduce NOx emissions.
Non-Continuous Monitors for Compression Ignition Vehicles (Diesel Engines):
- NMHC Catalyst Monitor: Evaluates the efficiency of Non-Methane Hydrocarbon (NMHC) catalysts used in diesel engines to reduce hydrocarbon emissions.
- NOx/SCR Aftertreatment Monitor: Monitors the Selective Catalytic Reduction (SCR) system and other NOx aftertreatment technologies used in diesel engines to reduce nitrogen oxide emissions.
- Boost Pressure Monitor: Checks the boost pressure control system in turbocharged diesel engines for proper operation.
- Exhaust Gas Sensor Monitor: Assesses various exhaust gas sensors used in diesel engines, such as NOx sensors or particulate matter sensors.
- PM Filter Monitor: Monitors the performance of the Particulate Matter (PM) filter (diesel particulate filter – DPF) in trapping and oxidizing soot particles from diesel exhaust.
- EGR and/or VVT System Monitor: Similar to gasoline engines, monitors the EGR and/or VVT systems for NOx emission control.
OBD2 readiness monitors status displayed using diagnostic software on a Windows laptop.
Monitoring Cycles: Since Reset and Current Drive Cycle
Historically, OBD2 systems primarily reported monitor status since the last time diagnostic trouble codes (DTCs) were cleared. This “since reset” status remains mandatory for all OBD2 compliant vehicles. It provides a long-term view of monitor completion after a check engine light reset or DTC clearing event.
Advancements in OBD2 technology have enabled newer vehicles to also report emission monitor status for the current driving cycle. These monitors initiate testing at the beginning of each new driving cycle (typically when the engine is started). Older vehicles may not support this current drive cycle monitoring feature. In such cases, OBD Auto Doctor and similar tools will typically indicate “NA” or “Not Available” for current drive cycle status.
Understanding Monitor Status: Complete, Incomplete, and Disabled
Each readiness monitor provides a status output indicating the test result. The possible completion statuses are:
- Complete (Ready): Indicates that the monitor test has been successfully completed, and the emission control system has passed the self-check. OBD Auto Doctor typically represents this status with a green checkmark. This is the desired status for passing emissions inspections.
- Incomplete (Not Ready): Signifies that the monitor test has not yet been completed. This could mean the OBD2 system has not yet run the test routine, or the test has failed. OBD Auto Doctor usually indicates this with a red exclamation mark. Incomplete monitors can prevent a vehicle from passing an emissions test.
- Disabled: Indicates that the monitor test has been temporarily disabled for the current monitoring cycle. Monitors may be disabled under certain conditions where it’s impractical for the driver to operate the vehicle in a way that allows the monitor to run. For example, extreme ambient air temperatures might prevent certain monitors from running.
- Not Available (NA): Means the vehicle does not support the specific monitor. It’s not mandatory for all vehicles to support every defined monitor. If a monitor is listed as “NA,” it simply means that particular test is not applicable to that vehicle’s emission control system.
Readiness monitors status shown on an Android app. The message highlights that readiness monitors are not supported for the current drive cycle in this vehicle.
Reasons for Incomplete or “Not Ready” Monitor Status
Clearing diagnostic trouble codes (DTCs), often done to turn off the Check Engine Light, also resets the readiness monitor statuses to “incomplete.” This commonly occurs during or after vehicle repairs when codes are cleared.
Monitor statuses are also reset in the event of a power failure, such as when the vehicle battery is disconnected. This is why disconnecting the battery is generally discouraged, especially before an emissions test. If battery disconnection is necessary (e.g., for replacement), understanding how to reset the monitors is crucial, which we will discuss later.
For the current monitoring cycle, monitors will typically be “incomplete” immediately after starting the engine or beginning a new drive cycle. This is a normal initial state, and the monitors will run their tests as the vehicle is driven and conditions are met.
Importantly, many regions have regulations regarding emissions inspections. An OBDII-equipped vehicle may fail its annual inspection if the required monitors since reset are not in a “complete” status. 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” to still pass inspection. These regulations highlight the significance of ensuring monitor readiness before an emissions test.
How to Get OBD2 Monitors to “Complete” or “Ready” Status
Since readiness monitors are self-check routines, the most effective way to get them to a “complete” status is by driving the vehicle. However, simply driving in a monotonous manner is unlikely to fulfill all the necessary conditions for all monitors to run. This is where the concept of an OBD drive cycle becomes relevant. But before delving into drive cycles, consider these initial steps:
- Check for Active MIL (Malfunction Indicator Light): Ensure the Check Engine Light (MIL) is not illuminated. Active or even pending diagnostic trouble codes can prevent monitors from running to completion. Address any underlying issues causing DTCs before attempting to complete monitors.
- Verify Sufficient Fuel Level: Some monitors, particularly the EVAP monitor, have fuel level requirements. The fuel level often needs to be between 35% and 85% for the monitor to initiate its diagnostic testing. Ensure adequate fuel in the tank.
- Perform an OBD Drive Cycle: Completing a drive cycle is often necessary to set non-continuous monitors to “ready.” Generally, about a week of combined city and highway driving under normal conditions is sufficient for most monitors to complete. The drive cycle is a structured driving pattern designed to meet the specific conditions required by various monitors. A generic drive cycle guideline is outlined below.
Generic OBD Drive Cycle Guideline
The purpose of an OBD2 drive cycle is to provide your car with the operating conditions needed to run its onboard diagnostics, enabling the readiness monitors to execute their tests and detect potential emission system malfunctions. The precise drive cycle for your specific vehicle can vary based on the car model, manufacturer, and the particular monitor in question.
Many vehicle manufacturers include specific drive cycle procedures in the owner’s manual. If a specific drive cycle is unavailable, the following generic drive cycle can serve as a guideline to assist in resetting monitors. However, it’s important to note that this generic cycle may not be effective for all vehicles and monitors.
It’s crucial to note that executing a drive cycle exactly as prescribed can be challenging under normal driving conditions. Therefore, it’s advisable to perform the drive cycle in a controlled or restricted area where safe driving maneuvers can be executed.
- Cold Start: Begin with a cold start. The coolant temperature should be below 50°C (122°F), and the coolant and intake air temperatures should be within 11 degrees Celsius (20 degrees Fahrenheit) of each other. Allowing the vehicle to sit overnight typically achieves these cold start 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 correctly.
- Idle in Drive (2.5 Minutes): Start the engine and allow it to idle in drive for two and a half minutes. Turn on the air conditioning (A/C) and rear defroster (if equipped) during this idle period 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). Maintain a steady speed of 90 km/h for three minutes.
- Deceleration (No Braking): Slow down to 30 km/h (20 mph) without applying the brakes. If driving a manual transmission vehicle, 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 this steady speed for five minutes.
- Controlled Stop: Slow down and come to a complete stop without braking if possible, allowing the vehicle to coast to a halt.
Prepare for Vehicle Inspection: Check Your Monitors
To prevent potential rejection during an annual vehicle inspection, proactively prepare your car by checking the readiness monitors yourself. Ensuring that the required monitors are in a “ready” status beforehand can save you from a near-certain inspection failure.
In addition to monitors, it’s also advisable to read the diagnostic trouble codes and confirm that no DTCs are present. You can conveniently check both monitor statuses and diagnostic trouble codes using OBD Auto Doctor diagnostic software. Even the free version of the software allows you to read monitor statuses and DTCs. Take advantage of this capability and try the software now!
Remember, addressing potential issues promptly is always better than waiting until the annual inspection. Early detection and action can save you significant time, prevent future repairs, and even improve fuel efficiency.
Editor’s note: This tutorial was updated in March 2020 to ensure accuracy and incorporate the latest information on OBD2 monitors.
