Readiness monitors are vital self-diagnostic checks within your vehicle’s OBD2 system. As an automotive repair expert at obd-de.com, I’ll explain how these monitors work and, importantly, how an Obd2 Scanner Monitor helps you understand your car’s emission system status.
The primary function of readiness monitors, also known as emission monitors, is to evaluate the effectiveness of your car’s emission control systems. These monitors continuously or periodically test various components to ensure they are functioning correctly and minimizing harmful emissions. Think of them as internal health checks for your car’s environmental responsibility.
Modern vehicles can perform up to 11 different system tests, each represented by a readiness monitor. The results from these monitors, conveniently displayed by your OBD2 scanner monitor, indicate whether your car’s computer has successfully completed these self-tests. This information is crucial for ensuring your vehicle is running efficiently and will pass emission inspections.
Types of Readiness Monitors
There are two main categories of readiness monitors: continuous and non-continuous. Their names reflect how frequently they operate.
Continuous monitors run constantly while your engine is operating. They are always evaluating key emission-related components in real-time. Non-continuous monitors, on the other hand, require specific driving conditions to be met before they can perform their tests. These conditions are often referred to as “drive cycles.”
The specific conditions needed for non-continuous monitors to run vary depending on the monitor and the vehicle manufacturer. Some monitors might require a predefined drive cycle, while others might need multiple drive cycles to allow for cool-down and warm-up periods between tests. Each emission monitor can have unique requirements to initiate its diagnostic routine.
Historically, the OBD2 standard (SAE J1979) explicitly classified each monitor as either continuous or non-continuous. However, the latest version of the standard has moved away from this strict categorization for all monitors. Consequently, modern OBD2 scanner monitors and diagnostic tools like OBD Auto Doctor also reflect this change and may not always categorize monitors in this way.
Continuous or Non-continuous Monitors (Manufacturer’s Discretion)
The following monitors can be classified as either continuous or non-continuous depending on the vehicle manufacturer’s design:
- Misfire Monitor
- Fuel System Monitor
- Comprehensive Components Monitor
Non-Continuous Monitors
Non-continuous monitors differ between gasoline (spark ignition) and diesel (compression ignition) engines due to the different emission control technologies they employ.
Spark Ignition Vehicles (Gasoline)
- Catalyst Monitor (CAT)
- Heated Catalyst Monitor
- Evaporative System Monitor (EVAP)
- Secondary Air System Monitor
- Oxygen Sensor Monitor (O2 Sensor)
- Oxygen Sensor Heater Monitor
- EGR (Exhaust Gas Recirculation) and/or VVT System Monitor
Compression Ignition Vehicles (Diesel)
- NMHC Catalyst Monitor
- NOx/SCR Aftertreatment Monitor
- Boost Pressure Monitor
- Exhaust Gas Sensor Monitor
- PM Filter Monitor
- EGR and/or VVT System Monitor
Viewing OBD readiness monitor status on a Windows-based OBD2 scanner software interface.
Monitoring Cycles: Since Reset and This Drive Cycle
Traditionally, OBD2 systems only reported the readiness monitor status since the last time diagnostic trouble codes (DTCs) were cleared. This “since reset” status is mandatory for all OBD2 compliant vehicles and provides a long-term view of monitor completion after the check engine light has been reset and DTCs cleared.
However, more advanced vehicles now also report emission monitor status for the current driving cycle, often labeled as “this drive cycle.” These monitors reset and begin testing anew at the start of each new driving cycle (typically when the engine is started). Older vehicles might not support this “this drive cycle” feature. In such cases, your OBD2 scanner monitor will typically indicate “NA” or “Not Available” for these monitors.
Understanding Monitor Status with your OBD2 Scanner Monitor
The output of each readiness monitor test is its status. Your OBD2 scanner monitor will display the completion status for each supported monitor. The possible statuses are:
- Complete (Ready): Indicates that the monitor test has been successfully completed, and the emission control system has passed the test. OBD2 scanner monitors often use a green checkmark or similar visual cue to represent this status.
- Incomplete (Not Ready): Means the monitor test has not yet been completed. This could be because the OBD2 system hasn’t had the opportunity to run the test, or the test has failed. OBD2 scanner monitors typically use a red exclamation mark or another indicator for this status.
- Disabled: Indicates that the monitor test has been temporarily disabled for the current monitoring cycle. This might occur if certain conditions necessary for the test are not met, such as extreme ambient air temperatures preventing the monitor from running reliably.
Your OBD2 scanner monitor will list all defined monitors, but it will only report the status for those supported by your specific vehicle. It’s important to note that not all vehicles are required to support every monitor. If a monitor is listed as “NA” or “Not Available” on your OBD2 scanner monitor, it simply means your car does not have that particular monitor and, therefore, it is not tested.
An example of readiness monitor status displayed on an Android OBD2 scanner app, showing “Not Available” for drive cycle monitors.
Why a Monitor Might Be Incomplete
Clearing diagnostic trouble codes (DTCs) and resetting the Check Engine Light will also reset all readiness monitor statuses to “incomplete.” This commonly happens during or after vehicle repairs when technicians clear codes.
Monitor statuses can also be reset by power loss, such as disconnecting the car battery. 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 is important.
For the “this drive cycle” monitors, it’s normal for the status to be “incomplete” immediately after starting the engine, as the tests have not yet had time to run in the new cycle.
Crucially, depending on your location’s emission regulations, your vehicle might fail its annual inspection if the required “since reset” monitors are not in a “complete” status. For instance, in the United States, EPA guidelines allow for up to two “not ready” monitors for 1996-2000 model year vehicles, and only one “not ready” monitor for 2001 and newer vehicles. Checking monitor status with your OBD2 scanner monitor before an inspection is therefore highly recommended.
Getting Monitors to “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 in a monotonous manner might not 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, let’s consider a few basic prerequisites:
- Check Engine Light (MIL): Ensure the Malfunction Indicator Light (MIL), or Check Engine Light, is not illuminated. Active stored or pending diagnostic trouble codes can prevent monitors from running to completion. Use your OBD2 scanner monitor to check for and address any DTCs.
- Fuel Level: Ensure you have sufficient fuel in your car. Some monitors, particularly the EVAP monitor, might require the fuel level to be between 35% and 85% to initiate testing.
- Complete a Drive Cycle: Typically, about a week of combined city and highway driving under normal conditions is sufficient for most monitors to reach a “complete” status. The OBD drive cycle, detailed below, is a more structured approach to expedite this process.
OBD Drive Cycle Explained
The purpose of an OBD2 drive cycle is to create the specific operating conditions needed for your car’s on-board diagnostics to run, allowing the readiness monitors to execute their tests and detect any potential malfunctions in your emission system. The precise drive cycle for your vehicle can vary significantly based on the car model, manufacturer, and even the specific monitor in question.
Many vehicle manufacturers now include recommended drive cycles in the owner’s manual. Generally, a few days of normal driving, encompassing both city and highway conditions, will allow the monitors to become ready. If a specific drive cycle for your car is unavailable, the following generic drive cycle can be used as a guideline. While it might not work for all vehicles and monitors, it can be helpful in resetting monitors when a car-specific drive cycle is unknown.
Due to the specific steps involved, it’s often best to perform a drive cycle in a controlled or restricted area where you can safely execute the maneuvers without disrupting normal traffic.
- 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. This condition is usually achieved by allowing the car to sit overnight.
- Ignition Timing: Do not leave the ignition key in the “ON” position before the cold start. Doing so may prevent the heated oxygen sensor diagnostic from running correctly.
- Idle: Start the engine and let it idle in drive (or neutral for manual transmissions with parking brake engaged) for two and a half minutes. Turn on the air conditioning (A/C) and rear defroster if equipped.
- Moderate Acceleration and Steady Speed: Turn off the A/C and rear defroster. Accelerate to 90 km/h (55 mph) under moderate, constant acceleration. Maintain a steady speed of 90 km/h (55 mph) for three minutes.
- Deceleration: 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.
- Heavy Acceleration and Steady Speed: Accelerate back up to 90-100 km/h (55-60 mph) using approximately 3/4 throttle. Maintain a steady speed of 90-100 km/h (55-60 mph) for five minutes.
- Stop: Slow down to a complete stop without braking.
Prepare for Your Emission Inspection with Your OBD2 Scanner Monitor
To avoid failing your annual emission inspection, proactively prepare your vehicle by checking the readiness monitors yourself using your OBD2 scanner monitor. Ensuring the monitors are “ready” beforehand can prevent a nearly guaranteed rejection.
Furthermore, use your OBD2 scanner monitor to read diagnostic trouble codes and verify that no DTCs are present. Diagnostic software like OBD Auto Doctor allows you to check both monitor statuses and DTCs, even with the free version. Why not try the software right now and equip yourself with the knowledge you need?
Remember, addressing potential issues promptly, rather than waiting until the emission inspection, can save you significant time, prevent future repairs, and even improve fuel efficiency. Regularly monitoring your vehicle’s readiness monitors with an OBD2 scanner monitor is a smart preventative maintenance practice.
Editor’s note: This tutorial was updated in March 2020 to ensure accuracy and reflect the latest information on OBD2 readiness monitors and diagnostic practices.
