OBD2 readiness monitors are essential self-diagnostic checks within your vehicle. These routines offer valuable insight into your car’s emission control systems, acting as a window into its self-diagnostics. This guide provides a detailed explanation of readiness monitors and their significance for your vehicle’s health and inspection readiness.
Readiness monitors, also known as Emissions Monitors, are designed to test and evaluate your car’s emission control systems. As their name suggests, they continuously or periodically observe the performance of emission-related components and systems.
Modern vehicles can perform up to 11 system tests, each represented by a readiness monitor. The results of these monitors indicate whether your car’s computer has successfully completed these crucial self-tests.
Types of OBD2 Readiness Monitors: Continuous and Non-Continuous
Readiness monitors are broadly classified into two categories: continuous and non-continuous. The distinction lies in their testing methodology. Continuous monitors operate constantly while the engine is running, providing real-time evaluation. Conversely, non-continuous monitors require specific conditions to be met before a test can be initiated and completed.
The conditions necessary for non-continuous monitor tests vary depending on the specific monitor and vehicle. Some monitors necessitate following a predefined drive cycle routine. Certain monitors may even require two drive cycles to accommodate cool-down and warm-up periods. Each emission monitor can have unique requirements for test conditions.
Historically, the OBD2 standard (SAE J1979) categorized each monitor definitively as either continuous or non-continuous. However, the latest standard edition has moved away from this strict categorization for all monitors. Consequently, OBD Auto Doctor, and this guide, adopt a more flexible approach that reflects the current industry understanding.
Continuous or Non-continuous Monitors: Flexibility in Design
The following monitors can be implemented as either continuous or non-continuous, depending on the vehicle manufacturer’s design choices:
- Misfire Monitor
- Fuel System Monitor
- Comprehensive Components Monitor
Non-Continuous Monitors: Varied by Engine Type
Non-continuous monitors differ between spark ignition (gasoline) engines and compression ignition (diesel) engines, reflecting the distinct emission control technologies employed.
Spark Ignition Vehicles (Gas Engines)
- Catalyst (CAT) Monitor
- Heated Catalyst Monitor
- Evaporative (EVAP) System Monitor
- Secondary Air System Monitor
- Oxygen (O2) Sensor Monitor
- Oxygen Sensor Heater Monitor
- EGR (Exhaust Gas Recirculation) and/or VVT System Monitor
Compression Ignition Vehicles (Diesel Engines)
- NMHC Catalyst Monitor
- NOx/SCR Aftertreatment Monitor
- Boost Pressure Monitor
- Exhaust Gas Sensor Monitor
- PM Filter Monitor
- EGR and/or VVT System Monitor
Monitoring Cycles: Since Reset and This Drive Cycle
Historically, OBD2 systems primarily reported monitor status based on the time since diagnostic trouble codes were last cleared. This “since reset” status remains mandatory for all OBD2 compliant vehicles. It provides a long-term view of monitor performance after the check engine light has been reset and DTCs cleared.
As OBD2 technology has advanced, newer vehicles now offer emission monitor status for the current driving cycle. These monitors initiate testing at the beginning of each new driving cycle. Older vehicles may not support this feature, in which case OBD Auto Doctor will display “NA” or “Not Available.”
Understanding Monitor Status: Complete, Incomplete, and Disabled
The result of a readiness monitor test yields a specific monitor status. Each monitor will have its own status, indicating the completion and outcome of its self-test. The possible statuses are:
- Complete or Ready: This indicates that the monitor test has been successfully completed, and the emission control system has passed the evaluation. OBD Auto Doctor visually represents this status with a green check mark.
- Incomplete or Not Ready: This signifies that the monitor test has not yet been completed. This could be because the OBD2 system has not yet run the test routine or because the test has been run and failed. OBD Auto Doctor uses a red exclamation mark to denote this status.
- Disabled: This status means that the monitor test has been intentionally disabled for the current monitoring cycle. Monitors may be disabled under certain conditions where the driver cannot easily operate the vehicle in a way that allows the monitor to run. For example, extreme ambient air temperatures might prevent certain monitor tests.
- Not Available (NA): OBD Auto Doctor lists all defined monitors within the software. However, the actual status can only be reported for monitors supported by your specific vehicle. “NA” indicates that the vehicle does not have that particular monitor and therefore, it cannot be tested.
Why is a Monitor Incomplete or “Not Ready”?
Several factors can result in an incomplete or “not ready” monitor status.
Clearing diagnostic trouble codes (DTCs) and turning off the Check Engine Light will invariably reset all monitor statuses to “incomplete.” This commonly occurs during or after vehicle repairs when technicians clear codes.
Monitor statuses are also reset in the event of a power failure, most often caused by disconnecting the car battery. This is why disconnecting the battery is generally discouraged unless necessary. If battery disconnection is required, for instance, for replacement, understanding how to reset the monitors afterward is crucial (discussed further below).
For the current monitoring cycle or “this drive cycle,” it is normal for monitor statuses to initially be “incomplete” upon starting the engine. These monitors begin their testing process anew with each engine start.
Crucially, depending on your local regulations, an OBDII-equipped vehicle might 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 model year 1996-2000 vehicles. For 2001 and newer vehicles, only a single monitor can be “incomplete” or “not ready” to pass inspection. This is particularly relevant for OBD2 IM (Inspection and Maintenance) readiness.
How to Get OBD2 Monitors to “Complete” or “Ready” Status
Since readiness monitors are self-check routines, the most effective way to achieve a “ready” status is by driving your vehicle. However, simply driving without variation might not fulfill all the necessary conditions for every monitor. This is where the concept of an OBD drive cycle becomes important. Before delving into drive cycles, let’s consider some fundamental steps:
- Verify No Active MIL (Malfunction Indicator Light): Ensure the Check Engine Light (MIL) is not illuminated. Active stored or pending diagnostic trouble codes can prevent monitors from running to completion. Address any underlying issues causing DTCs before attempting to get monitors ready.
- Maintain Adequate Fuel Level: Certain monitors, such as the EVAP monitor, are sensitive to fuel levels. They may require the fuel tank to be between 35% and 85% full to initiate diagnostic testing. Ensure your fuel level is within this range.
- Complete an OBD Drive Cycle: A period of combined city and highway driving, typically around one week of normal use, is often sufficient to allow most monitors to reach a “complete” status under typical driving patterns. For a more accelerated approach, or when specific monitors remain incomplete, a structured OBD drive cycle is recommended.
The OBD Drive Cycle: A Structured Approach to Readiness
The purpose of an OBD2 drive cycle is to create specific operating conditions that prompt your car’s on-board diagnostics to run and complete. This, in turn, enables the readiness monitors to operate and detect any potential malfunctions within your vehicle’s emission system. The precise drive cycle for your vehicle can vary significantly depending on the manufacturer, model, and even the specific monitor in question.
Many modern vehicle manufacturers include recommended drive cycles in the owner’s manual. Often, several days of normal driving, encompassing both city and highway conditions, will naturally set the monitors to “ready.” However, if a specific drive cycle is not available for your car, or for a more targeted approach, the following generic drive cycle can serve as a useful guideline. While not universally applicable to all vehicles and monitors, it can be effective in resetting monitors when a car-specific drive cycle is unknown.
Important Safety Note: The following drive cycle involves specific maneuvers that may be difficult or unsafe to perform in normal traffic conditions. It is strongly recommended to execute this drive cycle in a controlled environment, such as a restricted area or empty stretch of road, where safety can be ensured.
Generic OBD-II Drive Cycle Steps:
- Cold Start: Begin with a cold start. The engine coolant temperature should be below 50°C (122°F), and the ambient air temperature and coolant temperature should be within 11°C (20°F) of each other. This condition is typically achieved after the vehicle has been sitting overnight or for several hours.
- Avoid Key ON Before Cold Start: Do not turn the ignition key to the “ON” position prior to the cold start. Doing so may prevent the heated oxygen sensor diagnostic from running.
- Idle in Drive (2.5 Minutes): Start the engine and allow it to idle in “Drive” (for automatic transmissions) or in neutral with the clutch engaged (for manual transmissions) for two and a half minutes. Engage the air conditioning (A/C) and rear defroster if equipped, to increase engine load.
- Moderate Acceleration and Steady Speed (3 Minutes): Turn off the A/C and rear defroster. Accelerate moderately but consistently to 90 km/h (55 mph). Maintain this steady speed 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. Allow the engine braking to slow the vehicle down.
- Moderate to Heavy Acceleration and Steady Speed (5 Minutes): Accelerate again, this time using approximately 3/4 throttle, to reach 90-100 km/h (55-60 mph). Maintain this steady speed for five minutes.
- Controlled Stop (No Braking): Decelerate and come to a complete stop without using the brakes if possible. Again, utilize engine braking as much as possible.
Preparing for Your Vehicle Inspection (IM Readiness)
To avoid potential rejection during your annual vehicle inspection, proactive preparation is highly recommended. At a minimum, you should check your OBD2 readiness monitors to confirm they are in a “ready” state before going for the inspection. This simple step can prevent a near-certain rejection due to “not ready” monitors.
Furthermore, it is advisable to read your diagnostic trouble codes and ensure no DTCs are present. You can accomplish both of these checks – reading monitor statuses and DTCs – using OBD Auto Doctor diagnostic software. Even the free version of the software allows you to access monitor statuses and diagnostic trouble codes. Why not try OBD Auto Doctor right now to assess your vehicle’s readiness?
Remember, addressing potential issues proactively, rather than waiting until the annual inspection, can save you significant time, prevent future repair expenses, and even improve fuel efficiency.
Editor’s note: This guide was updated in October 2024 for accuracy and the latest information on OBD2 readiness monitors and IM readiness.
