OBD2 readiness monitors are essential self-diagnostic checks within your vehicle. They offer valuable insights into your car’s emission control systems and overall health. If you’re using an OBD scanner and see the term “monitors,” this article will explain everything you need to know. As an automotive repair expert at obd2scanner.store, I’ll break down what these monitors are, why they matter, and how to interpret what your OBD scanner is telling you.
Decoding OBD2 Readiness Monitors
Readiness monitors, also known as Emissions Monitors, are designed to test the effectiveness of your car’s emission control systems. Think of them as routine health checks for components that keep your vehicle environmentally friendly. Modern vehicles can perform up to 11 different system tests, each represented by a readiness monitor. The results from these monitors indicate whether your car’s computer has successfully completed these self-tests.
Continuous vs. Non-Continuous Monitors
There are two main categories of readiness monitors: continuous and non-continuous. Their names reflect how often they are active.
- Continuous Monitors: These monitors run constantly while your engine is operating. They are always evaluating critical systems in real-time.
- Non-Continuous Monitors: These monitors require specific conditions to be met before they can run their tests. These conditions can include factors like engine temperature, driving speed, and duration. Sometimes, a specific “drive cycle” is needed to activate these monitors.
Historically, the OBD2 standard (SAE J1979) explicitly categorized each monitor as either continuous or non-continuous. However, the latest OBD2 standards have become less rigid in this categorization for some monitors. Therefore, modern OBD2 tools like OBD Auto Doctor often don’t strictly adhere to this old categorization.
Examples of Monitor Types
The following lists provide examples of common monitors and whether they are typically continuous or non-continuous. Keep in mind that manufacturers have flexibility in assigning monitor types.
Monitors that can be Continuous or Non-continuous:
- Misfire Monitor
- Fuel System Monitor
- Comprehensive Component Monitor
Non-Continuous Monitors:
The specific non-continuous monitors differ slightly between gasoline (spark ignition) and diesel (compression ignition) engines.
For Spark Ignition (Gas) Vehicles:
- 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
For Compression Ignition (Diesel) Vehicles:
- NMHC Catalyst Monitor
- NOx/SCR Aftertreatment Monitor
- Boost Pressure Monitor
- Exhaust Gas Sensor Monitor
- PM Filter Monitor
- EGR and/or VVT System Monitor
Image alt text: OBD2 scanner software interface displaying readiness monitor statuses on a Windows computer.
Understanding Monitoring Cycles: Since Reset and This Drive Cycle
OBD2 systems track monitor status in different ways, depending on the vehicle’s age and sophistication.
- Monitor Status Since DTCs Cleared (Since Reset): This is the traditional and mandatory monitor status for all OBD2 compliant vehicles. It reflects the long-term status of monitors since the last time diagnostic trouble codes (DTCs) were cleared and the check engine light was reset. This status is crucial for vehicle inspections.
- Monitor Status For This Drive Cycle: Newer vehicles often provide monitor status for the current driving cycle. These monitors reset and begin testing anew each time a new driving cycle starts (typically when you start your engine). Older cars might not support this feature, in which case your OBD scanner will likely display “NA” or “Not Available.”
Interpreting Monitor Status: Complete, Incomplete, and Disabled
Each readiness monitor reports a status indicating the outcome of its self-test. Your OBD scanner will display one of the following statuses for each monitor:
- Complete (Ready): A “Complete” or “Ready” status, often shown as a green check mark in OBD Auto Doctor, means the monitor has successfully run its test and the emission control system has passed. This is the desired status for all monitors.
- Incomplete (Not Ready): An “Incomplete” or “Not Ready” status, often indicated by a red exclamation mark, means the monitor test has not yet been completed. This could be because the necessary conditions to run the test haven’t been met, or the test has run and failed.
- Disabled: A “Disabled” status means the monitor test has been temporarily deactivated for the current monitoring cycle. Monitors might be disabled if environmental conditions, such as extreme ambient temperatures, prevent the test from running reliably under normal driving conditions.
- Not Available (NA): If your OBD scanner shows “NA” or “Not Available” for a monitor, it means your vehicle is not equipped with that particular monitor. Not all vehicles are required to support every possible monitor.
Image alt text: Screenshot of an OBD2 Android app displaying readiness monitor status, indicating “Not Available” for the current drive cycle.
Why Monitors Show “Incomplete” or “Not Ready”
Seeing “Incomplete” monitors on your OBD scanner is a common occurrence, and it doesn’t necessarily mean there’s a problem with your car. Here are the main reasons why monitors might be incomplete:
- Recent DTC Clearing: When diagnostic trouble codes (DTCs) are cleared, or the check engine light is reset (often after repairs), all readiness monitor statuses are also reset to “Incomplete.” This is normal.
- Battery Disconnection: Power loss, such as disconnecting your car battery, can also reset monitor statuses to “Incomplete.” This is why disconnecting the battery unnecessarily is generally discouraged, especially before a vehicle inspection.
- New Monitoring Cycle: For “This Drive Cycle” monitors, the status will always be “Incomplete” at the beginning of a new drive cycle, right after starting the engine. They need time to run their tests as you drive.
Importance for Vehicle Inspections: Many regions have emission testing requirements as part of vehicle inspections. Typically, your vehicle must have a certain number of readiness monitors in the “Complete” or “Ready” status to pass inspection. Regulations vary by location and vehicle model year. For example, in the United States, EPA guidelines generally allow one or two monitors to be “Not Ready” depending on the vehicle’s model year, but stricter local rules may apply.
Getting Monitors to “Complete” or “Ready” Status
Since readiness monitors are self-check routines, the primary way to get them to “Complete” is by driving your car. However, simply driving randomly might not activate all monitors. Here are steps to help get your monitors ready:
- Check for Check Engine Light and DTCs: Ensure the Malfunction Indicator Light (MIL), or check engine light, is not illuminated. Active or pending diagnostic trouble codes can prevent monitors from completing. Address any DTCs first.
- Fuel Level: Make sure you have sufficient fuel in your tank. Some monitors, particularly the EVAP system monitor, require the fuel level to be within a specific range (e.g., 35% to 85%) to initiate testing.
- Complete a Drive Cycle: The most effective way to get monitors ready is to perform an OBD drive cycle. Normal daily driving over about a week, including a mix of city and highway conditions, will often be sufficient. If you need to expedite the process or have just reset your monitors, a specific OBD drive cycle procedure can be helpful.
Understanding the OBD Drive Cycle
An OBD drive cycle is a predefined driving pattern designed to run your car’s onboard diagnostics and activate readiness monitors. The exact drive cycle varies depending on your car’s make, model, and the specific monitor you’re trying to set.
Many vehicle manufacturers include specific drive cycle instructions in the owner’s manual. If a specific drive cycle isn’t available, a generic OBD-II drive cycle can be used as a guideline. Important: Drive cycles often involve specific speeds and accelerations, so it’s best to perform them in a safe, restricted area where you can safely follow the steps.
Generic OBD-II Drive Cycle (Guideline):
- Cold Start: Begin with a cold start. The coolant temperature should be below 122°F (50°C), and the coolant and air temperatures should be within 11°F (6°C) of each other. Letting the car sit overnight typically achieves this.
- Ignition Key Position: Do not leave the ignition key in the “ON” position before the cold start, as this might prevent the heated oxygen sensor diagnostic from running.
- Idle in Drive (2.5 minutes): Start the engine and let it idle in drive for two and a half minutes. Turn on the A/C and rear defroster (if equipped) during this idle period.
- Moderate Acceleration and Steady Speed (3 minutes): Turn off the A/C and rear defroster. Accelerate moderately and steadily to 55 mph (90 km/h). Maintain a constant speed of 55 mph for three minutes.
- Deceleration (No Braking): Slow down to 20 mph (30 km/h) without using the brakes. If you have a manual transmission, do not depress the clutch during deceleration.
- Moderate Acceleration and Steady Speed (5 minutes): Accelerate again to 55-60 mph (90-100 km/h) at approximately 3/4 throttle. Hold a steady speed for five minutes.
- Stop (No Braking): Slow down to a complete stop without using the brakes.
Prepare for Your Vehicle Inspection
To avoid failing your vehicle inspection due to emission monitor issues, it’s wise to check your readiness monitor status beforehand. Using an OBD scanner to read your monitors is a simple step that can save you from a failed inspection.
You should also use your OBD scanner to read diagnostic trouble codes and ensure there are no active DTCs. OBD Auto Doctor, even the free version, allows you to check monitor statuses and DTCs. Try the software today to proactively manage your vehicle’s health.
Addressing potential emission issues early is crucial. Don’t wait until your annual inspection. Taking prompt action can save you time, prevent more costly repairs, and even improve your fuel economy.
Editor’s note: This article was updated in March 2020 for accuracy and the most current information.
