OBD-II Scanner: What Does the Question Mark Mean? Understanding Readiness Monitors

For car owners and enthusiasts keen on vehicle maintenance, understanding your car’s onboard diagnostics system is essential. OBD2 scanners have become indispensable tools for reading and interpreting vehicle data. One crucial aspect of this data is Readiness Monitors. These self-check routines are your car’s way of ensuring its emission control systems are functioning correctly. But what does it mean when your OBD-II scanner displays a question mark in relation to these monitors? Let’s delve into the world of OBD2 readiness monitors and decipher the question mark mystery.

Readiness monitors, also known as Emissions Monitors, are essentially self-tests performed by your car’s computer to evaluate the emission control systems. Think of them as health checks for components that keep your car environmentally friendly. These monitors diligently observe the performance of emission-related systems, ensuring your vehicle adheres to emission standards. Modern cars can perform up to 11 different system tests, each being a readiness monitor. The result of these monitors indicates whether your car’s computer has successfully completed these tests.

Continuous vs. Non-Continuous Monitors: Understanding the Difference

Readiness monitors are broadly classified into two types: continuous and non-continuous. Their primary difference lies in their testing methodology.

Continuous Monitors: These monitors are always active, constantly testing and evaluating systems whenever the engine is running. They are designed for real-time monitoring of critical emission components.

Non-Continuous Monitors: Unlike their continuous counterparts, non-continuous monitors require specific conditions to be met before a test can be initiated and completed. These conditions can vary widely depending on the monitor and the vehicle manufacturer. Some might need a predefined drive cycle – a specific set of driving conditions – to run, while others might require multiple drive cycles due to cool-down and warm-up periods. Each non-continuous emission monitor can have unique requirements for these conditions.

Historically, the OBD2 standard (SAE J1979) explicitly categorized each monitor as either continuous or non-continuous. However, the latest iterations of the standard have moved away from this rigid classification for some monitors. Consequently, modern OBD2 tools often don’t strictly adhere to this categorization.

Examples of Monitor Types

Here’s a breakdown of common monitors and their typical categories:

Monitors that can be Continuous or Non-continuous (Manufacturer Dependent):

Misfire Monitor: Detects engine misfires, which can significantly increase emissions.
Fuel System Monitor: Checks the fuel delivery system for proper operation and fuel mixture control.
Comprehensive Component Monitor: Monitors various sensors and components related to emissions control for electrical circuit integrity and rationality.

Non-Continuous Monitors (Further categorized by engine type):

Spark Ignition Vehicles (Gasoline Engines):

Catalyst (CAT) Monitor: Evaluates the efficiency of the catalytic converter in reducing harmful emissions.
Heated Catalyst Monitor: Checks the performance of heated catalytic converters, which reach optimal operating temperature faster.
Evaporative (EVAP) System Monitor: Tests the evaporative emission control system to prevent fuel vapor leaks into the atmosphere.
Secondary Air System Monitor: Monitors the secondary air injection system, which helps reduce emissions during cold starts.
Oxygen (O2) Sensor Monitor: Assesses the functionality of oxygen sensors, crucial for air-fuel mixture regulation.
Oxygen Sensor Heater Monitor: Checks the heaters in oxygen sensors to ensure they 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.

Compression Ignition Vehicles (Diesel Engines):

NMHC Catalyst Monitor: Evaluates the Non-Methane Hydrocarbon Catalyst, used to reduce hydrocarbon emissions in diesel engines.
NOx/SCR Aftertreatment Monitor: Monitors the NOx Aftertreatment system, often Selective Catalytic Reduction (SCR), to reduce nitrogen oxides emissions.
Boost Pressure Monitor: Checks the boost pressure control system in turbocharged diesel engines for optimal emission control.
Exhaust Gas Sensor Monitor: Monitors various exhaust gas sensors specific to diesel emission control systems.
PM Filter Monitor: Evaluates the Particulate Matter Filter (Diesel Particulate Filter – DPF) for soot accumulation and regeneration efficiency.
EGR and/or VVT System Monitor: Similar to gasoline engines, this monitors EGR and/or VVT systems for NOx control in diesel applications.

Monitoring Cycles: Since Reset and This Drive Cycle

OBD2 systems have evolved in how they report monitor status. Traditionally, the primary monitor status was “since DTCs cleared”. This status reflects the long-term readiness of the monitors after the check engine light has been reset and diagnostic trouble codes (DTCs) have been cleared. This is the mandatory monitor status for all OBD2 compliant vehicles.

Newer vehicles often report monitor status for the “current driving cycle” as well. These monitors initiate testing at the beginning of each new driving cycle – essentially every time you start your car. Older vehicles might not support this feature, in which case your OBD2 scanner will typically display “N/A” or “Not Available.”

Decoding Monitor Status: Complete, Incomplete, Disabled, and the Question Mark

The result of a readiness monitor test is its status. Each monitor reports its individual status, which can be one of the following:

Complete (Ready): This status, often indicated by a green check mark on OBD2 scanners, signifies that the monitor test has been successfully completed and the emission control system has passed the test.
Incomplete (Not Ready): This status, often represented by a red exclamation mark or a question mark on some scanners, indicates that 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. This is where the “question mark” you’re seeing likely comes into play. The question mark is often a visual cue indicating an incomplete monitor status.
Disabled: In certain situations, a monitor can be temporarily disabled for the current monitoring cycle. This typically happens when specific conditions necessary for the monitor to run cannot be met. For example, extreme ambient air temperatures might prevent certain monitors from operating correctly.
Not Available (N/A): This status indicates that the vehicle does not support a particular monitor. Not all vehicles are required to support every defined monitor.

Why is a Monitor Incomplete or “Not Ready”?

Understanding why a monitor might be incomplete is crucial for troubleshooting and ensuring your vehicle is ready for emission testing. Common reasons include:

Recent DTC Clearing: Clearing diagnostic trouble codes (DTCs) and resetting the Check Engine Light will also reset all readiness monitor statuses to “incomplete.” This usually happens during or after vehicle repairs.
Battery Disconnection: Power failures, such as disconnecting the car battery, will also reset monitor statuses. This is why disconnecting the battery unnecessarily is generally not recommended. If battery disconnection is required (e.g., for replacement), you’ll need to perform a drive cycle to reset the monitors.
New Monitoring Cycle: For the “current monitoring cycle” status, it’s normal for monitors to initially be incomplete when starting the engine. They need time and specific driving conditions to run their self-tests.

Importance for Vehicle Inspection: Many regions have emission testing requirements for vehicle registration or annual inspection. Failing to have the required monitors in a “complete” state can lead to inspection failure. For instance, in the United States, EPA guidelines allow a limited number of “not ready” monitors depending on the vehicle’s model year.

Getting Monitors to “Complete” or “Ready” Status

The most effective way to get readiness monitors to complete their tests is by driving your car under conditions that allow the monitors to run. Simply driving monotonously might not be sufficient to meet all the necessary conditions. This is where the concept of an OBD drive cycle comes into play.

Before embarking on 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. Active or pending diagnostic trouble codes can prevent monitors from completing. Address any DTCs before proceeding.
Fuel Level: Some monitors, particularly the EVAP monitor, require the fuel level to be within a specific range (e.g., 35% to 85%) to initiate testing. Ensure your fuel level is appropriate.
Drive Your Car: Normal combined city and highway driving over about a week is often sufficient to allow most monitors to reach a “complete” status. However, for faster results, or if specific monitors remain incomplete, a more structured drive cycle might be needed.

The OBD Drive Cycle: A Step-by-Step Guide

An OBD2 drive cycle is a predefined set of driving conditions designed to trigger the onboard diagnostics and allow readiness monitors to run. The exact drive cycle can vary significantly depending on your car’s make, model, and the specific monitor in question. Some vehicle manufacturers include specific drive cycle instructions in the owner’s manual.

If a car-specific drive cycle isn’t available, the following generic OBD-II drive cycle can serve as a guideline. However, remember it might not work for all vehicles and monitors. It’s best performed in a safe, restricted area due to the driving maneuvers involved.

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 of each other. Allowing the car to 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 interfere with the heated oxygen sensor diagnostic.
Idle in Drive (2.5 minutes): Start the engine and idle in drive for two and a half minutes. Turn on the 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 and steadily 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 using the brakes. If you have a manual transmission, do not depress the clutch during deceleration.
Moderate Acceleration and Steady Speed (5 minutes): Accelerate back to 90-100 km/h (55-60 mph) at approximately 3/4 throttle. Maintain this steady speed for five minutes.
Stop (No Braking): Slow down to a complete stop without using the brakes.

Preparing for Vehicle Inspection

Before taking your car for an annual inspection, it’s prudent to check the readiness monitors yourself using an OBD2 scanner. Ensuring all required monitors are “complete” will prevent potential rejection due to emission system readiness.

Furthermore, it’s always a good idea to read diagnostic trouble codes and address any present issues before inspection. OBD Auto Doctor and similar diagnostic software allow you to read both monitor statuses and DTCs, even with free versions. Proactive maintenance and addressing issues promptly can save you time, money, and future repair costs.

By understanding OBD2 readiness monitors and what the question mark on your scanner signifies, you can take better care of your vehicle’s emission system and ensure it’s ready for inspection.