Understanding OBD2 Monitors

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OBD2-compliant vehicles are equipped with special diagnostic routines, called “Monitors”, to help ensure the vehicle’s emission-related components and systems are functioning properly.

Monitor Overview

To ensure the correct operation of the various emissions-related components and systems on OBD2-compliant vehicles, the vehicle’s on-board computer is equipped with a specialized diagnostic program. The program incorporates several procedures and diagnostic strategies, each of which is designed to monitor the operation of, and perform diagnostic tests on, a specific emissions-related component or system. These tests ensure the tested component or system is operating properly within the manufacturer’s specifications. On OBD2-compliant vehicles, these procedures and diagnostic strategies are called "Monitors."

Currently, a maximum of eleven Monitors are used in OBD2 systems. Additional monitors may be added as a result of Government regulations as the OBD2 system grows and matures. Not all vehicles support all eleven Monitors.

Monitor operation is either "Continuous" or "Non-Continuous," depending on the specific monitor.

Continuous Monitors

Three of the Monitors are designed to continuously monitor their associated components and/or systems for proper operation. Continuous Monitors run whenever the engine is running. The Continuous Monitors are:

• Comprehensive Component Monitor (CCM)
• Misfire Monitor
• Fuel System Monitor

Non-Continuous Monitors

The remaining eight Monitors are "non-continuous" Monitors. "Non-continuous" Monitors perform and complete their testing once per trip. The "non-continuous" Monitors are:

• Oxygen Sensor Monitor
• Oxygen Sensor Heater Monitor
• Catalyst Monitor
• Heated Catalyst Monitor
• EGR System Monitor
• EVAP System Monitor
• Secondary Air System Monitor
• Air Conditioning (A/C) Monitor

Terms and Definitions

The following terms are used throughout this article, and are defined here to aid in the understanding of OBD2 Monitors.

• Monitor Has/Has Not Run - The term "Monitor has run" means the vehicle’s computer has commanded a particular Monitor to perform the required diagnostic testing on a system to ensure the system is operating correctly (within factory specifications). The term "Monitor has not run" means the vehicle’s computer has not yet commanded a particular Monitor to perform diagnostic testing on its associated part of the emissions system.
• Rationality – The determination that a signal received from a monitored component or system appears rational. Each input signal is compared against all other inputs and against information in the computer’s memory to see if it “makes sense” under the current operating conditions. Example: The signal from the throttle position sensor indicates the vehicle is in a wide-open throttle condition, but the vehicle is really at idle, and the idle condition is confirmed by the signals from all other sensors. Based on the input data, the computer determines that the signal from the throttle position sensor is not rational (does not make sense when compared to the other inputs). In this case, the signal would fail the rationality test.

How Monitors are “Set”

Enabling Criteria

Each Monitor is designed to test and monitor the operation of a specific part of the vehicle’s emissions system (EGR system, oxygen sensor, catalytic converter, etc.). A specific set of "conditions" or "driving procedures" must be met before the computer can command a Monitor to run tests on its related system. These "conditions" are known as "Enabling Criteria." The requirements and procedures vary for each Monitor. Some Monitors only require the ignition key to be turned "On" for them to run and complete their diagnostic testing. Others may require a set of complex procedures, such as, starting the vehicle when cold, bringing it to operating temperature, and driving the vehicle under specific conditions before the Monitor can run and complete its diagnostic testing.

Trip Drive Cycle

A Trip Drive Cycle for a given Monitor requires that the vehicle is driven in such a manner that all the required "Enabling Criteria" for the Monitor to run and complete its diagnostic testing are met. The "Trip Drive Cycle" for a particular Monitor begins when the ignition key is turned "On." It is successfully completed when all the "Enabling Criteria" for the Monitor to run and complete its diagnostic testing are met by the time the ignition key is turned "Off." Since each Monitor is designed to perform its diagnostic testing on a different part of the engine or emissions system, the "Trip Drive Cycle" needed for each individual Monitor to run and complete varies.

A Monitor may be designated as either a “One Trip” Monitor or a “Two Trip” Monitor. A “One Trip” Monitor will set a DTC and command the MIL “On” the first time a fault is found. When a “Two Trip” Monitor detects a fault on the first trip, the vehicle’s computer temporarily saves the fault in its memory as a Pending Code. The computer does not command the MIL “On” at this time. If the fault is detected again on the second (subsequent) trip, the computer commands the MIL “On” and saves the code in its long-term memory.

OBD2 Drive Cycle

An OBD2 Drive Cycle is an extended set of driving procedures that takes into consideration the various types of driving conditions encountered during “real life” driving. These conditions may include starting the vehicle when it is cold, driving the vehicle at a steady speed (cruising), accelerating, etc. An OBD2 Drive Cycle begins when the ignition key is turned "On" (when cold) and ends when the vehicle has been driven in such a way as to have all the "Enabling Criteria" met for all its applicable Monitors. Only those trips that provide the Enabling Criteria for all Monitors applicable to the vehicle to run and complete their individual diagnostic tests qualify as OBD2 Drive Cycles. OBD2 Drive Cycle requirements vary from one model of vehicle to another. Vehicle manufacturers set these procedures. Consult your vehicle’s service manual for OBD2 Drive Cycle procedures.

Do not confuse a Trip Drive Cycle with an OBD2 Drive Cycle. A Trip Drive Cycle provides the "Enabling Criteria" for one specific Monitor to run and complete its diagnostic testing. An OBD2 Drive Cycle must meet the "Enabling Criteria" for all Monitors on a particular vehicle to run and complete their diagnostic testing.

Monitor Descriptions

The following paragraphs provide a brief description of the purpose and function of each of the eleven Monitors.

Comprehensive Component Monitor (CCM) - This Monitor continuously checks all inputs and outputs from sensors, actuators, switches and other devices that provide a signal to the computer. The Monitor checks for shorts, opens, out of range values, functionality and "rationality."

The CCM may be either a "One-Trip" or a "Two-Trip" Monitor, depending on the component.

Fuel System Monitor - This Monitor uses a Fuel System Correction program, called Fuel Trim, inside the on-board computer. Fuel Trim is a set of positive and negative values that represent adding or subtracting fuel from the engine. This program is used to correct for a lean (too much air/not enough fuel) or rich (too much fuel/not enough air) air-fuel mixture. The program is designed to add or subtract fuel, as needed, up to a certain percent. If the correction needed is too large and exceeds the time and percent allowed by the program, a fault is indicated by the computer.

The Fuel System Monitor may be a "One-Trip" or "Two-Trip" Monitor, depending on the severity of the problem.

Misfire Monitor - This Monitor continuously checks for engine misfires. A misfire occurs when the air-fuel mixture in the cylinder does not ignite. The misfire Monitor uses changes in crankshaft speed to sense an engine misfire. When a cylinder misfires, it no longer contributes to the speed of the engine, and engine speed decreases each time the affected cylinder(s) misfire. The misfire Monitor is designed to sense engine speed fluctuations and determine from which cylinder(s) the misfire is coming, as well as how bad the misfire is. There are three types of engine misfires, Types 1, 2, and 3.

• Type 1 and Type 3 misfires are “Two Trip” monitor faults. If a fault is sensed on the first trip, the computer temporarily saves the fault in its memory as a Pending Code. The MIL is not commanded on at this time. If the fault is found again on the second trip, under similar conditions of engine speed, load and temperature, the computer commands the MIL "On," and the code is saved in its long term memory.
• Type 2 misfires are the most severe type of misfire. When a Type 2 misfire is sensed on the first trip, the computer commands the MIL to light when the misfire is sensed. If the computer determines that a Type 2 misfire is severe , and may cause catalytic converter damage, it commands the MIL to "flash" once per second as soon as the misfire is sensed. When the misfire is no longer present, the MIL reverts to steady "On" condition.

Catalyst Monitor - The catalytic converter is a device that is installed downstream of the exhaust manifold. It helps to oxidize (burn) the unburned fuel (hydrocarbons) and partially burned fuel (carbon monoxide) left over from the combustion process. To accomplish this, heat and catalyst materials inside the converter react with the exhaust gases to burn the remaining fuel. Some materials inside the catalytic converter also have the ability to store oxygen, and release it as needed to oxidize hydrocarbons and carbon monoxide. In the process, it reduces vehicle emissions by converting the polluting gases into carbon dioxide and water.

The computer checks the efficiency of the catalytic converter by monitoring the oxygen sensors used by the system. One sensor is located before (upstream of) the converter; the other is located after (downstream of) the converter. If the catalytic converter loses its ability to store oxygen, the downstream sensor signal voltage becomes almost the same as the upstream sensor signal. In this case, the monitor fails the test. The Catalyst Monitor is a "Two-Trip" Monitor.

Heated Catalyst Monitor - Operation of the "heated" catalytic converter is similar to the catalytic converter. The main difference is that a heater is added to bring the catalytic converter to its operating temperature more quickly. This helps reduce emissions by reducing the converter’s down time when the engine is cold. The Heated Catalyst Monitor performs the same diagnostic tests as the catalyst Monitor, and also tests the catalytic converter’s heater for proper operation. This Monitor is also a "Two-Trip" Monitor.

Exhaust Gas Recirculation (EGR) Monitor - The Exhaust Gas Recirculation (EGR) system helps reduce the formation of Oxides of Nitrogen during combustion. Temperatures above 2500°F cause nitrogen and oxygen to combine and form Oxides of Nitrogen in the combustion chamber. To reduce the formation of Oxides of Nitrogen, combustion temperatures must be kept below 2500°F. The EGR system recirculates small amounts of exhaust gas back into the intake manifold, where it is mixed with the incoming air/fuel mixture. This reduces combustion temperatures by up to 500°F. The computer determines when, for how long, and how much exhaust gas is recirculated back to the intake manifold. The EGR Monitor performs EGR system function tests at preset times during vehicle operation. The EGR Monitor is a "Two-Trip" Monitor.

Evaporative System (EVAP) Monitor - OBD2 vehicles are equipped with a fuel Evaporative system (EVAP) that helps prevent fuel vapors from evaporating into the air. The EVAP system carries fumes from the fuel tank to the engine where they are burned during combustion. The EVAP system may consist of a charcoal canister, fuel tank cap, purge solenoid, vent solenoid, flow monitor, leak detector and connecting tubes, lines and hoses.

Fumes are carried from the fuel tank to the charcoal canister by hoses or tubes. The fumes are stored in the charcoal canister. The computer controls the flow of fuel vapors from the charcoal canister to the engine via a purge solenoid. The computer energizes or de-energizes the purge solenoid (depending on solenoid design). The purge solenoid opens a valve to allow engine vacuum to draw the fuel vapors from the canister into the engine where the vapors are burned. The EVAP Monitor checks for proper fuel vapor flow to the engine, and pressurizes the system to test for leaks. The computer runs this Monitor once per trip. The EVAP Monitor is a "Two-Trip" Monitor.

Air Conditioning (A/C) Monitor - The A/C Monitor senses leaks in air conditioning systems that utilize R-12 refrigerant. Vehicle manufacturers have been given two options:

• Use R-12 refrigerant in their A/C systems, and integrate an A/C Monitor in the OBD2 systems of these vehicles to detect for refrigerant leaks; or
• Use R-134 refrigerant instead of R12. The A/C Monitor is not required on these vehicles.

To date, all vehicle manufacturers have opted to use R-134 in their A/C systems. As a result, this Monitor has not yet been implemented.

Oxygen Sensor Heater Monitor - The Oxygen Sensor Heater Monitor tests the operation of the oxygen sensor’s heater. There are two modes of operation on a computer-controlled vehicle: "open-loop" and "closed-loop." The vehicle operates in open-loop when the engine is cold, before it reaches normal operating temperature. The vehicle also goes to open-loop mode at other times, such as heavy load and full throttle conditions. When the vehicle is running in open-loop, the oxygen sensor signal is ignored by the computer for air/fuel mixture corrections. Engine efficiency during open-loop operation is very low, and results in the production of more vehicle emissions.

Closed-loop operation is the best condition for both vehicle emissions and vehicle operation. When the vehicle is operating in closed-loop, the computer uses the oxygen sensor signal for air/fuel mixture corrections.

In order for the computer to enter closed-loop operation, the oxygen sensor must reach a temperature of at least 600°F. The oxygen sensor heater helps the oxygen sensor reach and maintain its minimum operating temperature (600°F) more quickly, to bring the vehicle into closed-loop operation as soon as possible. The Oxygen Sensor Heater Monitor is a "Two-Trip" Monitor.

Oxygen Sensor Monitor - The Oxygen Sensor monitors how much oxygen is in the vehicle’s exhaust. It generates a varying voltage of up to one volt, based on how much oxygen is in the exhaust gas, and sends the signal to the computer. The computer uses this signal to make corrections to the air/fuel mixture. If the exhaust gas has a large amount of oxygen (a lean air/fuel mixture), the oxygen sensor generates a "low" voltage signal. If the exhaust gas has very little oxygen (a rich mixture condition), the oxygen sensor generates a "high" voltage signal. A 450mV signal indicates the most efficient, and least polluting, air/fuel ratio of 14.7 parts of air to one part of fuel.

The oxygen sensor must reach a temperature of at least 600-650°F, and the engine must reach normal operating temperature, for the computer to enter into closed-loop operation. The oxygen sensor only functions when the computer is in closed-loop. A properly operating oxygen sensor reacts quickly to any change in oxygen content in the exhaust stream. A faulty oxygen sensor reacts slowly, or its voltage signal is weak or missing. The oxygen sensor is a "Two-Trip" monitor.

Secondary Air System Monitor - When a cold engine is first started, it runs in open-loop mode. During open-loop operation, the engine usually runs rich. A vehicle running rich wastes fuel and creates increased emissions, such as carbon monoxide and some hydrocarbons. A Secondary Air System injects air into the exhaust stream to aid catalytic converter operation:

• It supplies the catalytic converter with the oxygen it needs to oxidize the carbon monoxide and hydrocarbons left over from the combustion process during engine warm-up.
• The extra oxygen injected into the exhaust stream also helps the catalytic converter reach operating temperature more quickly during warm-up periods. The catalytic converter must heat to operating temperature to work properly.

The Secondary Air System Monitor checks for component integrity and system operation, and tests for faults in the system. The computer runs this Monitor once per trip. The Secondary Air System Monitor is a "Two-Trip" monitor.

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