OBD2 Monitors: Continuous and Non-Continuous Monitors

What is an OBD Monitor?

An Onboard Diagnostic (OBD) Monitor is a computer that performs a series of tests to determine if your vehicle's emission control system is failing, which can affect the result of an upcoming emission test. 

What is an OBD Monitor Test?

An OBD Monitor test is a test of the different inspection monitors that must be diagnosed by perming a complete drive cycle before an emission test. 

Note: Self-diagnostic tests are performed when your vehicle is driven. 

What are Inspection Monitors? 

When driving your vehicle, your car will continuously monitor systems to determine possible issues affecting its emissions control equipment. Inspection Monitors are also called readiness monitors, which are read by an OBD2 scanner during a vehicle's self-diagnostic test. 

OBD2 Monitors: Continuous Monitors

Three of these monitors are designed to constantly monitor their associated components and/or systems for proper operation. Continuous monitors run constantly when the engine is running. The continuous monitors are:

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

OBD2 Monitors: Non-Continuous Monitors

The other twelve 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

Note: The following monitors became standard beginning in 2010. The majority of vehicles produced before this time will not support these Monitors

  • NMHC Monitor
  • NOx Adsorber Monitor
  • Boost Pressure System Monitor
  • Exhaust Gas Sensor Monitor
  • PM Filter Monitor

A brief explanation of the function of each Monitor

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 value, functionality, and rationality.

Rationality: 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 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.

The CCM is supported by both spark ignition vehicles and compression ignition vehicles. 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 onboard 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 percentage. 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 is supported by both spark ignition vehicles and compression ignition vehicles. 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) misfires. 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.

The Misfire monitor is supported by both spark ignition vehicles and compression ignition vehicles.

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 supported by spark ignition vehicles only. The Catalyst Monitor is a Two-Trip monitor. If a fault is found on the first trip, the 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 sensed again on the second trip, the computer commands the MIL ON and saves the code in its long-term memory.

Heated Catalyst Monitor

The 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 downtime 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.

The Heated Catalyst Monitor is supported by spark ignition vehicles only. 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 supported by both spark ignition vehicles and compression ignition vehicles. The EGR Monitor is a Two-Trip monitor. If a fault is found on the first trip, the 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 sensed again on the second trip, the computer commands the MIL ON, and saves the code in its long-term memory.

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 the solenoid design). The purge solenoid opens a valve to allow the 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 supported by Spark ignition vehicles only. The EVAP monitor is a Two-Trip monitor. If a fault is found on the first trip, the 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 sensed again on the second trip, the PCM commands the MIL ON, and saves the code in its long-term memory.

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 an 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 an 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.

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 an 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 an 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.

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 supported by Spark ignition vehicles only. The Oxygen Sensor Heater monitor is a Two-Trip monitor. If a fault is found on the first trip, the 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 sensed again on the second trip, the computer commands the MIL ON, and saves the code in its long-term memory.

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 a 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 Monitor is supported by Spark ignition vehicles only. The Oxygen Sensor Monitor is a Two-Trip monitor. If a fault is found on the first trip, the 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 sensed again on the second trip, the computer commands the MIL ON, and saves the code in its long-term memory.

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:

  1. 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.
  2. 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 the 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. If a fault is found on the first trip, the computer temporarily saves this fault in its memory as a Pending Code. The computer does not command the MIL on at this time. If the fault is sensed again on the second trip, the computer commands the MIL ON, and saves the code in its long-term memory.

Non-Methane Hydrocarbon Catalyst (NMHC) Monitor

The non-methane hydrocarbon catalyst is a type of catalytic converter. It helps to remove non-methane hydrocarbons (NMH) left over from the combustion process from the exhaust stream. To accomplish this, heat and catalyst materials react with the exhaust gases to convert NMH to less harmful compounds. The computer checks the efficiency of the catalyst by monitoring the quantity of NMH in the exhaust stream. The monitor also verifies that sufficient temperature is present to aid in particulate matter (PM) filter regeneration.

The NMHC Monitor is supported by Compression ignition vehicles only. The NMHC Monitor is a Two-Trip monitor. If a fault is found on the first trip, the 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 sensed again on the second trip, the computer commands the MIL ON, and saves the code in its long-term memory.

NOx Aftertreatment Monitor

NOx after treatment is based on catalytic converter support that has been coated with a special washcoat containing zeolites. NOx Aftertreatment is designed to reduce oxides of nitrogen emitted in the exhaust stream. The zeolite acts as a molecular Sponge to trap the NO and NO2 molecules in the exhaust stream. In some implementations, injection of a reactant before the after treatment purges it. NO2, in particular, is unstable and will join with hydrocarbons to produce H2O and N2. The NOx Aftertreatment Monitor monitors the function of the NOx after treatment to ensure that tailpipe emissions remain within acceptable limits.

The NOx Aftertreatment Monitor is supported by Compression ignition vehicles only. The NOx Aftertreatment Monitor is a Two-Trip Monitor. If a fault is found on the first trip, the 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 sensed again on the second trip, the computer commands the MIL ON, and saves the code in its long-term memory.

Boost Pressure System Monitor

The Boost Pressure System serves to increase the pressure produced inside the intake manifold to a level greater than atmospheric pressure. This increase in pressure helps to ensure complete combustion of the air-fuel mixture. The Boost Pressure 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 Boost Pressure System monitor is supported by Compression ignition vehicles only. The Boost Pressure System monitor is a Two-Trip monitor. If a fault is found on the first trip, the 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 sensed again on the second trip, the computer commands the MIL ON, and saves the code in its long-term memory.

Exhaust Gas Sensor Monitor

The Exhaust Gas Sensor is used by a number of systems/monitors to determine the content of the exhaust stream. The computer checks for component integrity, and system operation, and tests for faults in the system, as well as feedback faults that may affect other emission control systems.

The Exhaust Gas Sensor monitor is supported by Compression ignition vehicles only. The Exhaust Gas Sensor Monitor is a Two-Trip Monitor. If a fault is found on the first trip, the 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 sensed again on the second trip, the computer commands the MIL ON, and saves the code in its long-term memory.

PM Filter Monitor

The particulate matter (PM) filter removes particulate matter from the exhaust stream by filtration. The filter has a honeycomb structure similar to a catalyst substrate, but with the channels blocked at alternate ends. This forces the exhaust gas to flow through the walls between the channels, filtering the particulate matter out. The filters are self-cleaning by periodic modification of the exhaust gas concentration in order to burn off the trapped particles (oxidizing the particles to form CO2 and water). The computer monitors the efficiency of the filter in trapping particulate matter, as well as the ability of the filter to regenerate (self-clean).

The PM Filter monitor is supported by Compression ignition vehicles only. The PM Filter monitor is a Two-Trip monitor. If a fault is found on the first trip, the 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 sensed again on the second trip, the computer commands the MIL ON, and saves the code in its long-term memory.

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