The Basics of OBD2 Readiness Monitors

Bykpausche

An Introduction

Every gasoline or diesel passenger car or light truck sold in North America is required to meet OBD2 diagnostic standards as defined by the SAE (Society of Automotive Engineers).

In addition to the functions you may already know about such as diagnostic trouble codes (DTC), there is a subset of tests that engine control modules must execute. These are known as “readiness monitors”. We will refer to them simply as “monitors” herein.

Among these monitors are two types: continuous and non-continuous.

Continuous Monitors

Three monitors are specified and designed to constantly monitor systems for proper operation. Each vehicle design varies, but logic is used to alert the driver and set a DTC should specific criteria be met.

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

Non-Continuous Monitors

For many other functions of the vehicle, continuous monitoring may not be appropriate. Specific systems like the catalytic convertor require pre-determined conditions such as road speed and exhaust temperature to be at ideal levels. For these, we have non-continuous monitors.

  • 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

Setting and Resetting

Non-Continuous monitor status will be reset whenever the vehicle’s battery is disconnected or if the diagnostic information is cleared with a scan tool. In OBD2 terminology, a monitor status is “incomplete” if its test parameters are not met. These test parameters are referred to as a “drive cycle.”

Drive Cycles

Each non-continuous monitor will have a specific drive cycle outlined by the manufacturer. Below is an example method to complete a catalyst monitor on a Japanese car.

Note that if the road speeds and timing is not met, the catalyst monitor will not complete. Simply driving this vehicle around city traffic would not meet the criteria of a 50 mph road speed, for example. Other monitors such as the EVAP System Monitor (evaporative emission system) are notoriously difficult to run. In some cases it could take several weeks of driving to complete, even depending on external factors such as ambient temperature.

Monitor Count

State and provincial emissions testing programs such as the IM240 or “smog” test have used completed monitor counts as a factor into testing vehicles for emission law compliance. A fully functioning vehicle, for instance, would have an incomplete count of zero. In other words, all monitors will have completed and passed. This is a good indication that the vehicle systems are running normally.

In the following example, ALD CHECKBOX is showing that 5 monitors are incomplete. Note the highlighted area next to the clipboard icon. This, in addition with the DTC count, has caused a FAIL condition.

Incomplete Monitor Count

As a reminder, disconnecting the battery or clearing the diagnostic DTC informaiton will reset the monitor count. Scan tools such as CHECKBOX will report the monitor count to the user. In a case where there are diagnostic trouble codes set, but the incomplete monitor count is high then one could assume that the diagnostic information has been cleared recently.

Conclusion

Being aware of OBD2 readiness monitors is useful knowledge for anyone responsible for checking and appraising vehicles. The technical specifics may not be required to know, but a tool such as CHECKBOX makes it easy to understand just by connecting to the OBD2 port and viewing the display.