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OBDII Readiness Monitor

What are readiness monitors and how do they work?

An OBDII  readiness monitor is a software protocol in the car’s computer that self-tests every section of the emissions system. The system was developed to prevent drivers from gaming their state’s emissions testing. In the past, drivers would get a trouble code showing a problem with a sensor or a system issue that was outside of spec. They would then disconnect the battery cables or use a scan tool to erase the stored codes and do a hard reset on their computer. Then they would drive it to the emissions testing station with no codes present. Those drivers can still try that trick, but they’ll instantly fail the emissions testing because all the readiness monitors will show NOT READY.

How does a readiness monitor work

Let’s say we’re talking about a catalytic converter. For a catalytic converter to work properly, it must heat up to a certain temperature. The computer knows how long the engine has been running and it knows the engine coolant temperature reading. Some of the newest cars have an exhaust gas temperature sensor, but the computer can still test cars that don’t. To test if the catalytic convert is working properly, the computer will command a rich mixture. It will look at the readings for the upstream oxygen sensor to make sure it sees the rich mixture. Then it will look at the downstream sensor to see if it shows a rich mixture as well. If the downstream sensor shows a rich condition, that means the catalytic converter isn’t hot enough OR the catalytic converter isn’t doing its job properly. If the readings fail the test, the computer will repeat the test under a set driving condition, called a Drive Cycle until it passes the test or determines that there’s a true failure. If it passes the test, the computer will turn the catalytic converter readiness monitor to READY/PASS. If it fails, it will read NOT READY/FAIL.

Two types of OBDII readiness monitors

According to the rules established by OBDII, cars must have continuous and non-continuous monitors. As the name implies, a continuous readiness monitor is constantly checking the system. The misfire monitor, for example, will constantly look for misfires. If the misfire count exceeds a certain set number of times/minute, it will set a trouble code. Non-continuous monitors, as the name implies, may only test the system once per drive.

Some examples on NON-continuous OBDII readiness monitors:

Comprehensive Component Monitor (CCM) See this article for more information
Misfires — See this article for more information on the misfire monitor
Fuel system mixtures — See this article for more information on Fuel Trim and how it works

Some examples on NON-continuous OBDII readiness monitors:

Oxygen Sensor Monitor — See this article for more information on the oxygen sensor monitor
Oxygen Sensor Heater Monitor — See this article for more information on the oxygen sensor heater monitor
Catalyst Monitor — See this article for more information on the Catalyst Monitor
Heated Catalyst Monitor — See this article for more information on the Heated Catalyst Monitor
EGR System Monitor — See this article for more information on the EGR monitor
EVAP System Monitor — See this article for more information on the EVAP Monitor
Secondary Air System Monitor — See this article for more information on Secondary Air Monitor

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

NMHC Monitor — See this article for more information on NMHC Monitor
NOx Adsorber Monitor — See this article for more information on NOx Adsorber monitor
Boost Pressure System Monitor — See this article for more information on the Boost Pressure System Monitor
Exhaust Gas Sensor Monitor — See this article for more information on the Exhaust GAs Sensor Monitor
PM Filter Monitor — See this article for more information on the PM filter monitor

©, 2019 Rick Muscoplat

Posted on by Rick Muscoplat



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