Rick's Free Auto Repair Advice

OBDII Readiness Monitor

What is a readiness monitor 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:

Fuel system mixtures

Some examples on NON-continuous OBDII readiness monitors:

Catalytic converter
Evaporative emission system
Secondary Air System
Oxygen (O2) Sensor function test
Oxygen Sensor Heater test
EGR (Exhaust Gas Recirculation)
Variable valve timing VVT System

©, 2019 Rick Muscoplat

Posted on by Rick Muscoplat


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