another question about readiness codes

[bob]

my scan tool tells you if your car will pass emissions in the readiness mode it tells me that the car needs emissions attention but their are no trouble codes or check engine lights. Should I be concern that my emission are not working properly. Thanks Eric for your help. I know I`m a pain.

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[Mike]

First off, no codes, no emissions problem.

Second, let me try to clarify the codes/readiness language.

The car has 7-10 self tests (depending on the car) that it runs to make sure that it’s emissions controls systems are working. These tests are called “monitors”. When all the monitors have been run and they all passed, the car is now “ready” or the “readiness has been set”. The wording in generally the each given monitor is either “complete” if it has been run or “incomplete” if it has not been run since the last time codes were “cleared” which includes clearing all readiness tests. Codes cannot be cleared without also clearing the monitors, setting all the readiness monitors to “incomplete”.

In NY, cars are allowed to have one monitor to be incomplete and still pass inspection. In some cases this can be exploited to allow a car that is guaranteed to have a CEL come on to pass a state inspection. Cars older than 2001 are allowed to have two incomplete monitors. These allowances vary from state to state, so you might want to find out how it is where you are.

If any monitor fails, a “diagnostic trouble code” (DTC) will be generated. The moment the monitor fails, the computer will generate a “pending” DTC. The car will then run the monitor a second time. If the same failure happens again, a “hard” DTC will be generated and the check engine light will come on. You can have the same code be set hard and be pending at the same time. That means that the failure has already been confirmed, and is also happening at the time you are checking the codes.

This whole system of monitors and code generation is called the “on board diagnosis” (OBD) system. The check engine light is actually a check emissions light, as the only reason it will come on is if it can confirm that the car is failing to meet it’s designed emissions standard. It has nothing to do with if there is an engine problem. Your engine can be completely destroyed by lack of lubrication and you may never see a check engine light. It is a emissions control system, and only emissions problems can trigger a DTC.

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