It sounds like you have an intermittent problem that is approaching constant. If the check engine light is not on when you hook up the OBD code reader, you will not see the code.

My earlier description of how the code keeps the check engine light on was less than complete. When a problem occurs that causes the check engine light light to come on, the code is "latched" into the ECM memory which in turn will keep the check engine light on even if the problem is fixed or goes away. Until the code is cleared from the ECM memory, a code reader will display the code.

 

Thanks. That clarifies the process to me. Again, thanks.