Toyota Repair: engine light, negative battery cable, continuous problem

Question
Andrew,

Rav4 with 105,000 miles.  My engine light came on and was on for a day and a half, then went off.  From what I've read, it sounds like the light will come back on again and go off again, over and over, eventually til the problems fixed.  I though once the engine light came on, it stayed on until it has be reset.  What causes this???
I did get the code the day the light was on.  It was PO401, which is EGR flow insufficient.

Thanks You,

Rick

Answer
There are 2 types of codes that are atored in your cars ECU. A 'hard' code and a 'soft' code. A hard code means that there is a continuous problem and the engine light will be illuminated all the time. A soft code means that the engine light has come on at some point but has turned off. You are experiencing symptoms of a soft code. To erase these codes, disconnect the negative battery cable for no less than 60 seconds, then re-attach it.