Subaru Repair: Check Engine Light, subaru legacy wagon, oxygen monitor

Question
I have a 2002 Subaru Legacy wagon.  The check engine light keeps coming on. The first and third times were bogus, but it costs $50 each time to have it checked and turned off.  The second time I did have the oxygen sensor replaced.  Thanks so much for any help you can give!

Answer
Hello Sue,

I will try to answer your question as best as possible but you may not like what I have to say.  There is a lot of confusion about what a check engine light means, this includes many automotive technicians and shops alike.  When the light comes on there is a corresponding code set that must be diagnosed by a skilled professional. There are two types of codes set in an OBDII equipped vehicle(1996 to present).  Component and conditional.  Conditional codes can be very hard to diagnose if the technician working on the car is not very familiar with Subaru or not at the top of his or her field. While I don't want to discount you spending $50.00 to have a diagnoses done I don't think you are getting any real value for your money, especially if the light is just being turned off.  What is also always lost is that when the check engine light is cleared what really happens is that the memory is cleared and all of the emission system monitors are turned to not ready status.  Once an emissions monitor is turned to not ready status an OBDII drive cycle must be performed to reset the emissions monitors back to ready status.  The whole time that a O2(oxygen) monitor is in not ready status the computer is not monitoring the O2 sensor.  It can take months for the monitor to go back to ready status if the specific drive cycle is not completed.  This is why the light can be turned off and stay off for months with out the car ever truly being repaired. While I don't know all of the specifics in regards to the issues you have had recently I suspect that that either the car has never been truly diagnosed properly or you have had a few different issues which is possible but would also require either a part being replaced or a computer reflash, or a procedure being performed.  It doesn't sound to me like that has been the case. Please remember that there is no such thing as a "bogus code".  If the computer turns on the light it has seen a problem with an emissions or engine control device or a specific condition has been met in accordance with the computers programming that will cause the computer to command on the check engine light.

What needs to happen is a "real diagnoses" needs to be done and it will most likely cost more than $50.00.  I would encourage you to seek out a Subaru Specialist in your area or take it to the Subaru dealer service department.  Having a reoccurring check engine light and no real answers is bad for both the health of you Subaru and the health of the environment.   

I know this is a wordy reply, but there is just not enough people providing the drivers of cars with real factual information about their check engine light and what it really means.