Subaru Repair: check engine light, vehicle speed sensor, subaru factory

Question
If I was to start this process what/where would you suggest I begin?  It sounds like I should begin by replacing the O2 sensors and then the N switch.  What is an N switch?  The speedo works so that may not be a problem.
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-----Question-----
Experts,  I have recently purchased a 97 impreza sport 2.2 and got a very good deal.  I guess the reason is that the check engine light was on but unplugged.  When I went to Auto Zone and had the code checked it read p1120,p0500,p1540,p0130,p1101,p1121,p0136, and p1100.  I was shocked. I went to my friendly mechanic who put his reader on and read the same codes.  He then cleared it two times but the codes would not clear. The codes remained in the system even without starting the engine. He suggested the ECM may be at fault and need to be replaced.  What is your idea of my problem?
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I wouldn't through parts at it if a vss or vehicle speed sensor is faulty when you try to clear it from memory it wont clear same goes for any other sensor if it is out of range .A lot of the listed codes are affected or can be set by the same things. such as p0130 and p0136 are the front and rear o2 sensor codes most likely whatever is affecting one may also be affecting the other one as well.p01121,1101,1120,1100 are all related to the "n"switch and starter circuit.I would start by using the manufactures flow chart and diagnose each code.Chances are we have two or three bad components or based on the history of the vehicle maybe a harness issue.While it is possible that an ECM has failed i would still diagnose the codes and symptoms first. Hope that helps you

Answer
Please don't just replace parts.The vehicle really needs to be diagnosed first.You may waste alot of time and money in a parts toss.It sounds like you want to do it your self so i would start with buying a Subaru Factory service manual or there are some web sites that will allow you to download info for a price.Then follow the steps and test the components and you should have good results.There may be a fuel trim problem causing both o2 sensors to read out of range.If you put o2 sensors in it you may still have a fuel trim problem that over time can cause engine damage by allowing the engine to run to rich or to lean.Your vehicle has more then one vehicle speed sensorAn "N" switch is the neutral safety switch