Am I'm Getting Screwed ??
 

Any feedback here be appreciated:
a) the engine warning light comes on (again - seems like every 3 months the light comes on if I accelerate on the highway?!!)
b) take car to Nissan, they put on tester and repalce Oxygen sensor - engine warning light goes off.
c) that evening car barely starts.
d) next morning car barely starts - seems like battery dying - take car to where I bought battery 6 months ago. they put testing equipment on car and test for 20 + minutes (??!!) - they say battery 80% but starter motor has high (amperage I think) that I need new starter. I wanted to take car to Nissan to get second opinion - but car will hardly turn over once and then nothing at all - so that starter is replaced.
e) when I come to pick up car - on the invoice for the starter - there is a note the engine warning light is on?
f) I take car back to Nissan - who try to put tester on - but tester won't go on because they say something is now wrong with the wiring that connects to tester??

Anyone here know what's happened?