So a couple weeks ago, I was driving and my check engine light came on and O/D off flashed a number of times. I brought it to my regular shop the next day (nope, Im not a do it yourself-er - I've changed my own hubcaps, windshield wipers, and put cleaning fluid in... thats about it) and they told me that it was a transmission code (which I basically knew based on the O/D off) that means the the tranny input & output speed sensors did not match (I have an automatic transmission)

He said it was the sensor and not the transmission, so they cleared the code & special ordered me a new transmission input speed sensor. $300 + labor was charged in advance (since its a special order) and I waited.

Almost week later i called back and they looked up the order and found out the Mazda had somehow messed it up (or so they said), and they corrected it. A few days after that (now 1 week and 4 days after the light originally went on), and I finally went back in to replace the part and they did so.

Now, 2 days later, I'm driving home from work and... *#!&#^@!... the CEL comes on again and O/D flashes.

I have to wonder...
1) Is it possible that I had just completed another 'drive cycle', and that the light came on because, for the majority of the code cycle (~11 of 14 days), the sensor was still broken? I know I haven't given the specific code (I'll call back tomorrow and ask if they know what it was), but is this possible with some codes?

2) Also, how exactly does a mechanic determine that its the sensor at fault? How likely is it to make this mistake? And yes, this is a mechanic I trust.

3) If the answer to 1 is not possible (or very unlikely), and the answer to 2 is also not possible (or very unlikely), then what's left?

One last thought. When I got the sensor replaced I also got my battery changed (unrelated, it was struggling to start up the car sometimes, even in warm weather). Would this reset drive cycles?