Need advice here.... weird problem.

The check engine light came on a couple of weeks ago. Easy enough to pull the code: knock sensor. Local Mazda dealer wanted CAD$220 for a new one, so I asked my Rotary mechanic if he had a spare. He gave me a used one, looks identical to the one I removed. Funny thing was, the lead was about an inch shorter than the original, so I ended up having to splice an extra piece of wire on to the connector so it could reach the harness. Anyway, after putting everything back together, the light still came on. Rotary mechanic then told me to go to a Mazda dealer who, he said, had all the necessary test equipment to diagnose this properly.

Mazda dealer now says its the ECU, which is basically the last step if the "if then else" logic in the FSM, ie if nothing else works, its the ECU. New ECU is "about $1800," Mazda dealer says, "but I can fix it for you for around $600!".

OK.... so now I'm thinking about this again, and I looked at the part number on the original knock sensor on the car. It says "N3A1" on it which kinda matches up to the number in the parts fiche. But the knock sensor that I put in (the one with the spliced wire) says NF01 (I think... don't have it in front of me but pretty sure that's what it was).

Is it possible that I replaced the 3rd gen knock sensor with a 2nd gen one? This might explain the shorter wire.... and if it is a 2nd gen one, would it cause the ECU light to go off as the ECU is expecting a different knock sensor? I'm kind of in a dilemma here, which I guess I could resolve in one way by just getting a brand new knock sensor from the dealer for $220, but if the light still comes on after that, then it is the ECU and not the sensor and I've just blown $220.