[Thanks a bunch to all y'all. Since it's the weekend, I'll get back with them Monday and show them what I found out. QUOTE=Snowbelt Spyder;1231595]Yeah, it would help to know which code you're getting to make sure we're talking knock sensor codes, but let's go over the basics for a second.
Think of the knock sensor as a specially tuned microphone whose job it is to listen for the sounds of detonation - and ignore the rest. When it "hears" detonation occurring, it takes control of the ignition timing and retards it so that the detonation clears. There's a limit to how far it can go. This override of the timing continues, with the sensor keeping control right on the verge of detonation, until the situation goes away and the sensor can go out of control without detonation re-occuring. It's not like a switch, there's nothing to reset, and it does all this in the background without lighting any lights or tripping any codes. Unless you have an extremely sensitive butt dyno, you'll never know it happened at all.
So, your mechanic might be mistaken because it typically doesn't throw codes when it thinks it's doing its job, but only when there is something wrong. HOWEVER, a check engine light could also be lit if the noise level is particularly high or if the detonation cannot be controlled by retarding the timing the set amount, such as what might occur if there is a loose part or excessive vibration external to the engine, but being picked up by the sensor. The codes listed in the repair manual refer to wiring, damage, excessive noise or mounting problems.
You have two knock sensors - one on the right hand side of the front cylinder, and the other on the left side of the rear cylinder. It's that little round black sensor with the bolt going down through the middle of it. The code will tell you which sensor is acting up. Number 1 is front, 2 is rear. You'll want to check the mounting, has it come loose - but don't over tighten. and you'll want to check harness connections - especially just disconnecting them and then reconnecting them a couple times. Some contact cleaner never hurt. You'll also want to think about the timing of the knock sensor problem related to installation of the cat bypass, to think for yourself if the two are related. I'm not sure an exhaust leak can fool a knock sensor into thinking it's hearing pinging, but.....check that pipe joint. And they can respond to a loose part or vibration external to the engine. Any metal to metal contact under there between the pipe and frame? Did you do any other work that could have damaged or disconnected wiring. If the Cat bypass were causing a lean condition, you'd typically be getting a lean code. A knock sensor code could be thrown if the sensor was sending a signal ALL THE TIME, even when the ECM is not programed to expect it, or if there is no minimum noise level. And as I said above, a code can be thrown if the detonation could not be eliminated even though it tried. If you get the code thrown with the key ON but engine OFF, again, an indication of a faulty / damaged sensor.
But Pitbull is essentially correct. There's any number of folks out there riding with a cat bypass on and knock sensor issues don't seem to be related. And who did the work, you or your mechanic? I don't find any forums postings relating the two, either. And it's always a good idea to change those two vacuum hoses going to the MAP sensor if they are still the original ones - they have caused more weird issues - and make sure you're using the proper grade of fuel and there's nothing loose / vibrating under there that the sensor could be picking up. The whole thing might just be due to a faulty sensor, too. It happens.[/QUOTE]
