Originally Posted by Diseasel300

Knock sensors can and do fail (BMW guys know about them quite well). Usually the failure mode is excessive activation from internal fractures or corrosion which then causes drivability problems, CEL, or low power from retarded ignition. Usually "Open circuit" codes are the ECU complaining about an incomplete circuit. It's possible that something got unplugged, a wire broken off, or something pinched in the bellhousing when the transmission was replaced. Could also just be coincidence. Be aware that knock sensors are delicate and fragile things, don't bother with used parts they will already be bad or fail shortly after being installed.