First off, it's not the plugs.
I suspect that while working on the engine, you may have accidentally damaged the knock sensor connector or wiring. "sensor high" is either an open, or short depending on the design of the system itself. Usually it means "open" (disconnected, burned out, wire cut...)
Find the sensor itself and then consider how the wiring may have been damaged.
If I was the shop, at a minimum, I'd plug in a known good sensor and fire up the engine. If it doesn't throw a code then the sensor went south. If it does, then it's the wiring. Buy a new sensor and be prepared to eat the cost. It's still cheaper than an oscilloscope.
