Knock Sensor - 2142

[Mohammed Bhatti <Mohammed.Bhatti@wcom.com>]

Resending since I have not seen appear on digest.

I've got 2142 coming up on my 88 80q.  Bentley says:
"Defective sensor or sensor wiring.  Check wiring between knock sensor
and ignition control unit according to wiring diagram."

A few questions:
1. Has anyone been through the test procedure for a defective knock
sensor?
2. If it's the knock sensor and not the wiring, how much does a knock
sensor cost?
3. What damage do I cause if I continue to drive with a defective knock
sensor?  Check engine light comes on sometimes, in particular when
the car is in stop-n-go traffic or the weather hot.

Ok, now for a dumb question:
What is the purpose of a knock sensor, and why do some have 1 vs 2?
I assume my 80q has one.

Thanks

mkb 
88 80q