Had same problem on my wife's 1994 855 turbo. Is a common problem in Ontario due to slushy snow and road salt getting past connector seals on knock sensors and causing high resistance or intermittent contact. Solution is to reach in under intake manifold, depress retaining spring and pull each connector off. Spray some WD40 into the connector on the wire harness and work the connector back and forth several times onto the sensor. This cleans the contacts and keeps the moisture out. Did this over a year ago and haven't had the codes come back. Accessibility to the connectors is by feel only as you can barely see them. You also need small hands. Newer models apparently have a different wire harness that relocates the connectors.

On '94 850 turbo's ONLY, the problem is not really a knock sensor at all and if you replace them, you'll just be wasting your time and money. There's an upgraded 2 in one kit from Volvo (old TSB) BUT the code will still come back.
For that car (only), the real cause that I've seen many times now is a bad ECU. We get used ECUs from a local used Volvo parts supplier ($250) and the problem is solved w/o replacing knock sensors.

Sure they do go bad. But before changing one, exhaust other possibilities first. Ticking sound will not be picked by the sensor as it is a piezoelectric device that converts engine stress caused by fuel ignition into electric signal. A ticking valve will not do. However I had a similar code popped up that was caused by ignition problems (distributor cap), not enough to set misfire but still something that ECU interpreted as a knock sensor malfunction.
Good luck.