The first thing I'd try would be to wiggle the connector to make sure that it's making a proper connection. The knock sensor is located on the back of the engine, on the firewall side, underneath the intake manifold. You should be able to get your hand behind there relatively easily.
Once you've tried that, either remove the 15A EFI fuse from the fusebox in the engine bay or remove the negative battery cable for a couple of minutes. Either of these methods will clear any codes stored in the ECU. Please note that removing the battery cable will obviously lose any stored radio stations/settings etc, so if you want to retain these I'd suggest just pulling the EFI fuse instead. Reinstall the fuse or the battery cable and check for codes again. If code 52 is still flashing up then it's time to inspect the sensor to see if it's faulty.
You can give it a try by using the probe of the multimeter to reach the terminal of the connector when it's unplugged. Unplug the knock sensor and test the actual sensor itself by checking that there is no continuity between the knock sensor and the body. If there is then the sensor will require replacement, but if there isn't then the sensor is fine and the fault is likely to be within the wiring or the ECU.
Give those steps a try first before considering replacing the sensor as you'd hate to go to all the trouble of removing the intake manifold for nothing. If the sensor turns out to be faulty. You can test it by continuing.
