CF6-80A and CF6-80C2 that use Systron fire detection systems have three equal resistance loop sensors. Each loop measures 4.5K Ohms. They are wired in a parallel circuit.
In a functional system 1.5K Ohms should be read at the fire detection card pins 19 & 20.
If 2.25K Ohms is read..... you've lost one loop. If 4.5K Ohms is read..... you've lost two.
The "only" way to verify an open loop is to disconnect one (or both) wires from the loop terminals and measure resistance at the terminals (if the wires are connected..... you're reading the other loops and possibly the card).
A loop is either good or bad. There is no halfway. 4.5K Ohms or open.
If a bad loop has been isolated, throwing a decade box across the terminals should at least give you a valid test from the cockpit.
Or..... just throw a 4.5K Ohm resistor across them.