How can I tell if a complex relay is intermittently failing?
Measure the actual resistance across contacts with a meter, when energised or not. You should see either &lt; 1 ohm (basically the same as you read with the meter leads shorted) or infinity. If you see readings fluctuating in the low ohms, the contacts are damaged and it's time to change it. If you power it from a bench supply, you can also check that the relay energises reliably from the specified voltage, if you can find the information.