Scenario

The relay is being supplied with a voltage signal, but the relay does not actuate or switch signals as it should.

Issue

There are several probable causes for this issue.

1. Disconnected coil: A previous application of a voltage in excess of the relay's rating may have caused the coil to become disconnected. Ultrasonic cleaning can also have this effect.
2. Insufficient coil voltage: The relay will not operate is the voltage is lower than the must-operate voltage.
3. Incorrect polarity of applied voltage: A relay equipped with a diode for absorbing counter-electromotive force will not operate if the positive and negative voltages being applied are connected incorrectly.

Solution

In order, the solutions are as follows.

1. Disconnected coil: Replace the relay.
2. Insufficient coil voltage: Measure the voltage being applied to the coil, and check this value against the must-operate voltage, which can be found in the relay's datasheet. If the value is below the must-operate voltage, increase the supplied voltage.
3. Incorrect polarity of applied voltage: Check the wiring of the relay, and ensure that the positive and negative voltages are being applied to the correct terminals.