Diagnosing a relay is simple with testing. On the exterior of most relays there is a diagram of the switches inside the relay. In the case of the starter relay, there normally be only one switch. Applying power and ground to the small circuit with proper polarity should make the relay click. This, however, is only the first half of the test. The secondary test is to measure the resistance across the relay, and compare that to the data on the relay or from the service manual. If the resistance is too high across a relay, the powered component will not receive the high amperage current it needs, and therefore, the relay must be replaced. If the relay does not click, the same applies, but the second part of the test will yield infinite resistance.

Replacing a relay is only a matter of identifying the relay according to diagrams in the owner's manual, under the lid of the power distribution or fuse block, or in the service manual for the vehicle. Once testing of the relay is complete, the failed relay is thrown away only after ensuring the new relay is exactly identical in function and size, and the new relay is firmly pressed in, pulled out, and once again pressed into the power distribution or fuse block. This will ensure that complete contact is made between the relay and the terminals on the vehicle.

Never attempt to repair a relay. There is generally nothing serviceable in a relay, and erroneous reconstruction will only lead to humidity entering the relay, a relay that does not function properly, or even melting of the entire relay. Relays are inexpensive sealed units that should be replaced, not tampered with.

If a relay is proven good, but still does not function, the terminals can be tested in the junction block with a test light or multimeter. A multimeter is prefered because of the accuracy of readings, but a test light will inform the technician if power is present or not. Also, the Ignition switch may provide a switched ground to the starter relay, or it may provide a switched hot (battery voltage). This should be known when testing the relay and the power distribution block. In the case of a switched ground, there will only be continuity between the battery negative cable and the relay when the ignition switch is turned to the 'start' position.

Relay testing and replacement is simple, straightforward, and could be a fun project to share with a family member. The issues start to emerge when the relay is diagnosed as operable, but is not activated when the ignition switch is turned on. If testing of electrical circuits is understood, and a multimeter with long leads is present, this repair can be accomplished at home, but if the circuit is not understood and the relay is good, leave the job to a professional. This will likely prevent further damage or electric shock.