The control wire harness usually lasts forever it has very low voltage and current on it. The ends of the power control wires loosen and heat up which definitely makes the relays heat up and fail sooner. Just to note, I have on very infrequent occasion received new relays that had a manufacturing defect and consequently did not last very long. Relays are fairly cheap and for studio work with six electric kilns we keep a decent stock of them and change them often. Almost all last for hundreds of