A heating element doesn't have to be broken in half to be considered "dead." Micro-cracks are the hidden precursors to catastrophic furnace failures. Because these cracks are often invisible to the naked eye, you need to rely on physical diagnostics.

Professional Diagnostic Techniques

• The Resistance Mapping (Most Accurate): Keep a log of your element resistance ($R_0$ at installation). Check it every 500 hours. If the resistance fluctuates more than 15% from the baseline, internal structural damage (micro-cracks) is almost certainly present.
• The Acoustic "Tap" Test: Gently tap the cold element with a small metal tool. A healthy, dense MoSi2 rod produces a high-pitched, metallic "ping." A cracked element sounds dull or "thuddy"—the sound is damped by the internal fracture.