Gary's thermal paste point is underrated. We've started adding thermal paste inspection to our annual PM after the same discovery — 5-7 years seems to be the typical lifespan before it dries out and cracks. The tricky part is you can't see it without pulling the module, so most people don't check until they get the fault.
One diagnostic shortcut for F30015 that's saved us time: look at parameter r0037 (power module temperature) trending over a few weeks. If you see a gradual upward drift of 1-2°C per week with the same production load, something is degrading — fan, paste, or blocked fins. A sudden jump usually points to a fan failure or sensor issue. The trend tells you which category you're in before you even open the cabinet.
Also worth checking: if you're running the S120 in a press application with high peak currents, the I²t calculation can trip F30015 even when the heatsink temperature looks fine. The IGBT junction temperature is estimated internally and can exceed limits during heavy forming hits even if the heatsink is cool. Parameter p0290 (thermal time constant) might need adjustment if you're doing short heavy hits with long pauses — the default assumes more continuous loading.