Hottub board undervoltage damage

Hello!

I have a sundance hot tub circuit board I believe I have damaged by not providing sufficient current to. I was wondering what sort of components are likely to have permanently failed given the circumstances below...

The short of it: The tub is for sale and I had run a heavy duty extension cord from a 15A circuit in the house to power it, for the purpose of demonstrating each of its components worked (individually, simultaneously it requires a 35A+ dedicated circuit). All worked fine for about 10 minutes, no CBs tripped. Eventually, while not being that careful obviously, I think I cycled two pumps at once and an error code displayed, and all stopped.

When powered up the usual behaviour is for the ciruclation pump to start (.4A) and run for ~15seconds then heater to start (10A), and nothing else (~11A measured). Now when powering up the circ pump will not start, and the same error is produced (no flow). I don't think the 12VDC relay that controls the circ pump is energising. Without removing the board I can't tell if 12V is available to the relay, or if its even available after the transformer before it is rectified, or if the signal never makes it from the I.C.

I will remove the board and work backwards from the relay tomorrow but my question is, what are the first things to look at after a voltage sag event...? Warning labels always say using too long an extension cord may damage the equipment, how exactly does this happen? I realize components will not like running below their nominated voltage, but, permanent damage?

Checked obvious things, fuses etc. I have run circ pump directly from Live/Neutral rails and know it works. Have also run it like this and observed flow switch closing, and it still produces no flow error, so I'm fearing the worst that an I.C. is damaged.
Note I'm in NZ here, 240V single phase mains. Any advice appreciated!