ATX PSU

[PlainName]

Got a CoolerMaster 600W ATX2 wired PSU. Was fine yesterday, PC won't power up today.

I think the 5V standby is OK (the motherboard start and reset buttons light up), although I am seeing 2.5V on PON. Pulling it down does nothing. However, putting an ATX PSU tester on it (which doesn't apply much, if any, load) sees the PSU come alive and all the voltages read correctly. Seems to be a decent load (like a motherboard) that stops it from starting.

Is there a typical fault that would cause these symptoms?

(To head off the obvious, I verified the motherboard will power up with a weedy ATX1 PSU.)