CPU fan running full speed after PSU replacement (2011 27 inch). Sensor problem?

I have a 2011 27 iMac (2.7, 12GB, 2TB HDD + 250GD SSD Fusion drive). A week ago the PSU failed (made clicking noise while iMac was switched off, quickly followed by a complete failure to switch on), which I replaced two days ago. Everything was fine for over 24 hours until suddenly in the middle of me using the iMac, the CPU fan suddenly ramped up to full-speed - and stayed there. The fan speeds up at the login screen phase of the boot process. I've tried SMC resets and NVRAM resets with no luck.

Geekbench 3 tests show that the iMac is running at about half its normal power (single core 1458, multi core 4550), so some sort of throttling is clearly taking place. I've got the fan under control using "Macs Fan Control" but obviously I would like fix the problem and get the computer working at full power. I've installed iStat Menus and can't see anything obviously amiss with the temp readings.

Running AHT produces the error code 4SNS/1/40000001:ID2S-0.00. From what I can work out, that relates to a DC current sensor error. Does anyone know where that sensor is? Is it on the PSU or the logic board? If it's on the PSU then hopefully the 'new' (secondhand) PSU is to blame and replacing that will solve the problem.

I'd be very grateful if anyone has any advice to offer or information about that AHT error code.

Thanks in advance.