AHD tells me) a faulty GPU temperature sensor

Not sure the bit about all the fans spinning up is accurate. I'm looking at a Mid 2011 iMac 27" with (AHD tells me) a faulty GPU temperature sensor (4SNS/1/C0000008: TGOH--124).

The ODD fan spins up (which makes sense as the GPU HeatSink is above the ODD fan/drive. However, the HDD fan doesn't. The CPU fan however does. Maybe it's only the big fans that spin up? Or I've also got a faulty CPU T sensor but just don't know it yet.

What I don't know is if replacing the GPU temp sensor will be enough to sort the CPU fan out too? The trouble is, AHD stops the test after the 1st error, so I won't know if any other sensors are faulty until I replace the 1st one, re-assemble and re-test. And given the amount of work involved in disassemly/re-assembly, I wish there was a way to test all the sensors in one go.

I've got the machine disassembled right now, so if someone can tell me a way to test a temperature sensor with a multimeter, that would be extremely helpful.