Model A1225 / Mid 2007 and Early 2008 / 2.4, 2.8, or 3.06 GHz Core 2 Duo processor

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Fan runs continuously after power supply replacement.

I've replaced the Power Supply on my 24" 8,1 iMac. Good news: booted right up. Bad news: fan runs continuously including whilst sleeping. Shutting down does shut fan off. I have reset the SMC a few times, including following the portables instructions (holding down power button for 5 seconds) one of the times. Have reset the NVRam. Any thoughts of what to try?

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At this point I think you'll need to see whats happening with the sensors. Download this app: Temperature Gauge Pro. Post a snapshot of what it shows here for us to see.

Update

At this point I would start off replacing the CPU fan unit. Here's the IFIXIT guide to follow: iMac Intel 24" EMC 2111 CPU Fan Replacement.

The other possibility which is much harder to replace is the CPU heat sink. While rare the copper tubes are filled with liquid and this liquid can leak out causing the CPU not to be cooled and why the fan is pegged as the heat sensor is on the CPUs heat sink pad. Here is the IFIXIT guide to follow if the replacement fan failed to fix things: iMac Intel 24" EMC 2111 CPU Replacement. I must warn you this is a very big job.

I have never seen a sensor failure in this series but its possible the connector on the sensor cable came off if someone every took the logic board out. As this is not easy to get to its the very last possibility.

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Thanks for your reply. Here'sthe screen shot of TPG PRO: http://postimg.org/image/4papg5x4t/

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Wow! Your CPU fan is pegged to max (3600RPM). As all of the other sensors are working and within the normal ranges. I suspect you have a bad fan unit.

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Sorry, but this seems like a misdiagnosis. As stated in the OP, I just replaced the PSU. There is a temp sensor on the PSU. TGP doesn't seperate-out this sensor. Why would I assume a fan that had worked fine is broken, but the new PSU is fine? Any way to test the PSU on its own?

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Review the Temperature Gauge snapshot you posted here. The Power Supply (PSU) thermal sensor is in the green range so the power supply is fine Power Supply Proximity Now look in the fan section on the bottom CPU is Red and pegged at the extreme and looking back up to the sensors we can see the CPU Heatsink is green and in the normal range as well How is this a PSU issue? It's clearly a bad fan and/or the fan was not correctly connected.

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