iMac fan running constantly

Good morning!

OK I purchased a mid 2011, 21.5, i5, 2.5ghz iMac from ebay the other day that was listed as slightly faulty. As I was looking to buy my first iMac and I had a tight budget I thought I'd give it a go. I work with computers and thought I could have a go at fixing the issues.

So this brings me to the problem, the seller said it was bought as part of an office clearance so not much history behind it and that when it was on one of the fans was noisy.

It's arrived today and true to form one of the fans is noisy. I've spent the past couple of hours researching the issue on the internet. First thing I did was run the online AHT. This brought back an error code of 4MOT/4/40000002:CPU-0. I think this refers to a faulty cpu fan but the noisy fan is coming from the right hand side of the screen?

I then downloaded smcfancontrol. Currently the ODD fan is running at 4300rpm and 46 degrees (I presume this is the noisy fan I hear). The HDD 1099rpm and 46 degrees and the CPU fan 0rpm and 46 degrees?

So I suppose my question is, where do I start to fix this? Why would a faulty cpu fan make the odd fan run at max?

Thanks in advance

It appears you have several problems with the fans. The error you received indicates a temperature sensor fault which makes sense since that's what controls the fan.

There are three fans in the 2011 21.5" models: Hard drive fan, Optical drive fan, and the CPU fan. If the CPU fan was indeed not running, the iMac would shut down in a very short order. I don't know what's up with that?

The hard drive and temperature sensor cable are proprietary to Apple. In other words, any changing of those components or others can cause the fans to run full RPM which is what you're experiencing. It's probably time to take the machine to an Apple store and let them run diagnostics on it. There's no charge for that provided you take it to an Apple store not an authorized Apple repair service.