how to reset smc on IMac late 2015 with error code PFM001

That code does indicate an SMC issue, see > https://support.apple.com/en-us/HT203747

To reset the SMC, see > https://support.apple.com/en-us/HT201295

Has this iMac been opened up for an upgrade or repair?

If so, then it may have a disconnected or damaged temperature sensor.

To view all of the temperature sensor read-outs, I use > https://crystalidea.com/macs-fan-control

When you have a hardware issue doing a reset will not fix it. You need to take the computer to your local AASP to have it repaired. If you are unaware of any AASPs in your area, please begin with the following link.

AASP Locator

I had this problem a couple of times on my old Mac Pro (desktop) and an SMC reset sorted it out.