I'm partially responsible for the heating problem

[sixgun]

Anchorage to Seattle flight. Around 200 miles from Seattle at cruise 20k feet got the heating problem warning. What did I do wrong to contribute to the problem. FE was in control. I had maintained manifold and rpm indicators at 12'oclock position in cruise. I made a 1500 fpm decent to 5k and changed course away from terrain. At 5k feet oat was -2c , cabin was +24c. I continued to KSEA and landed with applause and received a on the ground compliment form my copilot. Again, what did I do wrong ?

[TreeTops]

I had the same happen to me yesterday up at FL250.
Because I was a long way from land and needing the 50 knot tailwind to get there I stayed up high with the cabin at 16-18 degrees. A couple of comments from Betty but nothing mentioned in the career notes about being a poor captain. Perhaps the almost 13 hour flight and applause on landing made up for it.
I think it's just one of those random events that happen as part of COTS.