Slightly weird circuitry. This took longer to reverse engineer than expected, but yielded new information on the thermal sensing of Hakko-style soldering irons.
The default temperature of 500C (932F) is quite dramatic for a soldering iron, but there is a way to hack it to a lower temperature if desired.

The circuitry is a bit weird, in that the main control chip may have a stand-alone comparator section, but also monitor its activity externally and override it to shut the iron down. At least, that's my first guess at what's happening. Alternatively it could be ensuring the MOSFET is forced off in sleep mode, although the sleep current draw is very low at about 1uA.

The hack to set a lower bit temperature or allow it to be adjusted is to remove the zero ohm link shown and either solder a fixed value resistor in the two outer holes of the bypassed preset position, or put in a 100 ohm multi-turn preset. If using a single turn potentiometer I'd suggest a value of 56 ohms. I tested with various resistors and got the following results:-

Existing zero ohm link 500C.
10 ohms 460C
22 ohms 410C
33 ohms 360C - common standard electronic soldering temperature.
47 ohms 315C
56 ohms 277C
100 ohms 77C

The construction of the soldering iron seems quite robust, and the default bit temperature will probably make it ideal for automotive wire repairs or fast soldering.
The iron wins lots of points for using the very common soldering iron bit sold in bulk on eBay.

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