I have a Yihua 908D temperature-controlled soldering iron. One day the iron wouldn't work and the display said "S-E" which apparently means "sensor error". Unfortunately, while the iron was disassembled, I dropped it and the ceramic element shattered, precluding the possibility of taking measurements.

I purchased a replacement element for which the heating element and temperature sensor measure 200 ohms and 50 ohms but I'm not entirely certain which is which. I bought an entirely new 908D but the design has changed so I'm not sure the element should be the same either nor is the circuit board quite the same; it measures 250 ohms and 50 ohms. Although this means I now have a working unit, I am still compelled to try to repair the old one.

I copied the arrangement from the working unit to the one I'm repairing but when I turned it on, it seemed like the temperature reading was going down instead of up and the iron glowed red hot (before I realised and shut it down. Following the temperature excursion, the readings of the element/sensor are still 200 and 50 ohms, so I suppose that I did not cause any permanent damage except maybe to the tip. Are these observations consistent with the possibility that the heating element was connected to the sensor solder pads and the temperature sensor connected to the power solder pads?

Is it possible that the temperature sensor could survive such abuse as being connected to the power? Are they PTC?

 

The elements will be 50 ohms, the sensor will be 200 ohms, it's easy to test the sensor by measuring the resistance and heating it up with a flame or another soldering iron, if they are ptc types then as the temperature increases the resistance will increase.

 

Some soldering irons use thermistors and some thermocouples for temperature sensing.
Can you measure the old one to see if it has a 200R sensor or is that part broken too?
You need to check you have the correct one.
Can you test your iron on a working system if you know anyone who has one?
Your control board could well be faulty. See if you can measure if the triac is shorted.