GHD Mk3's do not suffer from this problem (as they don't have a thermistor - they control the temperature by monitoring the resistance of the heater elements).
Where is the Thermistor?
The thermistor is the brown wires coming from the heater plate on the non switch side arm:
The thermistor should measure 100k Ohms at 25degC. You will probably find that the only way to get a decent reading on the thermistor is to unsolder it from the PCB as the components on the circuit board mean you will get a constantly changing resistance measurement.
Problems that occur with the thermistor
When the control electronics decides the thermistor is not working you will hear a continuous beep and the red light will flash. The plates will not heat up.
1) The irons are too cold
Not a problem with the irons exactly, but if the irons are too cold when they are switched on you will get a continuous beep and flashing light and the irons will not heat up. GHD call this a feature and say it is to protect against damage occuring.
To fix it try holding the ceramic plate on the non switch side in your hand to warm it up. Once it is up to normal room temperature you should be able to turn the irons on and they work as normal. If that does not fix it, read on!
2) Bad solder joint
This is rare, but does happen. I've seen one pair where the solution was to simply resolder the thermistor onto the PCB. GHD don't use the screw terminals for the thermistor connections to the PCB.
3) Broken Thermistor
If the thermistor breaks (i.e. the resistance across it is infinite rather than 100k at 25degC) then the irons will beep continuously and not heat up. The thermistor therefore needs to be replaced. This is complicated by the fact that the thermistor is glued to the back of the heater element.
There is of course an easy method and a difficult method to replace the thermistor.
The easy method is to buy a new heater with the thermistor pre-attached which is available from our online shop. The thermistors we stock have the same resistance-temperature curve as the genuine thermistor which means that the irons will still operate at the correct temperature. There are some thermistors available on the internet that are sold for use in GHDs but are not exactly correct which means the plates will operate at ~200degC which is about 15degC too hot.
The difficult method for replacing the thermistor is to buy a thermistor and glue it onto the old heater element. The best thermistor we have found for the job is available from our online shop. This is 100kOhm at 25 degC. It is very important you use the correct thermistor, as thermistors all have different resistance curves, so if you use the wrong one, then they will either over heat or not get hot enough. To fit this thermistor you need to crimp on high temperature wire extensions (not solder, as that will melt), electrically insulate the legs, form the legs so the wires both head towards the PCB and then bond it to the heater with Silcoset 158 or some other high temperature glue.
4) Broken processor
Very occasionally intermittent cable connections will cause an electrical surge that damages the input pin on the processor. The irons will work normally, however you will hear the constant beep as if the thermistor was broken. Therefore if you have the continuous beep problem, but the thermistor measures 100k correctly, then you may need to replace the processor on the circuit board.
If the thermistor becomes detatched from the heater element it is supposed to be measuring the temperature of, then this will cause the GHDs to overheat and the thermal fuse will fail. It will be necessary to reattach the thermistor and then replace the thermal fuse.
There are two methods for resolving this problem:
- Reattach the thermistor using Heat Resistant Silicon Sealant available HERE. Be careful not to put too much glue between the old thermistor and the heater element as this will change the thermal properties and therefore the operating temperature of the irons.
- Replace the heater element for a new one with a preattached thermistor. Heater elements with thermistors for all models are available from our shop.
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