Pump Speed Controller or temp sensor

Hi experts

My GT 152 speed controller card isn't outputting any voltage to the HW pump that recirculates through the heat exchanger.

There is mains at the card, the fuse isn't blown and flow switch works ok.
Nothing is visibly blown or burnt on the card and the single LED occasionally illuminates (fast flashing - indicative of sensor faults???)
The pump also works well (as it is currently wired directly to a mains wall socket temporarily so we can have showers)

How would I trouble shoot if the failure comes from the sensors? What voltages are expected to be measured here?

I'm not sure if a new sensor or a new GT152 PCB is required. (new sensor is much cheaper but do you need to drain system to replace it?)

Any tips to help identify the culprit are truly appreciated.

many thanks in advance.

Phil

Hi the sensor used on this board is an active device that is to say it has a small chip in the sensor which send digital signals to the board so there is no practical way to measure the output with the likes of a meter. So as you say the sensors are less expensive than the board so you can buy a sensor and temporarily wire it in without fitting it in the heat exchanger and see what happens. You can cool the end of the sensor and in theory the hot water pump should kick in (cold damp rag should do it), if nothing improves then the pcb is likely faulty. When wiring in the sensor it must be wired in correctly as incorrect wiring can damage it as it has 5 volt supply to it. In respect of replacing the sensor in the system I reckon it is in the cold water circuit so with the system off, turn the cold water mains off open a hot tap to remove any pressure and it should be possible to change it with minimal water loss, old towel to catch any drips. However, I'm no plumber so happy to humbly defer to anyone with more knowledge on this. If in doubt as to your abilities to work safe pay a man (person (to be PC)) that can. Hope this helps a bit.

That is a great help, thank you for the insight and logic.