Pulsacoil 2000 Pump Runs Continuously

Hello All.  My son has just moved into an all-electric flat with hot water supplied from a Pulsacoil 2000, model PC185 controlled by a Horstmann Electronic 7 controller.  It is set up for Economy 7 overnight heating with no auto-boost.  Both immersion heaters seem fine (approx 17 ohms). The PSC board LED show the double blink indicating all is well. The green indicator LED on the indicator panel is broken, (awaiting replacement part). However, the pump runs continuously.  Hot water supply seems fine, so I'm assuming the heat exchanger is ok.  I would infer that the cause is:  a defective DHW or PHE Sensor, a faulty PSC board or by-pass of the pump controller thus sending continuous mains voltage to the pump.  
I would be most grateful for any step-by-step guidance that would allow me to determine which of the above is the cause of the problem (or advice of something I've overlooked).  Can the DHW/PHE sensors be tested without substitution by replacements?
Many thanks in anticipation of your help.

Check unit is full.
No way of testing PHE & DHW sensors.
Is the unit hot?
Does sound like PCB though.
Mike

Many thanks Mike,
I'm extremely grateful for your fast and insightful response.  The water level was ok (just), I've refilled the header tank to a more appropriate level. I'll try a replacement PSC board (I have been offered a tested second-hand one at a reasonable price) and, if that fails, will try replacing the sensors.  I see they both have different part numbers, do you know whether they are rated differently?

One is a wet sensor.
The other is a dry sensor
Mike

OK, PCB changed and continuous pump running problem remains.
* Is one of the sensors (DHW, PHE) more likely to be the culprit than the other? If both equally likely to be misbehaving, is one more easy to change than the other? I assume the dry bulb sensor can be changed fairly easily. Is it possible to isolate the wet bulb sensor or does the system have to be drained in order to change it?
* Am I correct in assuming there is no flow switch that calls for the pump to run, i.e. that the temperature change at the heat exchanger caused by the drawing of hot water causes the pump to run to provide hot water to the exchanger from the store?
* At what point should complete replacement of a 10 year-old unit (by a NextGen unit?) be considered?
Once again, very grateful for any further pointers.
ATB.

Dry sensor easy to change.
Wet one, turn off water, open hot and cold taps even flush the loo.
Undo sensor, have a cloth / towel must not wet the pump!
Ptfe around new DHW sensor and fit it.
The sensors tell / ask the pump to run.
And they also maintain the DHW around 50 to 55 C
Mike

id0mrg wrote:OK, PCB changed and continuous pump running problem remains.  
* Is one of the sensors (DHW, PHE) more likely to be the culprit than the other? If both equally likely to be misbehaving, is one more easy to change than the other? I assume the dry bulb sensor can be changed fairly easily.  Is it possible to isolate the wet bulb sensor or does the system have to be drained in order to change it?
* Am I correct in assuming there is no flow switch that calls for the pump to run, i.e. that the temperature change at the heat exchanger caused by the drawing of hot water causes the pump to run to provide hot water to the exchanger from the store?
* At what point should complete replacement of a 10 year-old unit (by a NextGen unit?) be considered?
Once again, very grateful for any further pointers.
ATB.

Hi
Check wehre the Pump get his power from, may someone messed around with the Boiler in the past, the brown ( L) has to be in the conector block, connection 2. If it is joined with the red one in position 5, change it to position 2 and check if working.
Regards Master Mario at Mario’s Boiler Service