Pulsing heat exchanger pump

Boilermate OV Model: ABA4200 Ser No 0020053959
With the heating and hot water off (either manually or by time clock and at night) there is a pulsing of the heat exchanger pump. It 'hums' for 60secs approx. then a click and switches off then after 10 - 20 secs switches on again. Cycling all the time. The two digit display shows, on the left, two horizontal bars (Top and middle). The right display pulses one top bar on and off with the pump pulse. With the heating/Hot water on the left display is a 'F'.
The system operates 'normally' otherwise.
Could this be a faulty temp. probe?
If it is the main pcb board, could it be one of the two electrolytic capacitors?
Any ideas please?
Thanks

Velcro49,

Can you confirm whether this is a Boilermate A-Class or a Boilermate 2000 please? I don't recognise the Model: "ABA4200".

Thanks,
Mike

Yes I can confirm this is a BoilerMate OV A- Class.
The model on the Installation instructions is a BMA 215 OV
The ABA4200 is on the label (stuck inside the casing) under Model No. and with the serial number, and capacity values.
I checked the error codes with S2 and S1
All Co values were FF
cO values: c1 to c4 were 29, all rest were FF
Oddly enough since cycling through the codes the pump has not been pulsing.
Any advice would be appreciated.
thanks
Velcro49