I’m being quoted £x,000 to replace this!

[BeBuLamar]

The board reports "Moto fault heat exchanger". Ok the motor is good but how does the board knows if the motor is faulted? Most of the time it only check on contact some where. I would try to find this out.
Designing a board is OK but there is safety involved.

[LinuxHata]

So it is quite easy to check, and no need for complicated knowledge.

From my practical experience of debugging such systems, most likely faults were due to either current sensor, pressure sensor or relay failure.
 
First of all, how system knows that motor is OK? it either checks the current consumption of a motor, or there is a pressure sensor next to motor, so when motor is on, pressure change is detected. We need to rule out both situations.

We know the location of motor, right? So it should be possible to connect small incandescent light bulb to parallel to it. So after it is done, we should check the following, as power is applied to system and moment, where motor should start spinning.

Does lamp come on or not?
If it does, so should do the motor, let's check motor too. If motor is ok, proceed to the next next step.

If it does not, this means that there's a fault in a way of the power, we need to find it. To do this, disconnect the board from mains, disconnect the motor and apply small DC voltage, say 3 volts, from battery or whatever, to motor terminals. Using the multimeter, trace the voltage backwards, to see from where it comes. At certain point, you should "meet" the relay or other kind of switching device, past which your battery current won't go, since device is in offline stage. After you have identified that device, you need to check the voltage on it's contacts while system is operating (do this carefully!) Motor should be connected as usually it is. So when system is started and motor should be on, if there's a voltage (usually, full or almost full mains) on contacts of that switching device, it is not operating properly, since it should be in engaged state and show zero voltage drop on it. So the switching device needs to be replaced, but still, there are some chances that it simply does not receive the statement to execute. Similarly, if there's a half mains voltage or whatever on open contacts of a switching device, it still might be faulty and needs replacement.

But what if there's no voltage at switching device at all? Most likely, there's a blown fuse somewhere on the way of the power line, so you should again disconnect from the mains, and continue to trace the power delivery path, but this time, bypassing the switching device. Once you see the interruption on the power delivery, here you are, here's the faulty component which needs to be replaced.

Another issue is, when motor spins, but system believes it does not. This usually happens either to faulty pressure switch or current sensor. The pressure switch can be easily checked - usually it is next to the motor, in line with piping and has two contacts,  which usually make contact under the required pressure only. So just connect multimeter to these contacts (disconnecting the wires going to it),and check for continuity - it should change when motor starts. If not, pressure sensor is faulty.

There are other, obvious possible causes of the problem, like clogged piping, but I guess, these were already ruled out?

 

[mikeselectricstuff]

My suspicion is that unless you can get hold of schematics for that board, you'd actually have an easier time (easier, not easy) designing a board from scratch to interface with the same hardware this one controls, than you would trying to debug a fault in a device you haven't enough information about.

Utter nonsense.
The thing did work, therefore it is a failed part or component. There is no way that even replacing pretty much every part would be anything like as much work than designing & debugging a whole new board.
I've fixed dozens, if not hundreds of things with no service information. OK it takes longer as you need to work out how it's supposed to work before you can figure out why it doesn't, but on a device like this it really shouldn't be that hard.

[pickle9000]

Mike is right. Don't get fancy.

Try not to make the situation worse. Don't play with the ribbon cable they can be delicate with age. Get a practice circuit board
with some thru hole connectors and a soldering iron and practice a bit so you know what you are getting into.

[Bud]

The board reports "Moto fault heat exchanger". Ok the motor is good but how does the board knows if the motor is faulted? Most of the time it only check on contact some where. I would try to find this out.
Designing a board is OK but there is safety involved.

Is it a fan motor? It may have an air pressure sensor and the board may be checking the sensor.

[Gyro]

If there is a pressure or airflow sensor, it could be physically blocked or stuck. Worth checking, it's probably more likely than an electronics fault.

[jmelson]

I see a board full of relays.  These have markings on them.  We had an old washing machine with similar relays that had no surge suppoessors on the contacts and they burned up the contacts.  A good guess is that those relays may be bad.  You may be able to order replacements and solder them in.
If the whole controller has gone bad, then maybe you SHOULD replace it.
Jon

[pickle9000]

I see a board full of relays.  These have markings on them.  We had an old washing machine with similar relays that had no surge suppoessors on the contacts and they burned up the contacts.  A good guess is that those relays may be bad.  You may be able to order replacements and solder them in.
If the whole controller has gone bad, then maybe you SHOULD replace it.
Jon

The controller gave some error message so it's probably just fine. As for sensors often the designer
will do a timeout error. For example on a furnace controller there may be a few temp sensors but none
on the blower motor. If the temp rise does not follow whats expected then the controller may say
blower motor failure and shut down. In reality a faulty relay or whatever on the output can cause the
same issue. Relay's offer very good protection to a controller and that is why they are often used.

[ttx450]

I cant get over the complexity and amount of wire plug ins.. for aq boiler.  You must have many zones and other stuff.  Crazy board replacement price. It would be interesting to know/see how this is wired and all the zones and what ever else is hooked to it.  Good luck, winter is coming.. post progress. 

[Nusa]

He did say up front it was for a house and other buildings on a farm.