I had the same problem on my A23 mffi and traced it to the ‘Temperature probe’.
Google 'A23 mffi' and click on one of the Harrian.com links for the EuroCombi A23 mffi (currently the top two). Make sure you are looking at the correct pdf document for the serial number of your boiler. The part I'm talking about is ‘Part 20, Code 569236 Temperature probe (CHW)’ in the 'a_23_mffi_from_2320015700001_to_.pdf' file and it's ‘Part 18, Code 569236 Temp probe (C.H.W.)’ in the 'a_23_27_mffi_from_983610001_to_2320015700001.pdf' file. Since they are the same part Code in these two documents I assume they are the same physical device. There may be another pdf somewhere for other serial numbers not covered by these two files.
To check if the Temperature probe in your boiler is faulty do the following:
• Switch off your boiler at the electricity mains and let the central heating system cool down fully (if its been on – apart from avoiding scalding yourself I think this may also be important to get a proper reading from the multimeter in the 3rd step).
• Disconnect the wires to the Temperature probe (pull on the connector, not the wires).
• Connect a multimeter, set to resistance (kilo ohms range), to the electrical tags protruding from the probe. The resistance that registers on the multimeter should be 5 kilohms. If it is less than 5 kilohms it is faulty and should be replaced. The part Code has now changed to 998458 and should cost about £3.60 + p&p. Note, the part is also known as a Thermistor (i.e. a resistor whose resistance varies with temperature).
To replace the Temperature probe you will need to drain the water from the boiler itself only (not the whole central heating system). There is a small ‘drain’ tap on the far right hand side of the bottom of the boiler (near where the mains cable goes into the boiler). Stick a bowl under the ‘drain tap’ and open it up (from inside the boiler). Once you have drained the boiler's ‘head’ of water, undo the faulty probe with a spanner, replace it with the new probe and reconnect the wires' connector. You then need to top up your boiler with water from the cold water mains. There should be a ‘top up’ tap (valve) coming off the cold water mains pipe that provides cold water to the boiler for heating up for ‘tap water’. Opening this 'top up' tap will begin to fill the central heating water circuit of the boiler (i.e. ‘pressurising’ your boiler). In order to purge as much air from the boiler while water is filling it, leave the 'drain tap' open until water starts coming out as a steady stream. If you can switch off the ‘drain tap’ to cut off this steady flow then the boiler will begin to pressurise properly and hopefully all the air should be purged. When the pressure gets to about 1.0 bar (i.e. in the middle of the blue range on the pressure gauge dial) close the ‘top up’ tap. If you can’t close the 'drain tap' while filling the boiler close off the ‘top up’ tap and then close to ‘drain tap’. Resume pressurising as before by opening up the ‘top up’ tap. Stop when the pressure reaches 1.0.
Close up the boiler and switch on the electricity mains. If it isn’t already, set the ‘Summer/Winter’ switch to ‘Winter’ (the picture of a radiator and tap). If the time you’re doing this is not at the normal time your heating comes on then use the manual select button on the boiler’s clock (the hand with the finger extended) to cycle through the options to switch the boiler on to ‘constant’. If the boiler fires up then let it run for a minute or so until any ‘gurgling’ stops. Switch off the boiler and then go around to the radiator nearest the boiler and ‘bleed’ any air out of the radiator (any air left in the boiler will probably have made its way through the pipes to the first radiator). Check the pressure guage and refill to 1.0 bar if necessary. Later when the heating has been running for a while carefully (they will be hot) check each radiator in turn to see if there is any air in them (the top of any radiator with air in will feel cool compared to the rest of the radiator). Once all (any) radiators are bled you may need to re-pressurise the central heating system again.
Good luck and if this doesn’t sort it out apparently there is a simple way to check that the printed circuit board responsible for temperature regulation is also working.
Mike, January 2009
