Ariston Microgenus 24 HE MFFI Error A77
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A bit more info in the post wouldn't have went amiss either 
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Hi, I just fixed this fault on my Micro-Genus 24 HE. The A77 fault is quite common on these boilers. Basically the boiler thinks that the sensor is open circuit (broken connection ) or short cicuit (Water across the 2 terminals in the recuperator). It is a safety device to check that the condensate pipe is not b locked and hence condensate backing all the way back up to the flue pipe. First thing to do is check that the condensate is able to run freely from the bottom of the recuperator down the pipe, into the condensate trap and out of the boiler. You do this by taking the rubber hose off the bottom of the recuperator and pouring water down the pipe. It should run out of the grey flexible pipe at the bottom of the boiler. Mine was fine.
Next remove the fan and remove the sensor across the two terminals on the side of the recuperator. I think this is just a resistor, so just check it's not open or short circuit. Clean any corrosion off the two terminals and reassemble and test. If this does not fix it then you will need to drain the boiler down un-clip the 2 pipes on the front of the recuperator and remove the 4 screws holing it in place. Remove it from the plastic housing. The metal fins on mine were corroded on the bottom edge and this corrosion was present on the bottom of the recuperator housing and between the 2 metal pins that connect to the sensor on the side of the recuperator. You need to remove ALL the limescale and corrosion and polish with P1200 Wet and dry. I also rubbed the bottom face of the plastic housing with a rag with a tiny smear of silicone grease. It's important not to get this on the face of the 2 metal pins that connect to the external sensor.
You know when the bottom face is clean, if when you flick water on to the inside bottom surface of the plastic housing it balls up and will run down to the recuperator exit pipe.
Ensure the gasket is in good order as it stops carbon monoxide leaking out of the recuperator.
Reassemble and it should work fine. I have not had any more A77 faults since doing the above.
NOTE: YOU MUST ONLY UNDERTAKE THIS TYPE OF REPAIR IF YOU ARE A CERTIFIED GAS ENGINEER.
Next remove the fan and remove the sensor across the two terminals on the side of the recuperator. I think this is just a resistor, so just check it's not open or short circuit. Clean any corrosion off the two terminals and reassemble and test. If this does not fix it then you will need to drain the boiler down un-clip the 2 pipes on the front of the recuperator and remove the 4 screws holing it in place. Remove it from the plastic housing. The metal fins on mine were corroded on the bottom edge and this corrosion was present on the bottom of the recuperator housing and between the 2 metal pins that connect to the sensor on the side of the recuperator. You need to remove ALL the limescale and corrosion and polish with P1200 Wet and dry. I also rubbed the bottom face of the plastic housing with a rag with a tiny smear of silicone grease. It's important not to get this on the face of the 2 metal pins that connect to the external sensor.
You know when the bottom face is clean, if when you flick water on to the inside bottom surface of the plastic housing it balls up and will run down to the recuperator exit pipe.
Ensure the gasket is in good order as it stops carbon monoxide leaking out of the recuperator.
Reassemble and it should work fine. I have not had any more A77 faults since doing the above.
NOTE: YOU MUST ONLY UNDERTAKE THIS TYPE OF REPAIR IF YOU ARE A CERTIFIED GAS ENGINEER.
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good job that you did not provide him with any info he could use to go into the combustion chamber area
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Hi Terry. The fault is not related to the primary combustion chamber. Whats your concern?
my concern is that your advice is so comprehensive he will think that he can attempt to work on any part of the boiler, especially after removing the fan to clean the sensor
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