Ariston A77 Fault Code: Condensate Trap and Sensor Problems
The Ariston A77 fault code means the boiler has detected a problem involving the condensate trap or its sensor circuit. It is mainly associated with Ariston Microgenus HE boilers rather than every Ariston model.
Official Microgenus HE documentation lists three direct reasons for A77: a condensate trap full of water, a short-circuited condensate sensor or an open-circuit condensate sensor. Try one reset using the boiler controls. If A77 returns, contact a Gas Safe registered engineer.
Quick Answer
Ariston error code A77 indicates one of the following:
The condensate trap is full of water.
The condensate sensor has short-circuited.
The condensate sensor circuit is open.
A blocked or frozen condensate outlet may also contribute by preventing water from draining away, but A77 does not automatically prove that the external pipe is frozen.
Which Ariston Boilers Display A77?
A77 is documented for the following Microgenus HE combi boilers:
Ariston Microgenus HE 24 MFFI
Ariston Microgenus HE 28 MFFI
Ariston Microgenus HE 32 MFFI
The installation manual lists GC numbers 47-116-37, 47-116-38 and 47-116-39 for these models. Other Ariston generations may use different fault codes for condensate drainage problems, so always check the exact boiler manual.
What Does the Condensate Trap Do?
A condensing boiler produces acidic wastewater, known as condensate, while recovering heat from combustion gases.
The condensate trap holds a water seal and allows this wastewater to drain safely away. The water seal also helps prevent combustion gases from passing through the condensate drainage route.
The Microgenus HE contains a condensate trap, a condensate trap tube and a dedicated condensate sensor. If the boiler detects an abnormal water level or an electrical problem with the sensor, it can stop operating and display A77.
Possible Causes of Ariston A77 Fault Code
| Possible cause | Why it may trigger A77 Fault | Who should investigate |
| Condensate trap full of water | The boiler detects that condensate is not draining from the trap correctly. | Gas Safe engineer |
| Blocked condensate outlet | Water may back up and fill the internal condensate trap. | Gas Safe engineer |
| Frozen external condensate pipe | Ice may prevent condensate from flowing to the drain. | Homeowner |
| Debris inside the trap | Contamination may restrict drainage or interfere with the sensor. | Gas Safe engineer |
| Contaminated condensate sensor | Deposits may cause an incorrect water-level signal. | Gas Safe engineer |
| Condensate sensor short circuit | The PCB receives an abnormal continuous electrical signal. | Gas Safe engineer |
| Condensate sensor open circuit | The PCB cannot detect the sensor circuit correctly. | Gas Safe engineer |
| Damaged sensor wiring or connector | The signal between the condensate sensor and PCB may be interrupted. | Gas Safe engineer |
| Incorrect condensate-pipe installation | Poor fall, sagging or unsuitable routing may restrict drainage. | Gas Safe engineer |
| PCB monitoring fault | The PCB may not interpret the condensate-sensor signal correctly. | Gas Safe engineer |
The manufacturer’s manual directly identifies a full trap and both open- and short-circuit sensor conditions. The remaining causes require diagnosis because they may create or imitate those conditions.
Safe Checks for Homeowners
A77 involves components inside the boiler, so homeowner checks should remain limited.
Reset the boiler once
Use the reset button described in the user manual for the exact model. The Microgenus HE user manual advises resetting the boiler when A77 appears. If the boiler shuts down again, it recommends contacting an authorised service provider.
Do not repeatedly reset the appliance. The servicing documentation states that the boiler limits reset operations within a short period.
Visually check the external condensate pipe
Where the plastic condensate pipe is accessible without using ladders, removing panels or disturbing fittings, look for:
Visible ice during freezing weather
A crushed or damaged section
A disconnected joint
Water backing up or leaking
An obvious obstruction around the drain outlet
Do not disconnect the pipe or insert tools into it.
Look for water around the boiler
Check for visible dripping or pooling beneath the appliance. Do not remove the casing to find the source. Record where the water appears and provide this information to the engineer.
Should You Check the Boiler Pressure?
Boiler pressure is not the main troubleshooting step for A77.
On the same Microgenus HE range, insufficient water pressure or poor primary circulation is identified separately as E02. A77 specifically relates to the condensate trap and condensate-sensor circuit.
Repressurising the boiler is therefore unlikely to resolve A77 unless another pressure-related warning is also present.
Can You Clean the Condensate Trap Yourself?
No. Removing the condensate trap requires access inside the boiler and disconnection of internal components. The Microgenus HE servicing instructions include trap removal as a servicing procedure rather than a user-maintenance task.
A homeowner should not:
Remove the boiler casing
Empty or dismantle the internal trap
Clean the condensate sensor
Disconnect sensor wiring
Alter the internal condensate hose
Replace the trap, sensor or PCB
A qualified Gas Safe registered engineer should inspect and repair the boiler. Gas Safe Register states that gas-boiler repairs and servicing must be carried out by an appropriately registered engineer.
What Should a Gas Safe Engineer Check?
The exact procedure should follow the Microgenus HE installation and servicing manual. Checks may include:
Whether condensate is leaving the trap correctly.
The external and internal condensate route.
Blockages, sludge or debris inside the trap.
The trap water level and sensor condition.
Sensor continuity and resistance.
Open- or short-circuit wiring.
Electrical connectors between the sensor and PCB.
The fall and routing of the condensate pipe.
Signs of leakage or damaged seals.
PCB monitoring of the condensate-sensor circuit.
The engineer should identify whether the trap is genuinely full or whether an electrical fault is falsely reporting that condition.
Does A77 Mean the Condensate Pipe Is Frozen?
Not always. A frozen external pipe can prevent condensate from draining and may eventually cause the trap to fill. However, A77 can also be caused by a short circuit, an open sensor circuit, damaged wiring or contamination around the condensate sensor.
The fault code alone should not be used to decide which component needs replacing.
Parts That May Be Related
Depending on the diagnosis, A77 may involve:
Condensate traps
Condensate sensors or electrodes
Internal condensate hoses
Trap seals and connectors
Wiring harnesses
Printed circuit boards
Compatibility must be checked carefully because parts can differ between Microgenus HE outputs and production versions.
Find Ariston Condensate Parts
FAQs
Yes. Try one reset using the boiler’s reset button. If the code returns, arrange an engineer visit.
No. On Microgenus HE boilers, A77 relates to the condensate trap or condensate-sensor circuit. Ignition failure is identified separately as A01.
Not directly. Insufficient water pressure is listed as E02 on this boiler range.
Only if testing confirms that the trap is blocked, damaged or unable to drain. The sensor, wiring or PCB may instead be responsible.
The boiler may remain locked out. Do not repeatedly reset or bypass the fault. Arrange professional diagnosis if A77 returns.
Key Takeaways
- The Ariston A77 fault code indicates issues with the condensate trap or its sensor circuit, primarily in Microgenus HE boilers.
- Common causes of A77 include a full condensate trap, short-circuited or open-circuit sensors, and potential blockages.
- Homeowners can reset the boiler once and visually check the external pipe, but should not attempt internal repairs.
- A Gas Safe registered engineer should diagnose the problem and check for blockages, sensor conditions, and wiring issues.
- Replacing parts may be necessary, but A77 can also result from faults in sensors or wiring rather than the trap itself.
