Error E99

Safety precautions:

1. Do not attempt internal electrical or gas repairs unless you are a qualified Gas Safe engineer. Working on gas appliances or mains electricity is dangerous.

2. Before any internal inspection, isolate the electrical supply to the boiler at the fused spur or consumer unit and, if you understand how, turn off the gas supply. Ensure the boiler is cool.

3. Wear suitable eye protection and avoid loose clothing. If you are unsure at any stage, stop and call a professional.

Initial homeowner checks you can do (no internal access required):

1. Note the exact fault code and any additional messages or behavior (e.g. boiler locked out, LEDs flashing). Try a single reset using the boiler reset button and see if the fault returns. Do not repeatedly reset more than once or twice.

2. Check the external flue terminal and air intake for obvious blockages, nests, snow or debris. A blocked flue can cause overheat and flue-related faults.

3. Check visually accessible wiring and connections to the boiler (externally) for obvious damage. Check that the mains supply to the boiler is present and stable (a significant voltage drop can cause faults).

4. Ensure condensate pipe/drainage is not blocked or frozen (if present), as other faults can sometimes appear with flue-related sensors.

If you are competent with basic electrical safety and the boiler is isolated you can perform limited internal checks (ONLY if comfortable and safe to do so):

1. Isolate electrical supply and, if possible, the gas supply. Allow the boiler to cool.

2. Remove the outer casing following the boiler manual instructions. Keep screws and panels safe. Take photos of connector positions if helpful.

3. Locate the flue sensor/flue protection thermostat (it is usually clipped to or positioned near the flue outlet or flue path). Identify its two push-on connector wires.

4. Visually inspect the sensor, wiring and connectors for signs of damage, corrosion, burning, water ingress or loose connectors. Re-seat the push-on connectors firmly.

5. If you have a multimeter and know how to use it safely: with the sensor disconnected from the PCB, measure resistance across the sensor leads. A short circuit is indicated by very low resistance (close to 0 ohms). An open circuit reads very high or infinite resistance. If you are unsure how to interpret readings, stop and call an engineer.

6. If a short is obvious (damaged wire touching earth, melted insulation, burned sensor), do not re-energise the boiler. Note the observed damage and call a Gas Safe registered engineer to replace the sensor/wiring and check the boiler electronics.

Possible fixes and next steps:

1. If reseating the connectors and clearing external blockages clears the fault and the boiler runs normally after a reset, monitor for recurrence and arrange a service if concerned.

2. If the fault persists after reseating connectors and basic checks, the flue sensor is likely faulty or there is a wiring/PCB fault. Replacement of the flue sensor or repair of wiring should be done by a qualified engineer. The troubleshooting manual and parts replacement procedures require isolating the appliance and removing the front panel and components — tasks for a professional.

3. If the engineer replaces the flue sensor and the fault still returns, further diagnostics by the engineer (checking wiring continuity back to the control PCB and testing the control box) will be required, as the problem may be in the harness or the control board.

When to call a professional:

1. Call a Gas Safe registered engineer if the fault does not clear after basic external checks and a single reset, if you find any internal damage, or if you are not confident performing the limited internal checks described above.

2. Always call a professional for sensor replacement, wiring repairs or any work that requires removing the boiler casing or accessing gas or mains electrical components.

Final note:

E99 is a safety-related lockout caused by a flue sensor short circuit. Do not ignore it. If simple visual checks and a single reset do not clear it, arrange for a Gas Safe registered engineer to diagnose and repair the fault to ensure the boiler operates safely and to protect your warranty and compliance.