Error D.98

Safety first: Do not open, repair or work inside the boiler unless you are a qualified Gas Safe engineer. Turn the boiler to standby and switch off the electrical supply at the isolator or fused spur before touching any parts. Do not attempt to isolate or work on the gas supply unless you are trained. If you smell gas, leave the property and call the gas emergency service immediately.

Initial homeowner checks (safe, non-invasive):

1. Record the error: note D.98 and any other codes or indicators, and whether the fault is constant or intermittent. This helps the engineer.

2. Reset the boiler: use the normal reset procedure or switch the boiler off, wait 30–60 seconds and switch back on. If the error returns immediately or within a short time, proceed with the checks below.

3. Check mains power: ensure the boiler has power (other displays light up) and your house fuse has not tripped.

4. Check system pressure: look at the boiler pressure gauge. If pressure is very low (well below 1 bar) top up per the boiler manual; very low pressure can cause odd sensor readings or pump problems. If you are unsure how to repressurise, do not attempt risky actions; call the engineer.

5. Listen for the pump and fan: when the boiler calls for heat you should hear the pump/fan running. If the pump seems silent when there should be flow, that may indicate pump failure or power issue.

6. Bleed radiators: airlocks can cause poor flow and temperature sensors to read unexpectedly. Bleed a radiator to release trapped air and then check pressure again.

7. Check external pipes: in freezing weather inspect the condensate pipe for freezing and any visible external piping for blockages. A frozen condensate or blocked flue can create related faults.

8. Visible connectors only: if there are any accessible external sensor leads or terminal blocks that are designed to be user-accessible (check the boiler manual first), ensure they look seated and dry. Do not remove the main casing or probe inside connectors unless you are qualified.

Diagnostic and likely fix steps a Gas Safe engineer will perform (what to expect):

1. Full fault readout and history: the engineer will read all stored diagnostic codes and operating history to identify linked errors (for example F.91 with D.98 points to DHW/cylinder sensor issues).

2. Electrical checks: with power isolated and then under controlled testing, they will check continuity and resistance of the NTC sensors and wiring harness. An NTC should show a measurable resistance that changes with temperature; a constant open or short indicates a faulty sensor or cable.

3. Inspect plugs and PCB connections: loose, corroded or wet plug connections to the flow/return or DHW sensors and to the actoSTOR electronics (where fitted) will be checked and reseated or replaced as necessary.

4. Check pump and hydraulic flow: the engineer will confirm the pump is operating correctly, check for blockages, air locks, or restricted flow in the primary circuit and may power-flush the system if there is debris restricting flow.

5. Check for related faults: they will inspect the condensate route, flue, ignition and earthing where relevant because multiple system faults can interact.

6. Replacement work: depending on findings the engineer may replace the faulty NTC sensor(s), the sensor cable/harness, pump or pump seals, or the main PCB if it is confirmed defective. After repairs they will test the system thoroughly and clear fault codes.

Important final notes: Do not attempt to replace sensors, wiring or electronic components yourself — these operations require isolation of gas/electrical supplies and commissioning by a Gas Safe registered engineer. If D.98 reoccurs after basic checks (reset, pressure, bleeding radiators, checking visible pipes), book a Gas Safe engineer and provide them the fault code and any symptoms you noted (no heat, no hot water, whether the pump sounds, recent power cuts, freeze conditions etc.). In cold weather treat this as urgent to reduce risk of frozen pipes.