Error S13

Safety first

1) If you smell gas or suspect a gas leak, do not touch the boiler or any electrical switches. Evacuate the building and call your gas emergency number immediately.

2) If the boiler shows a flame/ignition fault and you have any doubt about safety, isolate the gas supply (if you know how and it is safe to do so) and turn the boiler off at the isolator. Contact a Gas Safe engineer.

3) Never open the boiler casing or attempt to replace ignition electrodes, gas valves, PCB parts or internal wiring unless you are a qualified Gas Safe engineer.

Initial homeowner checks (safe, non-invasive)

1) Note the exact error: write down S13 and any additional codes or messages (for example an F.xx code shown at the same time). These additional codes help the engineer diagnose whether the problem is ignition, sensor, wiring or PCB related.

2) Power cycle: switch the boiler off at the mains or isolator, wait 30 seconds, then switch it back on. Try a single reset using the boiler reset button if the manual allows — do not repeatedly reset. If the fault clears and does not return, monitor for recurrence and still report it at service if it does return.

3) Check other gas appliances: confirm that other gas appliances in the property are working. If all gas is off, contact your gas supplier before calling an engineer.

4) Check the condensate discharge: look for frozen or blocked condensate pipe/trap (often outside the property). Thaw gently with warm (not boiling) water if frozen, clear any visible blockages. A blocked condensate can prevent ignition and cause lockout.

5) Check air intake and flue: ensure the flue terminal and air intake are free from obstructions (bird nests, debris, snow). Restricted flue air can stop ignition or cause flame loss.

6) Check boiler water pressure: view the pressure on the boiler display or gauge. If pressure is very low (below ~1.0 bar) some boilers will lock out. Re-pressurise the system only if you know how and it is safe; follow the boiler’s instructions. Do not over-pressurise.

7) Demand test: open a hot tap to force a DHW call and watch the boiler. Note whether the pump runs, whether the boiler attempts ignition (you may hear sparks), and whether the S13 or any F code returns. Record any lights or blinking sequences.

If the problem appears ignition-related (typical signs: boiler attempts to ignite but fails; F28/F29 or S13 shown)

1) Ensure gas is present and other appliances confirm gas supply. If gas is present and ignition still fails, do not attempt to replace ignition leads, electrodes, transformers or gas valves yourself.

2) A temporary reset may get the boiler running again, but repeated lockouts point to failing ignition components, spark generator, electrode corrosion or a faulty gas valve or PCB — these require a Gas Safe engineer to test spark, measure gas pressures, inspect electrodes and check combustion.

If the problem appears sensor/wiring-related (typical signs: other flow/return or cylinder NTC codes appear alongside S13)

1) Note which sensors are reported as faulty (flow NTC, return NTC, cylinder sensor). A wiring plug that has come loose, a broken harness or a shorted NTC can produce similar lockouts. Homeowners should not unplug or replug internal connectors inside the boiler. However, you can note if the boiler was recently serviced or moved, which might have disturbed a connector.

2) A Gas Safe engineer will need to test NTC resistances at ambient temperature, inspect harnesses and connections and replace defective sensors or wiring as required.

What to tell the engineer and when to call

1) If the fault persists after your safe checks, call a Gas Safe registered engineer. Provide: boiler model and serial, exact displayed codes (S13 and any F.xx), whether the fault is constant or intermittent, what initial checks you have performed (power cycle, condensate thaw, pressure top-up, gas supply check), and when the fault started.

2) Do not attempt internal repairs, replacements of ignition, gas or electrical components, or PCB swaps yourself.

3) If the boiler is under maintenance contract or manufacturer warranty, contact that service first and mention the recorded codes.

Urgency and final note

S13 means the boiler is locked out from lighting in DHW mode or has an associated sensor/wiring fault. While some homeowner actions can clear external or simple faults (condensate, frozen pipes, gas supply, power reset), most root causes—ignition components, gas valve, PCB, wiring harnesses or defective NTC sensors—require a Gas Safe qualified engineer to diagnose and repair safely. Do not attempt internal gas or electrical work yourself.