Error S.42

Safety precautions:

- If you smell gas or suspect a gas leak, leave the property immediately and call the gas emergency number. Do not attempt any checks.

- Turn off electrical power to the boiler at the mains before touching internal or external wiring. Do not open or attempt repairs on gas-carrying components, the gas valve, ignition parts, or the PCB unless you are a Gas Safe engineer.

- Work only on visible external items (condensate pipe outside, external pump box) and do not dismantle the boiler casing unless instructed by a qualified engineer.

Initial checks a homeowner can do:

1. Note the fault code and any display message. Try a normal reset of the boiler following the user manual. If the code returns immediately or after a single restart, continue with the checks below.

2. Check that the boiler has power and that no fuses/trips have occurred in your consumer unit. Confirm other electrical items on the same circuit are working.

3. Confirm gas supply is present: try another gas appliance (hob) to verify the gas supply is on. If there is no gas supply to the property, contact the gas supplier.

4. Check boiler pressure gauge. If pressure is very low (below about 1.0 bar) top up following the manual instructions. Low pressure can cause other faults; topping up may allow the boiler to attempt normal operation again.

5. Inspect the condensate discharge pipe (commonly runs to an outside drain): in cold weather it can freeze. If frozen, carefully thaw with warm (not boiling) water or a warm cloth. Do not use an open flame.

6. If your boiler has a separate condensate pump unit (usually a small plastic box with an electrical lead near the boiler or under a kitchen unit), listen for a hum or operation when the boiler attempts to run. Check for visible blockages or kinks in the clear plastic discharge tubing.

Specific diagnostic and basic fix steps:

1. Reset and observe: After performing the initial checks (pressure, gas, condensate pipe thawed), reset the boiler and observe for any change. If the S.42 code clears and the boiler runs normally, monitor for recurrence.

2. Clear visible condensate blockages: If the external condensate pipe is blocked or frozen, clear or thaw it and re-test. If the condensate discharge runs into a drain, make sure the drain is not blocked.

3. Check external condensate pump unit: If you can see the pump, ensure it has power (indicator LED). If the pump is powered but you hear no motor action when the boiler tries to run, the pump may be seized or faulty and will likely need replacing. You can unplug and plug back any external accessory module or pump to try a power-cycle (only if you can reach plugs without removing boiler covers). Do not open pump internals.

4. Inspect for obvious wiring/connectivity issues to any external accessory modules (loose plug-in connectors). If there is an accessory module (e.g., for a cylinder circulation pump) that is removable, a power-cycle of that module can sometimes clear a communication fault. Do not work on internal boiler wiring.

5. Check the flue and inlet area for visible obstructions or damage (bird-nesting, debris). Do not dismantle the flue or the boiler's fan; report any obstruction to your engineer.

When to call a professional:

- If the fault code persists after the basic checks, if the condensate pump appears to be dead or leaking, or if there are unexplained electrical/communication errors, call a Gas Safe registered engineer.

- Do not attempt to replace or repair gas valves, ignition components, PCB, internal wiring, NTC sensors, or the primary heat exchanger yourself. A competent engineer should check condensate pump condition, accessory module communications, sensor wiring, the PCB and any related components.

What the engineer will likely do:

- Remove covers and test condensate pump operation, check and clear any internal or external condensate blockages, test accessory module communication and wiring, and inspect flow/return sensors and PCB as needed. They may replace a faulty condensate pump or a defective accessory module or repair wiring/coding resistor issues.

If you complete the safe checks above, record what you tried and tell the engineer — that information speeds diagnosis. Do not continue attempting internal repairs if you are not qualified; call a Gas Safe engineer for inspection and repair.