Error F0 242

Safety precautions:

1) If you smell gas, evacuate the property immediately and call the gas emergency number. Do not attempt any checks. 2) Before touching any internal components isolate electrical power to the boiler at the mains (turn off the dedicated breaker or isolator). If you are not comfortable isolating power or opening the boiler casing do not proceed. 3) Do not attempt to work on the gas supply, ignition or replace the control board/HCM yourself unless you are Gas Safe registered.

Initial homeowner checks you can do (no tools or minimal action):

1) Record the exact displayed code(s), note whether a warning triangle or secondary cause code appears and take a photo of the display. 2) Try a controlled reset: press and hold the reset or tick/OK button for around 3–8 seconds (Greenstar i often needs about 8 seconds) to reboot the boiler. If the fault is transient this may clear it. 3) Cycle the boiler power off at the mains for 30–60 seconds and then back on, then check whether the error returns. 4) Check for obvious signs of water ingress, corrosion or damage around the boiler casing and at visible cable entry points. 5) Check that any external controls/room thermostats or timers are correct and did not recently change settings or wiring.

Specific diagnostic checks you can safely perform with power isolated (only if comfortable and confident):

1) Remove the outer case only if you are comfortable and have isolated power. Do not touch live parts. Visually inspect connectors to the control box and HCM for loose plugs, bent pins, corrosion or water damage. Take photos. 2) With power still off, gently reseat any clearly accessible multi-pin connectors (do not force) and ensure the coding plug (if fitted) is fully seated. 3) Restore power and re-check the display for the same error. If re-seating a connector changes behaviour, record what you did and show the engineer.

What to do next and information for your engineer:

1) If the error persists after reset and basic connector checks, stop and call a Gas Safe qualified engineer. Electronics faults usually require specialist test equipment, firmware checks and correct replacement parts. 2) When you call the engineer, provide the exact fault code (F0 242), any secondary cause code or flashing triangle, details of any steps you tried (reset, power cycle, reseated connectors), and photos of the display and internal connectors if you took any. 3) If any recent work, power surge, condensation or water leaks occurred, tell the engineer — water damage to the control box/HCM is a common cause.

Why you need a professional:

1) The control box and HCM interact with gas and ignition circuits and safety devices; replacing or testing them requires a Gas Safe engineer. 2) A professional will check correct HCM part coding (an incorrect HCM can also cause faults), test the control board, wiring harness, and sensors properly, and carry out safe replacements and commissioning. 3) After repair the engineer will clear lockouts and fully test the boiler under demand.

Summary: try the documented safe checks (record code, reset, power cycle, visual connector check), but if F0 242 returns you should call a Gas Safe engineer to diagnose and replace faulty control electronics. Do not attempt gas or live-electrical repairs yourself.