Overview
These F0 24x–F0 27x and related F0 codes on a Worcester Bosch GB162 indicate an internal communication/control fault centred on the burner control unit (often called the KIM or HCM). The boiler is telling you the control module that identifies/controls the burner is either not communicating correctly with the main control board, has poor connector contacts, or has failed outright. With these faults the boiler will usually lock out and refuse to run until the underlying fault is fixed and the code cleared. Causes range from simple (loose or corroded connectors, pinched wiring, water ingress) to more serious (failed KIM/HCM module or main control PCB). Severity is moderate to high because the fault prevents safe ignition and operation; it is not purely cosmetic. Basic non-invasive checks and a single reset can be done by a competent homeowner, but diagnosis beyond visual checks and reseating connectors or replacement of the module must be carried out by a Gas Safe registered engineer. Do not attempt gas or high-voltage electrical repairs yourself.
Possible Cause: The wiring of the burner control unit needs checking, also check the operating characteristics of the boiler by replacing the burner control unit.
Troubleshooting Steps
Safety precautions:
- If you smell gas: turn off the gas supply at the meter, leave the property, and call the gas emergency number immediately. Do not operate electrical switches or appliances.
- Do not repeatedly reset the boiler more than 2–3 times. Repeated resets can mask a dangerous fault and cause further damage.
- Unless you are a qualified engineer, do not attempt to work on gas valves, the burner, or internal high-voltage circuits. If in doubt, stop and call a Gas Safe engineer.
Initial homeowner checks (non-invasive):
1. Record the exact fault codes as shown (including any static cause code) and take a clear photo of the display for the engineer.
2. Try one controlled reset only: follow the boiler’s reset procedure (turn the selector to reset/press the reset button once, wait for the boiler to attempt to restart). Note whether the fault returns immediately or after a short run.
3. Check the mains power supply and RCD/fuse box to make sure the boiler is receiving power. Ensure the boiler isolator (if fitted) is ON.
4. Check the gas supply quickly by confirming other gas appliances (hob, cooker) work. If there is no gas supply to the house, contact your gas supplier.
5. Look for visible signs of water ingress, corrosion, burn marks or loose external cables around the boiler. Also check for recent flooding or heavy condensation near the boiler.
Specific diagnostic steps you can do only if competent and comfortable (isolate mains first):
1. Turn the mains electrical supply to the boiler OFF at the isolator or fuse spur and, if possible, turn off the gas supply to the boiler before removing covers.
2. Remove the outer case only if you are confident and comfortable doing so. Visually inspect the KIM/HCM module and its connector for loose plugs, corrosion, bent pins or signs of overheating.
3. If the connector is visibly loose and you are competent to do so, carefully unplug and re-seat the module/connector ensuring it goes in straight and clips home. Do not force or lever components. Make sure no wiring is trapped and that pins line up correctly.
4. Refit the outer cover, restore the mains supply and gas, then attempt a single reset and observe the result.
5. If the fault clears and boiler runs normally for an extended period, monitor closely. If the code reappears, do not continue to reseat or reset repeatedly — this signifies component failure or intermittent wiring.
When to call a professional / what the engineer will check:
- Call a Gas Safe registered heating engineer if the code returns after a single reset or after reseating connectors, if you are not comfortable opening the boiler, or if you find damaged wiring, corrosion, or signs of electrical burn.
- Provide the engineer with the exact fault codes (including any static cause code) and photos of the display and internal connectors you inspected. This speeds diagnosis.
- The engineer will perform safe electrical checks, run diagnostic tests, check connector continuity, measure flame/ionisation signals, inspect ignition electrodes, gas valve coils and may replace the KIM/HCM or main control PCB if faulty. They will also check for related causes (water ingress, incorrect module fitted, wiring damage).
Additional notes:
- Do not attempt to replace the KIM/HCM or control PCB yourself unless you are a Gas Safe engineer. These parts interact with gas and safety circuits and must be commissioned correctly.
- If the boiler shows additional signs such as gas smell, visible damage, or persistent lockouts, leave the unit off and get a qualified engineer to attend immediately.
- Keep a record of all fault codes and the actions you took; this helps the engineer diagnose and repair the fault more quickly.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Worcester Bosch GB162 Boiler.