Overview
The Keston C36 error codes E12 to E22 are grouped as control box / internal electronics faults. In practice this range can cover several specific issues: a blown thermal fuse or activated over‑temperature thermostat (commonly reported as E12), failed calibration or faults with the flow and return temperature sensors, communication problems between the user fascia and the main control PCB, or a failing control PCB itself. When the control box detects one of these faults the boiler will normally go to lockout and stop operating to protect the appliance and the system. Severity ranges from moderate to high. Some causes (a simple power glitch or a temporary sensor disconnect) can be cleared by a reset and a power cycle, but many of the failures indicated by codes in this range require access to live electronics and the gas system or replacement of the control PCB/thermal fuse. For safety and legal reasons any work that involves removing the boiler cover, testing live circuits, changing gas valves or replacing internal parts must be carried out by a qualified Gas Safe registered engineer. Homeowners can do a few basic external checks and resets, but internal diagnostics and repairs should be left to a professional.
Possible Cause: Control box errors – possible faulty control box
Troubleshooting Steps
SAFETY PRECAUTIONS:
1) If you smell gas, evacuate the property immediately and call the gas emergency number. Do not operate electrical switches or the boiler.
2) Do not remove the boiler case or attempt live electrical work unless you are a qualified Gas Safe engineer. Internal parts are live and gas-bearing.
3) Isolate electrical supply if instructed by a professional. For homeowner checks, only use external isolation (fused spur or consumer unit) and simple reset buttons; do not open the appliance.
INITIAL HOMEOWNER CHECKS (safe, non-invasive):
1) Record the exact error code shown and any other codes or messages. Note when it first appeared and what the boiler was doing then (e.g. firing for hot water, during reset, after a power cut).
2) Try a controlled reset: press the boiler reset button once (follow manufacturer handbook timing). Wait and observe if the code returns. Do not repeatedly reset more than twice — repeated resets can mask a persistent fault.
3) Power cycle the boiler at the fused spur or consumer unit: switch off for 1–2 minutes, then restore power. Check if the fault clears after a full reboot.
4) Check mains electrical supply to the property: are other appliances on the same circuit working? If you suspect low voltage or recent power surges, note this for the engineer.
5) Check visible external wiring and the fascia interface: ensure the display panel is seated correctly and the control knob has not been damaged or knocked. Do not unplug anything behind the fascia.
6) Check boiler system pressure on the pressure gauge. Although E12–E22 are control box errors, unrelated low pressure or repeated pressure faults can produce other codes; top up to the normal range (usually ~1–1.5 bar) only if you know how from the manual.
IF THE CODE PERSISTS AFTER HOME CHECKS:
1) Do not attempt to open the boiler. Contact a Gas Safe registered engineer and give them the exact fault code(s), history of resets, whether the code reappears immediately, and any recent events (power cut, freezing, service overdue).
PROFESSIONAL / DIAGNOSTIC STEPS (for engineer):
1) Confirm identity and isolate appliance: switch off electrical supply and gas supply per safe procedure before accessing internals.
2) Visual inspection: check PCB and harness for burn marks, blown components, water ingress, or corrosion. Check connectors from the fascia, sensors, fan, pump and gas valve are secure.
3) Thermal fuse and over-temperature devices: for E12 specifically test the thermal fuse and flue/overheat thermostat for continuity and correct operation; replace if confirmed blown.
4) Sensor checks: measure resistance/voltage of flow and return thermistors at known temperatures; check for open or short circuits (E30–E36 family). Verify sensors are fitted to correct pipes and have good thermal contact.
5) Communication testing: check the cable and connectors between the fascia PCB and main control box for continuity and correct pin assignment. E41/E99 style faults point to communication issues.
6) Mains supply verification: check incoming mains voltage and frequency at the control PCB (should be ~230 V and correct frequency). Low voltage or incorrect frequency can cause control faults.
7) Functional checks: observe fan start signals, gas valve drive signals and flame detection circuits (watch for E02/E26/E09 related interactions). Check for false flame or unstable flame conditions that may present as other intermittent faults.
8) Run the automatic calibration/test routines (if supported) to verify flow/return sensor calibration. Re-run after any repairs.
9) If PCB is confirmed faulty (component failure, burnt traces, unrepairable fault) replace the main control box following manufacturer procedure: isolate, disconnect multi-pin connectors and HT lead, remove retaining screws, swap unit, reassemble using correct gasketing and torque.
10) After repairs: restore gas and electrical supplies, purge and check gas tightness, run combustion checks, verify correct operation of DHW and heating modes, test for leak and correct system pressure, and document repairs and safety checks.
FINAL NOTES:
1) Many E12–E22 faults will require a trained engineer due to live electrical work, gas safety, and the need to replace or reprogram control electronics. Do not attempt internal repairs yourself.
2) When you call a service engineer, give them the code, the steps you have already tried, and any other codes or symptoms — this speeds diagnosis.
3) Keep service records and consider an annual boiler service to reduce recurrence of electronics and sensor problems.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Keston C36 Combi Gas Boiler.