Error E35

Safety first

1) Do not open the boiler casing or touch live terminals unless you are a qualified electrician. Isolate mains at the consumer unit before any internal work. Work on gas parts must only be done by a Gas Safe registered engineer. Frequent resets can cause other lockouts; avoid repeated resets.

Initial homeowner checks you can safely do

1) Note the exact behaviour and when the E35 appeared (after a power cut, generator start, new inverter/solar installation, recent electrical work, storm etc.).

2) Check other electrical appliances in the house. Are lights, sockets and other major appliances operating normally? If multiple items are affected, the problem is likely with the incoming supply.

3) Check the boiler’s local isolator/switch and the fuse/spur feeding the boiler. Make sure the mains switch to the boiler is ON.

4) Turn off any alternative power sources temporarily: if the property is using a generator, UPS, inverter, or uninterruptible power device, switch the boiler back to the utility supply and see if the error clears. Some inverter/generator outputs do not provide a stable 50 Hz and will trigger E35.

5) Power-cycle the boiler: switch off at the boiler isolator, wait 60 seconds, switch back on, then press Reset once. If the error clears and remains cleared, monitor for recurrence.

If the error persists — steps for professionals (call them)

1) Contact a qualified electrician to check the incoming mains frequency and voltage. The electrician should measure frequency at the consumer unit and at the boiler terminal using a meter capable of measuring Hz (digital multimeter with frequency function or a frequency meter). The mains should be approximately 50 Hz and the voltage around 230 V; large deviations or instability indicate a supply issue.

2) The electrician should also check for correct polarity, earthing continuity, neutral integrity and any signs of loose connections or corrosion on the supply conductors and the boiler spur. They should check for any nearby equipment or installed devices (solar inverters, VSDs, large drives, battery inverters) that could inject non-sinusoidal or wrong-frequency power.

3) If the measured frequency is out of tolerance at the property boundary, contact your electricity supplier or DNO. If the frequency at the consumer unit is correct but the boiler still shows E35, have the electrician measure at the boiler terminals to rule out wiring or local interference.

4) If mains frequency and voltage are within acceptable limits at the boiler but E35 persists, the fault may be in the boiler’s control electronics or the internal measurement circuitry. At this point arrange for a Gas Safe registered engineer (Keston-trained if possible) to inspect the boiler. The engineer will: verify incoming power at the boiler PCB, check the boiler earth and connections, inspect the control box and wiring harness, run internal diagnostics and, if required, replace the control PCB or fascia PCB.

5) If the engineer suspects electromagnetic interference or harmonics from nearby equipment causing a false frequency reading, work with the electrician to identify and mitigate the source (filtering, different wiring routing, or isolation of offending equipment).

Important notes and next steps

1) Do not attempt to repair or replace control PCBs, gas valves, or internal components yourself. Those tasks must be performed by qualified engineers. Incorrect intervention can create serious safety risks and may void warranties.

2) If an external supply problem is confirmed, notify your electricity supplier or DNO — they are responsible for correcting frequency/voltage issues on the network.

3) Keep a record of measurements and the sequence of checks (time, meter readings, actions taken) to help engineers and the electricity supplier diagnose the cause.

4) If you are in any doubt about safety or the origin of the fault, switch the boiler off at the isolator and call a Gas Safe registered engineer and an electrician. E35 is an electrical-supply related fault that should be resolved by professionals.