Error 201

Safety precautions:

- Before any checks isolate electrical supply at the boiler fused spur or main switch. Do not work inside the boiler with power applied.

- Never work on gas appliances unless you are a Gas Safe registered engineer. If you must turn off the gas supply in an emergency, only do so if you know where the service cock is and how to operate it safely.

- If unsure at any point, stop and call a qualified engineer.

Initial homeowner checks (safe, external checks you can do):

1) Reset the boiler: try switching the boiler off for 1–2 minutes then back on to see if the code clears. If it clears and the code returns, proceed with checks.

2) Visual inspection: look for obvious signs of water leakage around the hot water cylinder, pipework or boiler base and for any damaged, crushed or corroded sensor cable where it is visible. A wet connector or corroded plug can cause the fault.

3) Check system pressure on the gauge (boiler front). Very low or very high system pressure can cause other faults to appear; restore pressure to the normal operating range if needed following the boiler manual.

4) If your system uses an external cylinder thermostat or has obvious accessible sensor plugs on the cylinder, look for a removable connector that may be loose. If found, only with the boiler powered off, ensure the connector is seated and dry.

If you are technically competent and comfortable with basic electrical checks (only proceed if trained and safe):

5) With the boiler isolated from power, you may be able to check continuity of an accessible sensor lead using a multimeter. An open circuit indicates a broken wire; a short to earth indicates cable damage. Refer to the service manual for the expected resistance values for the DHW probe at a given temperature if you wish to compare readings. Do not attempt any live measurement inside the boiler while powered.

When to call a professional and what they will do:

- Call a Gas Safe / qualified heating engineer if the code remains after the simple checks, if you find a damaged cable/connector, or if you are not confident performing any of the above. Replacing the DHW probe usually requires gaining access to the boiler internals or the cylinder, fitting a replacement sensor, and verifying correct readings and parameters.

- The engineer will perform a full diagnostic: check probe resistance and continuity, inspect and test wiring harness and PCB connections (for example CN12 or other relevant connector), check for water ingress or corrosion, check relevant boiler parameters and, if necessary, replace the DHW probe and test the boiler through several DHW call cycles.

- If the engineer suspects a PCB or wiring loom fault (less common), they will isolate and test those components and advise on the correct repair.

Important notes:

- Do not attempt to operate the boiler if you suspect gas or combustion faults or if you smell gas; evacuate and call the gas emergency number immediately.

- Avoid trying to jury‑rig sensor connections or use non‑specified replacement parts. Use a qualified engineer to fit genuine or manufacturer‑recommended parts and to re-commission the boiler.

- If the fault appears intermittently and is associated with water leaks or condensation nearby, fix the leak and have the sensor/wiring inspected, as moisture is a common cause of probe faults.