Error E164

Safety precautions:

- Before doing anything, turn off electrical power to the boiler at the isolator switch and, if you are going to touch pipework, allow the unit to cool down. Do not open the boiler casing or attempt electrical repairs unless you are a registered heating engineer. Gas and high-voltage work must only be done by a Gas-Safe (or local equivalent) qualified engineer.

- If you smell gas, evacuate, ventilate, and call the gas emergency number immediately.

Initial checks a homeowner can do:

1. Note the exact code and any display messages, then switch the boiler off and back on (full power off, wait 20–30 seconds, then on). Many sensor faults clear with a reboot if they were transient.

2. Open a hot tap to check if the boiler attempts to fire. Observe whether the fault reappears immediately or after a short period.

3. Check the boiler pressure gauge. If system pressure is very low (<0.8 bar) or very high (>2.9 bar), repressurise or isolate as appropriate and re-test. Low pressure can cause circulation and sensor-related errors.

4. Visually inspect any external sensor wiring you can see (no casing opening): look for loose connectors or obvious damaged cables to external probes/sensors. Re-seat any plug-in connectors that are easily accessible and intended for user access.

5. If your system has an external hot water cylinder sensor or remote probe, ensure that probe wiring and terminals are secure and that the probe has not been knocked loose.

Specific diagnostic and next-step actions (for information; only certified engineers should perform electrical/inside checks):

1. If reboot and pressure checks do not clear E164, the likely causes are a faulty DHW NTC sensor, a faulty heating flow/pressure switch, wiring/connector fault, or (less commonly) a PCB input problem. An engineer will measure the sensor resistance/voltage with a multimeter and compare to expected NTC values at ambient temperature.

2. The engineer should check the flow switch: verify the switch changes state when a hot tap is opened (mechanical activation or differential flow), test continuity, and inspect for blockages or debris that can prevent operation. In some Promax variants the flow sensor can stick or fail electrically.

3. Check connections at the PCB: corroded or loose terminals between the sensor/flow switch and the control board can cause E164. The engineer should also inspect for signs of water ingress, burn marks, or other PCB damage.

4. Where the DHW heat exchanger is scaled or blocked, flow characteristics can change and trip flow/pressure sensors; an engineer may examine and descale or replace components as needed.

5. If replacement of the DHW sensor or flow switch is required, the engineer will fit the correct part, re-test operation, and clear the fault code. If the PCB input remains incorrect after replacing sensors, further PCB diagnostics or replacement may be required.

When to call a professional:

- Call a qualified Gas-Safe (or local equivalent) heating engineer if the fault persists after the basic checks, if you are not confident performing even the basic inspections, or if the boiler displays repeated lockouts. Any work involving opening the boiler, testing or replacing sensors, wiring, or the PCB must be carried out by a registered engineer. Explain that the fault code is E164 and mention whether you tried reset, pressure checks, and whether the fault appears only on DHW demand or continuously.

- Do not attempt electrical or gas repairs yourself. Leaving the boiler in a fault condition may lead to loss of hot water and nuisance lockouts; a professional will safely diagnose and repair the defective sensor, flow switch, wiring, or control board as required.