Error E31

Safety first:

- Do not work on the gas supply. If you suspect a gas or combustion issue, call a Gas Safe engineer.

- Isolate electrical power to the boiler at the fused spur before opening the casing. Wait a few minutes for internal components to discharge.

- If you are not confident working safely with mains electricity or removing the boiler casing, stop and call a qualified engineer.

Initial homeowner checks (simple, non-invasive):

1. Note the exact error code and when it first appeared. Try a single reset (follow the boiler display/reset procedure in your manual). If the code returns immediately or after a short run, do further checks.

2. Check for obvious signs: inspect the external wiring and visible connectors (no need to open the boiler for this) for loose plugs, damaged cables, water ingress or corrosion.

3. Make sure the boiler has normal mains power and system water pressure (very low pressure can cause other faults) and that no other error codes accompany E31.

If you are comfortable and competent with basic electrical checks (ONLY with power isolated):

1. Isolate the boiler electrically at the fused spur. Confirm power is off.

2. Remove the boiler front panel to access the control box and the flow sensor wiring (follow instructions in the boiler manual if available).

3. Locate the flow thermistor/sensor — it is normally clipped to the flow pipe from the heat exchanger. Take care not to disturb flue/gas parts.

4. Check the connector: ensure the push-on connector to the sensor and the connector on the control PCB are fully seated. Look for corrosion or trapped wires. Re-seat connectors and replace the cover, restore power and attempt a reset/test. If the fault cleared, monitor for recurrence.

5. If the fault remains, isolate power again and use a multimeter to test the sensor wiring: unplug the sensor and measure continuity/resistance across its terminals. An open circuit (infinite resistance) confirms a failed sensor or broken wire. A very low resistance would indicate a short (which would normally show as E30).

6. If the sensor shows open circuit, inspect the wiring back to the PCB for breaks, chafing or loose crimps. You can also measure for continuity from the sensor connector to the PCB connector pins to locate a wiring break. Repair or replace damaged wiring only if you are competent to do so; ensure correct routing and insulation.

7. If wiring is intact but the sensor is open, the sensor will need replacement. Replace with the correct Keston flow thermistor/part number. After replacement, reconnect, restore power and reset the boiler to test.

8. If the sensor and wiring test OK but the control still reports open circuit, the PCB input for the flow thermistor may be faulty; this typically requires an engineer/installer to diagnose and replace the control PCB if necessary.

When to call a professional:

- If you are unsure at any stage, if the problem requires removing gas-carrying parts, or if the diagnostics point to a control PCB fault, call a Gas Safe registered heating engineer.

- A professional should be called for sensor replacement if work requires removing panels or parts that affect the gas or flue, and definitely for any work on the gas supply, combustion parts, or control PCB replacements.

Notes and good practice:

- Record the fault, any actions you took and whether the error reappeared. This helps the engineer diagnose recurring or intermittent faults.

- Do not attempt to bypass safety devices or run the boiler with known sensor faults.

- Follow the boiler handbook wiring and safety instructions if you perform any checks. If in doubt, stop and call a qualified engineer.