Error D.72

Safety precautions:

- Do not open the boiler or work on gas or live electrical components unless you are Gas Safe / certified. Internal repairs are potentially dangerous.

- If you need to isolate power for visual checks, switch the boiler off at the programmer and the isolator. Do not isolate the gas supply yourself unless you are trained.

Initial homeowner checks you can safely do:

1. Note exactly when the fault happens (cold start OK but warm restart fails, time since last run, thermostat setpoint). This information is useful to the engineer.

2. Reset the boiler using the normal reset procedure and see if the code returns. If it is persistent, proceed with checks below.

3. Check system pressure on the boiler gauge. Typical cold pressure is around 1.0–1.5 bar. If the pressure is low, top up using the filling loop (only if you know how). Low pressure can cause circulation and sensor-related faults.

4. Bleed a few radiators to remove trapped air, then recheck pressure. Airlocks can cause incorrect flow/return readings and pump issues.

5. Ensure all external valves to the boiler and the pump (if isolation valves exist) are open. Check that the room thermostat or any external controls are not requesting rapid on/off behaviour.

6. Listen for pump operation when the boiler is running or in pump overrun: is the pump running continuously, making noise, or silent? Make a note — do not attempt to remove covers to access the pump wiring.

7. Check condensate discharge and visible pipework for obvious blockages or freezing if temperatures are low.

Step-by-step diagnostic and fix actions for an engineer (what they will typically do):

1. Read fault history and live parameters from the service menu to confirm D.72 events and any associated codes (flow temp, return temp, pressure faults, STB trips).

2. Visually inspect and test the flow and return NTC thermistor plugs and wiring. Look for unplugged, loose, corroded or damaged connectors and repair/replace wiring as needed.

3. Measure the resistance of the flow and return NTCs with a multimeter at known temperatures to confirm correct thermistor behaviour; replace any defective NTCs.

4. Check continuity and condition of the main wiring harness to the sensors and pump. Repair any broken or intermittent connections.

5. Test pump operation and flow rate under demand (measure differential pressure or flow). Replace the pump if it is failing or not providing the required flow.

6. Inspect for hydraulic issues: airlocks, blockages in heat exchanger, closed isolation valves, or restriction at the 3-port diverter valve. Perform bleeding, power-flush or a targeted clean/power-flush if debris is present.

7. Check the 3-port diverter valve for correct movement and no seizure or sticking; replace if faulty.

8. Verify the operation of the safety temperature limiter (STB) and flue temperature monitoring; clean or replace if soot/damage has caused nuisance trips.

9. If wiring, sensors and hydraulic parts check out but the fault persists, test the PCB and control logic for intermittent faults; replacement may be necessary.

When to call a professional:

- If the error returns after a reset or after the simple homeowner checks above, you should contact a Gas Safe registered engineer. Many root causes (NTC replacement, wiring repair, pump replacement, 3-port valve or PCB work) must be done by a qualified technician.

- Explain to the engineer that the issue appears on warm restart and provide the exact fault D.72 and any observations (pressure reading, pump noise, whether you had it power-flushed or a new pump fitted recently).

Temporary operational notes:

- Allowing the boiler to cool fully before restarting may avoid the immediate pump-overrun lockout but is only a temporary workaround.

- Avoid repeated resets if the fault is recurring — call a Gas Safe engineer to prevent damage or unsafe conditions.

Remember: diagnosing and repairing sensor wiring, gas, or internal electrical components is not a DIY job. A Gas Safe registered engineer should carry out the full fault finding and repairs.