Error F.72

Safety precautions:

Do not attempt gas-side repairs or internal PCB wiring work unless you are a Gas Safe registered engineer. Always isolate mains power to the boiler before opening the case. Be careful of hot surfaces and hot water when inspecting pipes. If you smell gas, leave the property and call the gas emergency service.

Initial homeowner checks (safe, simple steps):

1. Note the fault: record when F.72 appeared and any preceding symptoms (intermittent heating, noisy pump, recent work on system).

2. Try a basic reset: follow the boiler's reset procedure once. If the code returns immediately or reappears repeatedly, stop and call an engineer.

3. Check system pressure on the boiler display. Low pressure may cause other faults — if pressure is low (below about 1.0 bar), top up the system using the filling loop per your manual. If pressure is correct and stable, continue checks.

4. Check for obvious problems: visible leaks, frozen condensate pipe (in cold weather), or a blocked magnetic filter on the return pipe if you have one and it is safe/accessible to inspect. Clear obvious external blockages if safe to do so.

Diagnostic checks that usually require an engineer (but you can prepare for the visit):

1. Visual inspection of wiring and connectors: with the boiler isolated and ideally the cover removed by an engineer, the wiring harness to the NTC sensors (often red = flow, blue = return on many Vaillant models) and the connector to the PCB (X20) should be checked for secure seating, corrosion, damage or brittle insulation. Homeowners should not probe these with power applied.

2. Thermistor resistance check: a qualified engineer will measure each NTC sensor resistance with a multimeter. At approximately 20°C the Flow and Return NTC should each be around 12 kΩ (values change with temperature). Large differences between sensors or values out of expected range indicate faulty sensors. Engineers also check for intermittent signals which point to wiring faults.

3. Pipe and flow checks: the engineer will measure actual flow and return pipe temperatures with a probe thermometer to confirm the sensors’ readings. They will also check for restricted flow through the boiler (pump speed, blockages, sludge or limescale) and inspect any magnetic filter or scale reducer on the system.

4. Connector and PCB checks: the technician will inspect the X20 connector and trace wiring back to the PCB to ensure no partial breaks or poor contacts and check the PCB for related faults if necessary.

Possible fixes your engineer may perform:

1. Re-seat or replace damaged wiring/connectors if there is a poor electrical connection. This often resolves intermittent errors.

2. Replace one or both NTC thermistors if they are out of tolerance or intermittent. Thermistors are relatively inexpensive but replacement should be done by a qualified engineer.

3. Correct sensor positioning or wiring if sensors were installed on the wrong pipes (flow vs return swapped).

4. Restore correct system flow: clear blockages, flush sludge, clean/replace magnetic filter, treat or remove limescale if that is affecting temperature readings.

5. After repairs the engineer will reset the boiler and run it to verify the fault is cleared and the system is stable.

When to call a professional:

If the F.72 code persists after a reset, or if you find damaged wiring, loose connectors, or if you are not comfortable performing the basic safe checks above, call a Gas Safe registered heating engineer. Replacing sensors, opening the boiler, working on the PCB, gas or mains electrical isolation and carrying out system flushing or limescale work must be done by a qualified professional. Repeat or persistent F.72 faults should always be inspected by an engineer to prevent further breakdowns or incorrect repairs.

After repair:

The engineer will reset the boiler and test under normal demand. If the fault returns, the engineer should re-diagnose rather than repeatedly resetting. Keep records of the fault and repair work for warranty and future servicing.