Ferroli Fault Code A04 — Diagnose and Fix
Fault code A04 on a Ferroli boiler indicates diverter valve. Hot water works, heating doesn't (or vice versa). The diverter valve cartridge or motor has stuck. Common on Worcester Greenstar — £60–120 to replace. Here is the correct diagnostic order UK Gas Safe engineers follow before ordering parts.
Step-by-step method
1. Confirm the A04 lockout
Cycle the boiler off at the fused spur for 60 seconds, restore power and press the reset button. If A04 returns within 5 minutes, proceed with diagnosis. Intermittent A04 usually points to a sensor or connection, not the PCB.
2. Check pressure and condensate first
Every fault-code diagnosis starts here: system pressure 1.0–1.5 bar cold, condensate pipe unfrozen and draining, gas supply proven at another appliance. These three account for ~60% of lockouts.
3. Test the component A04 points at
Hot water works, heating doesn't (or vice versa). The diverter valve cartridge or motor has stuck. Common on Worcester Greenstar — £60–120 to replace.
4. Reset and test-fire
After any component test, purge the gas line, restore all covers (interlocks matter), press reset and observe a full ignition cycle. Confirm flame current with a multimeter on the flame sense wire (typically 1–8 µA DC).
5. Call a Gas Safe engineer for parts
Any gas-side work on a Ferroli requires Gas Safe registration. Have the fault code, boiler GC number and serial to hand — engineers can pre-order the right part before attending.
Frequently asked questions
Can I reset a Ferroli A04 myself?+
Yes — press and hold the reset key for 5 seconds. If A04 returns, the underlying fault is still present and needs diagnosis.
How much does fixing A04 cost?+
£120–£280 typical, depending on part. PCBs are £180–£320 fitted; sensors £60–£100. Get a fixed-price quote — refuse hourly for diagnostic-only work.
Is A04 covered by boiler warranty?+
Yes if within the Ferroli manufacturer warranty period AND the boiler has valid annual services logged. Missed services void most warranties.