Ferroli A42 Fault Code: Causes, Fixes & Repair Costs
What does the Ferroli A42 fault code mean?
The A42 code means the boiler's flow (heating) sensor has failed or is producing readings that fall outside the expected range, triggering the built-in protection system to shut the boiler down. It's worth knowing how A42 sits alongside its close relative A41: A41 indicates the flow sensor has become disconnected, whereas A42 points to the sensor itself being faulty and in need of replacement. Ferroli also pairs this with the F42 code under what the manufacturer describes as 'sensor control protection activation', which can involve either the flow sensor or the return sensor being damaged or disconnected. If your display is showing A41 rather than A42, start by checking sensor connections before assuming a full replacement is needed.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Failed flow (heating) sensor Common
The flow temperature sensor has developed an internal fault — typically a break in the thermistor element — and is sending readings the boiler's control board considers invalid. This is the most direct cause of an A42 code and usually means the sensor needs replacing rather than just re-seating.
- Damaged or faulty return sensor Common
Ferroli's sensor protection system monitors both the flow and return sides. If the return sensor is damaged, short-circuited, or giving erratic resistance values, the boiler may log an A42 rather than pointing specifically to the return side. An engineer will check both sensors during diagnosis.
- Wiring fault — broken, shorted, or loose connections Sometimes
The sensor itself may be perfectly functional, but damaged cable insulation, a corroded connector, or a broken wire between the sensor and the PCB can produce the same fault signal. This is especially worth considering on older Ferroli units where wiring looms have had years of heat cycling.
- Air in the system causing abnormal temperature differentials Sometimes
Trapped air in the heating circuit can prevent proper water circulation, leading to unusual temperature readings across the flow and return sensors even when the sensors themselves are undamaged. Bleeding radiators can sometimes resolve the underlying trigger, though the sensor fault code still needs clearing by an engineer.
How to fix it
- Check your gas supply is working DIY safe
Before assuming the boiler is at fault, confirm your gas supply is active. Check that other gas appliances in the property (hob, gas fire) are working normally. If there is no gas at any appliance, contact your gas supplier rather than attempting any boiler work.
- Reset the boiler once or twice DIY safe
Locate the reset button on the boiler fascia — on most Ferroli models it is marked with an 'R' or a flame symbol. Hold it for around five to ten seconds. If the boiler fires up and runs normally, monitor it over the next few hours. A single reset is reasonable; if the A42 code returns within a short period, do not keep resetting, as forcing the boiler to fire repeatedly with a faulty sensor can cause heat exchanger damage.
- Check and top up system pressure if low DIY safe
Look at the pressure gauge — on most Ferroli boilers the needle should sit between 1 and 1.5 bar when the system is cold. If it has dropped below 1 bar, use the filling loop (usually a silver braided hose beneath the boiler with two isolation valves) to top up slowly to around 1.2 bar, then close both valves. Low pressure on its own won't cause an A42, but it can contribute to poor circulation and abnormal temperature readings.
- Bleed radiators to remove trapped air DIY safe
If any radiators feel cold at the top but warm at the bottom, there is trapped air in the system. Use a radiator bleed key to release air from each radiator starting with the ground floor, working upwards. Re-check the boiler pressure afterwards and top up again if needed. This can help restore normal flow and return temperature differentials.
- Have a Gas Safe engineer inspect and test both temperature sensors Gas Safe engineer
An engineer will use a multimeter to measure the resistance of both the flow and return sensors at various temperatures, comparing readings against the manufacturer's specification chart. This confirms whether one or both sensors have failed internally or whether the fault is in the wiring harness connecting them to the PCB.
- Sensor replacement by a Gas Safe engineer Gas Safe engineer
If either sensor is confirmed faulty, the engineer will drain down the relevant section of the circuit, remove the defective sensor, fit a new Ferroli-compatible sensor, and refill and re-pressurise the system. On older or discontinued Ferroli models, lead times for genuine parts can occasionally be longer than for mainstream brands, so it's worth asking the engineer to confirm parts availability before booking.
- Wiring inspection and repair if required Gas Safe engineer
If both sensors test within specification, the engineer will trace the wiring from the sensors back to the PCB, looking for chafed insulation, corroded pins, or a broken conductor. Wiring repairs are typically quicker and cheaper than sensor replacement but require the same level of competence to do safely.
- Call a Gas Safe registered engineer if the fault persists or returns Gas Safe engineer
If the A42 code reappears after a reset, or if you have not been able to resolve the fault through the DIY-safe steps above, contact a Gas Safe registered engineer. You can verify any engineer's credentials at GasSafeRegister.co.uk. Do not leave the boiler running in a fault condition or attempt to bypass the sensor protection system.
Parts you may need
- Ferroli flow temperature sensor (NTC thermistor) · from £25
- Ferroli return temperature sensor (NTC thermistor) · from £25
- Sensor wiring harness / loom · from £35
- Ferroli PCB (if secondary fault develops) · from £180
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £100–£250, depending on the underlying cause.
Frequently asked questions
What is the difference between Ferroli fault codes A41 and A42?
A41 means the flow sensor has become disconnected — so the first step is to check the sensor is correctly seated and its connector is secure. A42 means the sensor itself has failed and needs replacing. In practice the two codes are related and an engineer may check both sensor connections and sensor resistance values before confirming which part needs attention.
Can I fix a Ferroli A42 fault myself?
The only steps that are safe to carry out yourself are: confirming your gas supply is on, attempting one or two resets, checking and topping up system pressure via the filling loop, and bleeding radiators. Everything beyond that — testing sensors with a multimeter, replacing the sensor, or inspecting the wiring loom — must be done by a Gas Safe registered engineer. Sensor replacement is not a complex job for a competent engineer, but it does involve draining part of the circuit and working near the boiler's electrical components.
How much does it cost to fix a Ferroli A42 fault in the UK?
For most households the repair involves a call-out, diagnosis, and a flow or return sensor replacement, which typically comes to between £100 and £250 including parts and labour. Ferroli sensors are relatively affordable components. If the fault turns out to be in the wiring rather than the sensor itself, the job may be at the lower end of that range. In the less common scenario where the PCB has been damaged by a prolonged sensor fault, costs can rise to £300–£450 — if that happens on an older boiler, it is worth discussing replacement versus repair with your engineer.
Should I keep resetting my Ferroli boiler if the A42 keeps coming back?
No. One or two resets are reasonable to see whether the fault is a one-off, but if the A42 code reappears repeatedly it means there is a genuine fault with the sensor or its wiring that resetting cannot cure. Repeatedly forcing the boiler to fire with a faulty flow sensor risks causing abnormal heat build-up in the heat exchanger, which is a much more expensive component to replace. Stop resetting and arrange an engineer visit instead.