Error A42 on Ferroli boiler: causes and solution

Introduction

The A42 error on a Ferroli boiler signals an anomaly in the temperature control of the heating circuit. When the control board receives an inconsistent or out-of-range thermal reading, the unit performs a safety lockout to avoid overheating, uncontrolled combustion or damage to sensitive components. This article explains what A42 means, the most frequent causes, how the boiler behaves, the checks to perform before replacing parts and the usual solutions.

What the A42 lockout really means

Ferroli boilers use self-diagnosis to supervise temperature, pressure, flame and exhaust. When A42 appears, the electronics have detected that the value coming from the probe does not match the expected behavior of the circuit. In practice, the boiler no longer trusts the thermal reference and stops operation to protect the heat exchanger, the pump and the control board.

Consistency is the key concept: the alarm does not always indicate an irreversibly broken sensor. Sometimes the reading becomes unstable because of a loose connection, oxidation, moisture or poor water circulation. If a basic inspection does not restore a stable reading, the lockout will persist and a deeper diagnosis is required.

Common causes

Heating (flow) probe or return probe failure

The most frequent cause is a worn or damaged NTC probe (flow or return). If the thermistor ages, cracks or loses internal continuity, resistance values become erratic and the board interprets that as a thermal monitoring failure. A damaged or intermittent probe can cause sudden or sporadic lockouts.

Electrical connections and wiring

A disconnected or loose connector, pinched cable, heat-damaged insulation, oxidized terminals or moisture in the connector can alter resistance and produce false readings. Because the control unit only sees the electrical signal, a bad connection can trigger the same lockout as a failed probe.

Poor water circulation (hydraulic causes)

When the pump is weak, a valve is partially closed or there is air in the circuit, heat can concentrate locally and the probe may measure a non-representative temperature. The board may interpret that as a risk and shut down. In many cases replacing the sensor alone does not fix the issue if circulation remains inadequate.

Other causes

• Corrosion or overheating around terminals and board contacts.
• Intermittent faults in the control electronics (less common).
• Installation errors that produce abnormal thermal behavior.

How the boiler behaves when A42 appears

Typically the boiler stops heating and, depending on the model, may also limit or interrupt domestic hot water. The display shows the error code and ignition is interrupted without the normal cycle. A reset may clear the warning temporarily if a contact returns by chance, but if the underlying cause persists the code will reappear. Intermittent behavior (error changes when moving a cable or after different running times) is a strong diagnostic clue.

What to check before replacing parts

1. Visual inspection: Check connectors, wiring routing, pinched or chafed cables, corrosion on terminals and signs of moisture or overheating around the board and probes.
2. Connections: Verify plugs are fully seated, terminals are clean and free of oxidation and connectors are dry.
3. Hydraulics: Confirm pump operation, circuit pressure and bleed the installation to remove air pockets. Check that valves are correctly positioned.
4. Electrical continuity: Inspect wiring for breaks or intermittent contact; move harnesses while watching the display for changes.
5. Sensor test: Measure the probe resistance and compare it to the manufacturer’s table at the actual temperature (see next section).

Measuring sensor resistance

Measuring the sensor’s resistance across its terminals is the most useful technical test. Compare the measured value with the reference values provided by Ferroli for that NTC type at the current temperature. If resistance does not change consistently as temperature varies, the probe is failing. If the probe responds correctly, focus on wiring, connectors, pump and installation hydraulics before replacing the sensor.

Why lack of circulation can trigger the lockout

A boiler measures water temperature within a circuit that must circulate. If water barely moves, heat concentrates locally and the probe can register an abnormally high or erratic value. The control board interprets this as a hazardous condition and locks the unit. Therefore circulation checks (pump, bleeds, pressure, and valve positions) are integral to the diagnosis—not a secondary detail. Even a new probe can continue to cause A42 if the hydraulic environment remains faulty.

What to do and what to avoid when the fault occurs

• Do: Turn the unit off if the lockout repeats and avoid continuous restarts. Perform the visual and hydraulic checks described above. Measure probe resistance and inspect connectors.
• Do: Bleed the system, verify pump operation and check system pressure before replacing components.
• Avoid: Replacing the probe blindly without confirming wiring and circulation. Repeated resets without diagnosis—this delays proper repair and can stress the appliance.
• Call a qualified technician if the code persists after the checks, or if you are not confident performing electrical and hydraulic diagnostics safely.

Quick diagnostic checklist and usual solutions

• A42 — Protection activated by sensor control. Cause: anomalous reading in flow or return probe (or connector/wiring/circulation). What to check: connector, wiring, sensor resistance, water circulation. Usual solution: repair connection or replace faulty probe.
• F42 — Sensor control protection activated (related alarm). Cause: inconsistent signal in the thermal circuit. What to check: sensor condition, electrical continuity, possible poor contacts. Usual solution: repair wiring or replace affected probe.
• F41 — Protection associated with thermal control. Cause: temperature reading out of range or unstable (sensor, water return or air pockets). What to check: installation hydraulics and sensor. Usual solution: check installation and replace component if necessary.

Final recommendations

Start diagnosis with simple, low-cost checks: visual inspection, reconnection of plugs, bleeding the system and confirming pump operation. Use a multimeter to verify sensor resistance against the manufacturer’s table. Replace the sensor only after confirming it is truly out of specification or if wiring and circulation have been ruled out. If the fault returns after these checks, switch the unit off and contact a qualified technician to avoid repeated stress on the appliance.

Tip: If you have other error codes or need guided support, use our free error code finder to locate and solve errors efficiently.