Vaillant F.22 Fault Code: Causes & What To Do

F.22 indicates a fault with the flow/return temperature sensor, or a shortage of water in the system. The boiler can't confirm the system is safely filled and reading temperature correctly, so it locks out to protect itself.

The most common causes, in order of how often we see them:

• • Low system pressure — if pressure has dropped below roughly 0.6 bar, the boiler may read it as a water shortage and lock out with F.22.
• • Faulty flow or return NTC sensor — the temperature sensor can fail or drift, reporting a reading the boiler doesn't trust even when the system is full.
• • Air in the system — trapped air in radiators or pipework causes erratic temperature and pressure readings, triggering F.22 intermittently.
• • Wiring or connector fault — a loose, corroded or damaged connection between the sensor and the PCB interrupts the signal and throws F.22.

Repressurising the system to around 1.2 bar and resetting the boiler can clear F.22 if low pressure was the cause. If the pressure gauge already reads 1.0–1.5 bar and the fault persists, a reset won't help — the sensor or its wiring needs replacing. Don't reset repeatedly; if it returns, book an engineer.

• • Check the pressure gauge — a healthy system sits between 1.0 and 1.5 bar when cold. If it's below 0.6 bar, low pressure is likely your F.22 trigger.
• • Repressurise via the filling loop to around 1.2 bar, then reset the boiler. If the fault clears and pressure holds, the fix may be complete — but watch for it dropping again.
• • Bleed radiators if they're cold at the top or you hear gurgling — trapped air causes erratic readings. Re-check pressure afterwards, as bleeding lowers it.
• • If pressure reads 1.0–1.5 bar and F.22 persists, the fault is with the sensor or its wiring — not something to attempt yourself. Book a Gas Safe engineer.

The flow/return sensors and their wiring sit inside the boiler casing. Replacing a sensor, testing the PCB signal, or any work inside the casing is gas/electrical work that must be done by a Gas Safe Registered engineer. If pressure is normal and F.22 persists, stop and book a repair.

• • Confirm system pressure on the gauge and with a calibrated reading.
• • Compare the flow and return sensor readings against actual system temperature.
• • Inspect the wiring and connectors between the sensors and the PCB.
• • Test the PCB input signal to confirm whether the fault is a sensor or the board.
• • Replace the faulty sensor using a manufacturer-approved part.
• • Re-commission the boiler and verify safe combustion after the repair.

Our Gas Safe Registered (594547) engineers repair every Vaillant model — ecoTEC, ecoFIT, atmoTEC — across Surrey, Berkshire, Buckinghamshire and West London, and carry common sensors and ignition parts on the van. Most Vaillant fault-code repairs are completed in a single visit, with any part quoted and agreed before fitting.