Vaillant F.73 Fault Code: Causes & What To Do

F.73 indicates a problem with the water pressure sensor or the system pressure itself. The boiler can't confirm there's enough water in the system to fire safely, so it locks out to protect itself. It's one of the more common Vaillant ecoTEC and atmoTEC fault codes we attend.

The most common causes, in order of how often we see them on call-outs:

• • Low system pressure — if pressure has dropped below roughly 0.6 bar, the sensor can't confirm water is present and the boiler locks out with F.73.
• • Faulty pressure sensor — the sensor can fail or drift out of calibration, reporting a reading the boiler doesn't trust even when pressure is normal.
• • Air in the system — trapped air in radiators or pipework causes erratic pressure readings at the sensor, triggering the fault intermittently.
• • Wiring or connector fault — a loose, corroded or damaged connection between the pressure sensor and the PCB interrupts the signal and throws F.73.

Repressurising the system to around 1.2 bar and resetting the boiler can clear F.73 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 the fault 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 almost certainly your F.73 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 you hear gurgling or they're cold at the top — trapped air causes erratic readings. Re-check pressure afterwards, as bleeding lowers it.
• • If pressure reads 1.0–1.5 bar and the F.73 persists, the fault is with the sensor or its wiring — not something to attempt yourself. Book a Gas Safe engineer.

The pressure sensor and its wiring sit inside the boiler casing. Replacing the 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.73 persists, stop and book a repair.

• • Confirm system pressure on the gauge and with a calibrated reading.
• • Compare the sensor's reported pressure against actual system pressure.
• • Inspect the wiring and connector between the sensor and the PCB.
• • Test the PCB input signal to confirm whether the fault is the sensor or the board.
• • Replace the pressure sensor if faulty, 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.