How to Fix a Vaillant F73 Fault Code

The Vaillant F73 fault code usually points to a problem with the boiler's water pressure sensor or the wiring connected to it. It commonly affects Vaillant ecoTEC Plus, aquaTEC and ecoMAX boilers.

When this fault appears, the boiler normally enters lockout mode to protect itself. That means no heating or hot water until the fault is resolved.

You can carry out a few basic checks yourself, but repairs involving electrical components or gas appliances should always be left to a Gas Safe registered engineer.

What Does the Vaillant F73 Fault Code Mean?

The F73 error means the boiler is receiving an invalid signal from the water pressure sensor.

The pressure sensor constantly reports system pressure to the printed circuit board (PCB). If the signal is missing, incorrect or interrupted, the boiler cannot confirm that it's safe to operate, so it shuts down.

If your boiler displays this fault repeatedly, professional diagnosis is normally required.

What Happens When the F73 Fault Appears?

Once the fault is detected, the boiler locks out. You may notice:

• No heating
• No hot water
• Cold radiators
• The F73 error displayed on the control panel

Leaving the fault unresolved can sometimes lead to further electrical damage if water is leaking onto internal wiring.

Common Causes of a Vaillant F73 Fault Code

Faulty water pressure sensor

The most common cause is a failed pressure sensor. Age, wear and internal failure can all prevent it from sending accurate readings.

Water-damaged wiring

Leaks inside the boiler can damage wiring between the pressure sensor and PCB. Corrosion or moisture often interrupts communication.

Loose electrical connections

Vibration and heat cycles can gradually loosen plugs or electrical terminals.

Low system pressure

Low pressure rarely causes F73 on its own, but ongoing pressure problems can contribute to sensor faults. Check the pressure gauge and top up the system if it's below the manufacturer's recommended level.

How to Troubleshoot a Vaillant F73 Fault

Reset the boiler

Reset the boiler once using the manufacturer's instructions. If the fault immediately returns, further investigation is needed.

Check the pressure gauge

Most Vaillant boilers should sit around 1.0–1.5 bar when cold. If pressure is low, repressurise the system carefully using the filling loop.

Look for visible leaks

Check beneath the boiler and around pipework for signs of water. Internal leaks often lead to damaged wiring and failed sensors.

Inspect accessible wiring

Look for obvious signs of corrosion or disconnected plugs, but don't dismantle the boiler casing or attempt electrical repairs yourself.

When Should You Call a Gas Safe Engineer?

If the fault returns after a reset or the pressure is correct, it's time to call an engineer. A professional can test:

• Water pressure sensor
• PCB communication
• Wiring continuity
• Internal water leaks
• Other components that may be contributing to the fault

For ongoing faults, book a boiler repair in London before further damage develops.

Safety Advice

Do not attempt repairs inside the boiler.

The F73 fault often involves electrical testing and internal components that should only be handled by a qualified Gas Safe engineer.

Never bypass safety devices or continue resetting the boiler repeatedly.

Preventing Future F73 Faults

Regular servicing helps identify pressure sensor issues, loose wiring and small leaks before they become expensive repairs.

Keep an eye on boiler pressure and arrange repairs promptly if you notice repeated pressure loss.

If you're seeing several different fault codes, our guide to Vaillant boiler fault codes explains what each one means and when professional repairs are needed.

Need a Vaillant Boiler Repair in London?

Serviceteam's Gas Safe engineers diagnose and repair Vaillant boiler faults across London.

We can identify whether the issue is a failed pressure sensor, damaged wiring or another underlying fault, then carry out the necessary repair to get your heating working safely again.

Frequently Asked Questions - (FAQs)

It means the boiler is receiving an invalid signal from the water pressure sensor.

Usually not. Simple pressure checks are safe, but repairs involving sensors or wiring require a Gas Safe engineer.

It depends on whether the sensor, wiring or another component needs replacing. A Gas Safe engineer can confirm the exact cost once the fault has been diagnosed.

No. The boiler normally enters lockout mode until the fault is repaired.

Many repairs can be completed within one to two hours once the fault has been diagnosed.