Vaillant Boiler F73 & F74 Fault Code


The Vaillant fault code F73 & F74 boiler Low Pressure Water Sensor problem Q & A or require an engineer for an emergency boiler repair.


The Vaillant Boiler Fault Code F73 & F74 Error Common Q&A


Vaillant Boiler F73 & F74 Fault Code

Vaillant Boiler F73 & F74 Fault Code Error

  • What does it mean when there’s a F73 or F74 fault code displayed on my boiler?
  • Why are there two different error codes for the same fault?
  • What is a low pressure water sensor?
  • What will happen if the low pressure water sensor is faulty on my Vaillant boiler?
  • What causes a low pressure water sensor fault?
  • Will I need to replace my boiler if the F73 or F74 fault codes are displayed?
  • Can I fix the low pressure water sensor on a DIY basis?
  • What other fault codes are likely to be displayed with the F73 & F74 errors?
  • Can the engineer confirm the low water pressure sensor is causing the F73 & F74 error codes?
  • Will the engineer fix or replace the low pressure water sensor to clear the F73 & F74 fault codes?
  • How much will a new low water pressure sensor cost, and how long will it take to fit?
  • How will the boiler engineer know if it’s wiring that’s causing the fault?
  • If water damage is the issue, how will we find the cause?
  • What can be done to fix leaks to clear the F73 & F74 fault codes on my Vaillant boiler?

What does it mean when there’s a F73 or F74 fault code displayed on my boiler


According to Vaillant, the F73 and F74 boiler fault codes relate to a low-pressure water sensor fault.

The problem is the same for both fault codes.

Why are there two different error codes for the same fault


Over the years, boilers will be updated. Alongside this, new models will be released.

The fault code list for different Vaillant boilers will vary, as the company constantly improve their database of error codes.

So, depending on your boiler’s list of fault codes, it will display either the F73 or the F74 fault code

What is a low pressure water sensor


Each boiler is fitted with various gauges and sensors. These devices help to monitor the boiler’s air pressure, water pressure and even the temperature of the water.

When a sensor recognises a boiler working outside of recommended pressures, it will lock out and display a fault code.

In this case, the water pressure sensor looks for a heating system that drops below a certain pressure; usually around 0.6 bar.

This signals there’s a leak in the system. And, to avoid the boiler operating as a dry fire, it will lock out.

But, it’s not just a safety precaution to protect internal boiler parts.

A leak means property damage. And a heating system that is running produces more pressure, will release more water via the leak, and create more property damage.

So, the lockout protects your boiler, and your property.

What will happen if the low pressure water sensor is faulty on my Vaillant boiler


So, we know the low pressure water sensor is designed to stop the boiler from working, given a low pressure scenario.

A faulty pressure sensor can give off intermittent readings. If you’re lucky, this might mean that the PCB (printed circuit board) thinks only on occasion, water pressure is lower than it should be.

This could lead to a boiler temporarily locking out, and needing to be reset.

However, pressure sensors that have a permanent fault, are likely to lockout the boiler and even a reset isn’t going to be able to fix the problem.

Your boiler is being given a signal to show pressure is too low, even when it isn’t.

What causes a low pressure water sensor fault


The problem will be due to:

  • A loose, broken or damaged connection between the sensor and the PCB
  • A connection that has water damage somewhere between the sensor and main PCB
  • A faulty low water pressure sensor

Will I need to replace my boiler if the F73 or F74 fault codes are displayed


Typically, this is classed a minor fault. The problem is limited to basic wiring faults, and a faulty low water pressure sensor.

If there are no other fault codes implying a multitude of problems, the fix is usually cheap and simple. And therefore, there’s no need to replace the boiler.

Can I fix the low pressure water sensor on a DIY basis


All boilers should be worked on by a Gas Safe engineer. This isn’t just for safety reasons. Misdiagnosing boiler parts can be expensive.

The real problem here arises when more than one fault code is displayed. This makes diagnosing and fixing the fault a lot more complex.

What other fault codes are likely to be displayed with the F73 & F74 errors


One of the most common fault codes we see alongside the F73 and F74 faults, is the F75.

But, this isn’t as specific. It can be a circulation based fault, or a pressure sensing fault.

This makes the fault finding process a little more difficult.

Can the engineer confirm the low water pressure sensor is causing the F73 & F74 error codes


The low water pressure sensor can be tested, given it is an electrical component.

However, your Vaillant boiler’s internal fault finding system has determined that this is the fault.

Will the engineer fix or replace the low pressure water sensor to clear the F73 & F74 fault codes


The main cost of boiler repairs, is labour. Many parts are inexpensive to replace. But, dismantling the boiler and checking parts for faults is what takes the time.

After spending cash on labour, it makes sense to replace the pressure sensor with a new unit.

How much will a new low water pressure sensor cost, and how long will it take to fit


The cost of a new low water pressure sensor could be as little as £50, but that’s for parts only.

Expect to pay a boiler engineer 1-2 hours for a diagnostic.

And, a further 1-2 hours to replace the faulty pressure sensor with a new one.

How will the boiler engineer know if it’s wiring that’s causing the fault


Low water pressure sensors as a unit, don’t come with new wiring.

So, your engineer needs to be certain that the issue is the sensor, not the wiring.

On older boilers, a visual inspection will highlight damaged or broken wiring. It will also help to highlight any loose connections.

To back up their findings, they can test wiring using a multi-meter.

These loose connections will need to be secured. And, any damaged connections or wires will need to be replaced.

If water damage is the issue, how will we find the cause


Boiler leaks are the most common cause of damage to electrical components on boilers.

Once the gas engineer has removed the boiler casing, the leak will become obvious. In fact, you might have noticed drips of water beneath your boiler.

Boiler leaks will usually come from a broken heat exchanger, or from a pump where the seals have blown.

Many electrical units are sealed. But if they’ve degraded over time, or they’ve been dripped onto directly, this is enough for water to enter the unit and cause damage.

On older boilers, the electrical units are not as well sealed.

Small leaks that condense (due to internal boiler heat) can settle on components and cause damage, leading to intermittent signals to and from the low water pressure sensor.

What can be done to fix leaks to clear the F73 & F74 fault codes on my Vaillant boiler


The first step is to isolate and fix all boiler leaks. As an example, if the pump’s seals have blown, they can be replaced. A fully functioning pump (minus the seals) can be repaired, rather than replaced.

The next step is to find the problems the leaks have caused. That means inspecting all wiring and electrical components.

And, this isn’t limited to just to low water pressure sensor and the wiring to/from it. Water and condensation can find its way into other important boiler parts, such as the PCB.

Checking these components and repairing any damage will help to preserve the life of these parts.

Other Vaillant Fault Codes And Error


If your looking for the list of error codes relating to the Vaillant boiler then please read the article Vaillant Boiler Fault Codes And Cures