F.73 Error on Vaillant Boiler? Comprehensive Fix Guide

• Difficulty: Intermediate (Requires opening the front casing and basic electrical testing).
• Estimated Time: 30 to 60 minutes.
• Tools Needed: Phillips-head screwdriver (No. 2), Digital Multimeter (for testing continuity), and a dry cloth or paper towels.
• Estimated Cost: £0 (if it’s just a loose wire) to £45 (for a replacement sensor part).

The Vaillant Boiler Error F.73 indicates a specific “Water Pressure Sensor Signal Range Fault.” This occurs when the boiler’s internal computer (the PCB) receives an electrical signal from the pressure sensor that is outside of its expected operating range—typically signifying a short circuit to ground, an open circuit, or a complete sensor failure.

If you’re staring at your boiler right now, you might be dealing with lukewarm water that suddenly turns ice cold, or perhaps your radiators are staying stubbornly chilly while the unit makes frustrated clicking noises. It is definitely a nuisance, especially on a cold morning! But please, take a deep breath and don’t worry. This is one of the more common “logic” errors in the Vaillant range, and I’m going to walk you through exactly how to diagnose and potentially fix it yourself. You’ve got this!

How to Fix Vaillant Error F.73 (Step-by-Step)

Step 1: Safety First & Power Down
Before you even think about touching a screwdriver, you must turn off the power to the boiler. Find the fused spur (the switch on the wall, usually with a small fuse drawer) and flip it off. Safety Warning: Working inside a boiler involves proximity to high-voltage components. Always verify the power is off using your multimeter or a voltage tester pen before touching internal wires.

Step 2: Remove the Outer Casing
Most Vaillant Ecotec models have two screws at the very bottom of the unit. Unscrew these Phillips-head screws and put them somewhere safe (like a magnetic tray). Pull the bottom of the front panel toward you slightly and then lift upward to unhook it from the top. Set the panel aside carefully so it doesn’t get scratched.

Step 3: Locate the Water Pressure Sensor
The sensor is usually located on the left-hand side or toward the rear, tucked into the plastic hydraulic block. It is a small, cylindrical plastic component with a three-wire plug (typically red, black, and blue or white) clipped into the end of it. It looks a bit like a spark plug for a car, but made of plastic.

Step 4: The “Wiggle Test” and Visual Inspection
Check the connector. Is it pushed all the way in? Unplug the plastic harness by squeezing the tabs and pulling gently. Look inside the plug for green crusty bits (corrosion) or water. If it’s wet, dry it thoroughly with paper towels and a hairdryer on a cool setting. If a wire has snapped off the terminal, you will need to re-strip the wire and re-seat it, or replace the wiring harness.

Step 5: Testing with a Multimeter
If the wiring looks fine, set your multimeter to DC Voltage. With the power back on (be extremely careful here!), check the voltage between the ground and the power wire. You should see a steady 5V supply coming from the PCB. If you have 5V but the boiler still shows F.73, the sensor itself is almost certainly dead and sending back a “garbage” signal. Turn the power back off before proceeding.

Step 6: Replacing the Sensor (The Final Fix)
If you’ve determined the sensor is faulty, you’ll need a replacement (Part No. 0020059717 for most Ecotec models).

1. Drain the boiler: Close the isolation valves and drain the water from the internal circuit using the drain-off cock.
2. Remove the retaining clip: There is usually a metal “U” clip holding the sensor in place. Pull it out with pliers.
3. Swap the part: Pull the old sensor out (have a towel ready for a small splash of water) and push the new one in.
4. Re-secure: Slide the “U” clip back in, reconnect the wiring, and refill the boiler to 1.5 bar.

Step 7: Reset and Test
Close the casing, turn the power back on, and press the “Flame/Reset” button for five seconds. The error should clear, and you should hear the boiler begin its ignition sequence. Success!

Why is my Vaillant showing Error F.73?

To fix the problem, we first have to understand why it happened. Unlike an “F.22” (which usually just means you need to add more water), the F.73 is an electrical communication breakdown. Here are the primary culprits:

1. Internal Sensor Component Failure: Inside the pressure sensor is a tiny ceramic diaphragm that flexes as water pressure changes. Over years of constant heating and cooling, or due to “sludge” (magnetite) buildup in the system, this diaphragm can crack or seize. When the physical part fails, the electrical signal it sends back to the PCB becomes nonsensical, triggering the F.73 code.

2. Wiring Harness Issues and Vibration: Boilers are mechanical beasts; they vibrate when the pump and fan run. Over time, these micro-vibrations can cause the wiring harness connected to the sensor to wiggle loose. Alternatively, if a wire was pinched during a previous service, it might have finally frayed through, creating an “open circuit” where the signal simply cannot travel.

3. Moisture and Corrosion: Even a tiny, microscopic leak from a nearby auto-air vent or a pump seal can drip water onto the electrical connector of the sensor. Water and electricity don’t mix! This moisture creates a “short circuit to ground,” confusing the boiler’s brain into thinking the pressure is at a level that is physically impossible.

4. PCB Logic Error: While rarer, sometimes the Printed Circuit Board (the “brain”) suffers a minor stroke due to a power surge or aging capacitors. If the PCB can no longer interpret the 0-5V signal coming from the sensor, it defaults to the F.73 error as a safety precaution.

Symptoms

Before we dive into the guts of the machine, let’s make sure we are looking at the right problem. If your Vaillant is suffering from an F.73 fault, you will likely notice the following behaviors:

• The Digital Display: The screen will alternate between the current temperature and a flashing “F.73” code.
• Lockout Mode: For safety reasons, the boiler will likely enter a “lockout” state, meaning it will refuse to ignite the burner to prevent damage.
• Inconsistent Pressure Readings: If the display does show pressure, it might jump wildly from 0.0 bar to 3.0 bar in seconds, or stay stuck at 0.0 despite the system being full.
• The “S.27” Status: If you check the status codes, you might see S.27, which indicates the pump is running but the sensor isn’t detecting a pressure change.
• Lukewarm Water: If the sensor is failing intermittently, you might get brief bursts of heat followed by a total shutdown.

How to Prevent Error F.73

Once you’ve got your hot water back, you definitely don’t want to see F.73 ever again. Here is how to keep your system healthy:

• Install a Magnetic System Filter: Most sensor failures are caused by “black sludge” (iron oxide) clogging the sensor. A magnetic filter (like a MagnaClean) traps this debris before it reaches your delicate boiler components.
• Annual Inhibitor Checks: Every year, ensure your heating system has the correct level of chemical inhibitor. This liquid prevents the internal pipes from rusting and creating the grit that destroys sensors.
• Surge Protection: If your area is prone to lightning or power flickers, consider a surge protector for your boiler’s power supply. This protects the sensitive PCB from sending or receiving faulty voltage signals.

Frequently Asked Questions

Q: Can I still use my heating if I have an F.73 error?
A: Generally, no. The F.73 error is a safety lockout. Because the boiler cannot accurately “see” how much water is inside, it refuses to fire the burner. This prevents the heat exchanger from melting or exploding due to dry-firing. You must resolve the error before the heat will return.

Q: Is F.73 the same as F.22?
A: No. F.22 means the pressure is simply low (you just need to top up the water). F.73 means the sensor itself or the wiring is broken. Topping up the water will not fix an F.73 error because the boiler can’t read the signal regardless of how much water is in it.

Q: Do I need a Gas Safe engineer to fix this?
A: Legally, in many regions, you only need a Gas Safe engineer to work on the “gas side” of the boiler (the burner, gas valve, etc.). Since the pressure sensor is on the “water side,” a competent DIYer can often replace it. However, if you are uncomfortable opening the casing or working with a multimeter, always call a professional to ensure the job is done safely.