The models that this fault code is applicable to, are the Vaillant Ecotec Pro/Plus, Vaillant Ecotec Exclusive and Vaillant Ecofit Pure.
According to the Vaillant Installation and Service book, the F75 fault code means ‘Fault, no pressure change detection when starting-pump’ . The cause of it is ‘Water pressure sensor and/or pump defective, air in heating installation, too low water pressure boiler; check adjustable bypass, connect external expansion vessel to return’
The number one cause of this fault is some sort of blockage or sludge has has cause one of the mentioned components to break down or get blocked. When diagnosing which component has failed, you want to check the water quality of the system and see the presence of sludge. If there is sludge, this should be cleaned out or system powerflushed before changing components.
Overview: The pressure sensor has to read a pressure increase when the the pump is started. You will hear a humming sound once the pump starts. If it not audible then removing the case and using a long screwdriver to act as a sound transmitter from the pump will allow you to hear if it is operational.
1. Try resetting the boiler to see if it is simply a random occurrence. Following is the reset procedure:
Press reset button upto 3 times just in case it is a one off occurrence, and keep an eye on future occurrences.
2. The first lowest price component that throws this error is the pressure sensor. In some models you may find the older orange pressure sensor. This has been superceded by a black redesigned pressure sensor. Approx cost is £30. Whilst changing this sensor make sure the finger mesh is also clean in hydro block. In the Vaillant Ecotecs with rubber hoses, sludge formation is much higher, replace these with the new solid pipe versions. When the pump is active, you are looking for an increase of 0.3 volts on connecter X2 Black and Green, from approx 1.6 volts to 1.9 volts.
Update: Please note, you can sometimes simply remove the sensor, unclog it by tapping on a tissue or using a descaling / desludging chemical. This is more than likely a temporary solution. Also the F75 Upgrade Kit moves this sensor on the pipe from the pump to the heat exchanger, therefore minimising the chance of debris from the heat exchanger entering the sensor like it does in the older designs with the sensors lower down on the pipe flowing out of the main heat exchanger.
Old type and new type pressure sensors.3. Check that the pump is working and spinning. You can use a magnetic spin guage to do a crude test. For more accuracy on the pumps with the screw you can check that the shaft is spinning. If it is not, using a flat screwdriver give it a little spin. Please note the water may be hot that is expelled from the pump.
4. Check to see if your boiler has rubber hoses, that they are not blocked up. These can be replaced with solid copper ones as well as using the F75 upgrade kit.
5. Check to see if power is being supplied to the pump, if not there could be a possible PCB issue. If no power is being supplied to the pump, you can also test the functioning of the pump by running a temporary permanent live connection to the pump.
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Statistical Chances of faults
There is a 60% chance of the Pressure sensor being at fault.
There is a ~28% chance of the Pump being at fault.
Other causes amount to ~12% chances of being at fault.
F75 Additional Info:
Expansion vessel: Check to see the pressure is topped up whilst working as this can sometimes throw up the F75 error in conjunction with other components.Heat Exchanger: Make sure the heat exchanger pathways are clean from sludge and debris. Flushing this directly with a machine and chemicals using specially adapted connections is the most effective way.
There is also a option to install the F75 kit that is available from some merchants.
As sludge is usually the biggest cause for the F75 fault code, the most important thing you can do is to make sure that the system is free from contaminants ie flushed. Making sure the inhibitor levels are topped up to the correct levels will minimise the formation of sludge. Having dirt or magnetic filters will also prevent sludge from reaching the boiler and it’s components.
Case 1. – Vaillant Ecotec Plus 637
Boiler showing F75. Resetting the boiler made it work a few times. Pressure sensor checked, was found to have debris and partially blocked. Pressure sensor was replaced. Expansion vessel checked and recharged. Other basic checks and maintenance tasks carried out. Boiler continued working without any more F75 errors.
Case 2. – Vaillant Ecotec Plus 831
Boiler showing F75. Resetting boiler has no effect. Pressure sensor was clear. Pump tested using non contact tester. Power found to be going to pump, pump failing to start. Pump was replaced with the newer Grundfos version, the rubber hoses were replaced with solid F75 upgrade kit and solid connection set. Other checks were carried out. Boiler was left in working order.
Case 3. – Vaillant Ecotec Plus 831.
Boiler showing F75. Pressure sensor was previously replaced a few months prior. No rise in pressure when there is a heating or hot water demand.
We checked the pressure sensor to make sure it was still clean. Put the sensor back. Next step was to check if there was a voltage going to the pump. We checked this using a non contact voltage tester. This confirmed that the pump was receiving signal,however the pump was not working. It was an old type Wilo. In this situation, a reconditioned wilo pump was used as upgrading requires more time and more cost. The customer was not prepared to pay the extra expense of upgrading the pump and replacing the rubber hoses.
Vaillant Sensor (Water Pressure) 0020059717
Vaillant Ecotec Exclusive 832 – VUW 326/4-7 – Fan ASSISTED
Vaillant Ecotec Exclusive 838 – VUW 386/4-7 – Fan ASSISTED
Vaillant Ecotec Plus
612 – VU 126/3-5 – 2005-2005
612 – VU 126/3-5 R1 – 2005-2006
615 – VU 156/3-5 – 2005-2005
615 – VU 156/3-5 R1 – 2005-2006
618 – VU 186/3-5 – 2005-2005
618 – VU 186/3-5 R1 – 2005-2006
624 – VU 246/3-5 – 2005-2006
624 – VU 246/3-5 R1 – 2005-2006
630 – VU 306/3-5 – 2005-2005
630 – VU 306/3-5 R1 – 2005-2006
637 – VU 376/3-5 R1 – 2006-2006
824 – VUW 246/3-5 – 2005-2005
824 – VUW 246/3-5 R1 – 2005-2006
831 – VUW 316/3-5 – 2005-2005
831 – VUW 316/3-5 R1 – 2005-2006
837 – VUW 376/3-5 – 2005-2005
837 – VUW 376/3-5 R1 – 2005-2006
Vaillant Printed Circuit Board (PCB) 0020132764
Vaillant Ecotec Pro
24 (up to 2012)
28 (up to 2012)
Vaillant Ecotec Plus
824 (up to 2012)
831 (up to 2012)
837 (up to 2012)
937 (up to 2012)
Vaillant Turbomax Plus Printed Circuit Board (PCB) 0020034604
Vaillant TurboMax Pro
Vaillant TurboMax Plus 824
Vaillant TurboMax Pro 828
Vaillant TurboMax Plus 837
Vaillant EcoTec Plus PCB 0020036861
Vaillant EcoTec Plus
F75 Please note,a qualified gas engineer is required to work on gas appliances, however basic task such as pressing the reset button and refilling the system, can be carried out by home owners or tenants. If you are looking for an engineer to carry out a repair on your boiler, feel free to use the Vaillant Pro finder section on our website. Remember Vaillant Pros can diagnose F75 problems quickly and efficiently.