Vaillant ecotec 937 PLUS F81-F91 fault code

vaillant f81-f91 fault code explained and how to fix

The Vaillant F81 and F91 fault usually happen together and typically presents itself when you have an issue with the appliance domestic hot water pump and/or a blocked plate heat exchanger.

If you happen to have a Vaillant ecoTEC Plus 937 combination boiler, there is a very good chance you may experience the F81 and F91 fault codes at some point. It is a common problem and the secondary hot water pumps seem prone to failure.

The good news is that this problem can usually be repaired economically and you probably do not need a new boiler! Repairs for this problem is a service i provide.

The F81 & F91 faults explained

These two fault codes usually present themselves together, they are only specific to the Vaillant ecoTEC 937 combination boilers. They are usually noticed after a lack of hot water.

These faults usually relate to a faulty domestic hot water pump, or a blocked plate heat exchanger which is the heat exchanger that delivers the hot water. The ecoTEC 937 is a conventional combination boiler for the most part. Where it differs slightly from the other standard conventional combination boilers is that it has several extra components which include an unvented cylinder a second pump and an expansion vessel for the hot water side. This boiler is designed to give higher flow rates than a conventional combination boiler for a period of time.

The F81/F91 codes are only related to the hot water. If you are having problems with central heating as well then this should have its own fault code and its own cause usually.

On this particular model the first pump works in the usual way as the standard combi’s, one does the central heating and circulation on the dirty side of the plate heat exchanger. The second additional pump circulates water between the plate heat exchanger and the unvented cylinder located on the back of the boiler.

Vaillant ecoTEC 937 actoSTOR pump location underneath the boiler

If you look up the F81 fault code in the Vaillant installation manual it says;

“actoSTOR load pump defective; only in combination with F.91” then lists a number of possible causes;

– Cylinder is not fully charged after specified time.
– Check cylinder charging sensor and cylinder sensor
– Air in the actoSTOR pump
– Check the cable harness to the pump
– Check the impeller sensor and/or limiter in the boiler – Diverter valve defective
– Secondary heat exchanger blocked
– Pump defective

The F91 fault coe just says;

“Sensor/actuator fault on actoSTOR module”

The problem

9 times out of 10, the domestic hot water pump or “actoSTOR” pump as Vaillant likes to refer to it as has failed. Sometimes blockages in the plate heat exchanger(secondary heat exchanger) can cause problems.

Having issues with the plate heat exchanger should be welcomed as good news as this is usually cheaper and easier to deal with.

Unfortunately replacing these pumps is usually a bit difficult, as it is located in an awkward place on the boiler in the back right hand corner underneath the appliance. These pumps are of a special design and are not cheap as they retail for well over £200. In cases where the installation guide has not been followed, the pumps can be potentially impossible to replace without removing the boiler off the wall as it is impossible to get to it.

vaillant-ecotec-printed circuit board
Vaillant ecoTEC 937 secondary printed circuit board

The solution

Most of the time the actoSTOR secondary pump will be the problem. However making assumptions is never a good idea and one can end up wasting their time and money guessing. Once I have checked things over and ruled out the plate heat exchanger etc I will locate the secondary primary printed circuit board which supplies power(240 volts) to the actoSTOR pump.

There is a second additional smaller printed circuit board(0020111282) which is located behind a little metal plate heat exchanger at the top right-hand corner of the boiler near the domestic hot water expansion vessel. Once you are satisfied that you have power going to the pump from the main pcb, you need to check that there is power coming from the smaller pcb to the pump. This is a low voltage and is essentially what tells the pump to run.

If there is no power being supplied from one of the pcb’s then it is likely that the pump is not the issue and one of the circuit boards are maybe at fault. For the time it takes to check for power from these to pcb’s it is not worth the risk of not checking and making the assumption that the pump is the problem. Having said that I have yet to repair the F81/F91 fault where a circuit board was at fault but it is always worth checking.

Given the cost of the actoSTOR pump, the time and hassle it takes to replace it, and the fact that one could be stuck with a £200 part that is not needed, it makes sense to check things over properly and identify exactly what parts are needed. There is nothing more painful for an “engineer” than taking the time to fit an expensive part to a boiler which does not fix the problem!

Vaillant ecoTEC 937 actoSTOR hot water pump

The cost to repair

If the secondary pump is required then this typically retails at well over £200, although it can be purchased for around the £200 mark online. Whenever I get called to fix this problem on the 937 it is usually not just a case of replacing the faulty pump.

To diagnose the fault diligently it takes a bit of time. On top of that 9 times out of 10 there are other issues with the boiler. Poor maintenance means both expansion vessels are usually flat and require recharging at the very least. Other components like the pressure sensor(see F75 fault) are often at the point where they require replacing.

Replacing the actoSTOR pump is time consuming. The unvented cylinder needs to be drained completely which takes time. Acces to the pump is not easy anyway and a difficult installation can add time to the job.

I would say on average this repair usually costs between £330 – £350 if the pump is the problem.

As an end user experiencing these faults, your only option is to call someone competent and Gas Safe registered to fix the problem. Replacing parts on boilers based on guesswork is almost always an expensive endeavour for someone, either the “engineer” or the customer so it is best to try to and find someone who understands this problem and has experience fixing it.

This is not the typical problem I repair on a daily basis but I do have a fair bit of experience fixing this fault. If I live in your locality then I am sure I can help with this problem.

Richard contact numberemail address

Areas where I can repair your Vaillant ecotec Plus 837 F.81, F.91 fault

areas that are covered

I am located in Elsecar, Barnsley, South Yorkshire. I cover a large part of South Yorkshire including most of Sheffield, Rotherham and Barnsley. I also cover a small part of Doncaster.


I cover most of Sheffield including central Sheffield, Chapeltown, Ecclesfield, Grenoside, High Green, Oughtibridge, Thurgoland, Wortley, Aston, Aughton, Swallownest, Todwick, Hillsborough and Stannington.


I cover the whole area of Rotherham including central Rotherham, Brinsworth, Catcliffe, Canklow, Masbrough, Treeton, Whiston, Greasbrough, Kimberworth, Thorpe Hesley, Rawmarsh, Wentworth, Bolton-on-Dearne, Goldthorpe, Thurnscoe, Wath-on-Dearne, Mexborough, Swinton, Kilnhurst, Hooton Roberts, Ravenfield, Thrybergh, Bramley, Hellaby, Maltby, Thurcroft, Wickersley and Braithwell.


I cover most of Barnsley including central Barnsley, Birdwell, Stairfoot, Worsborough, Kendray, Ardsley, Athersley, Monk Bretton, Lundwood, Royston, Brierley, Cudworth, Cortonwood, Great Houghton, Shafton, South Hiendley, Brampton, Darfield, Wombwell, Hemingfield, Elsecar, Hoyland, Jump, Barugh Green, Cawthorne, Darton, Dodworth, Gawber, Mapplewell, Pogmoor, Staincross, Wilthorpe, Worsbrough, Thurnscoe and Tankersley.


I cover a small part of Doncaster including central Adwick, Conisborough, Edlington, Warmsworth and Mexborough.