Vaillant Boiler F85 Fault Code
The Vaillant fault code F85 boiler temperature sensor fault problem Q & A or require an engineer for an emergency boiler repair.
The Vaillant Boiler Fault Code F85 Error Common Q&A
Vaillant Boiler F85 Fault Code
- What does the F85 fault code mean on my Vaillant boiler?
- What causes the F85 fault code to be displayed on my boiler?
- How do the NTC thermistors work in my Vaillant boiler?
- What happens if the NTC thermistors are faulty, or they are incorrectly fitted?
- Is an NTC thermistor expensive to replace, and is it better to buy a new boiler?
- Can I fix the F85 fault without special tools, or a boiler engineer?
- How will the engineer know if the NTC thermistor is at fault?
- Is it hard to determine whether the NTC thermistors have been incorrectly installed?
- What happens if the NTC thermistors are fitted on the wrong pipes?
- Are loose and damaged connections easy to fix, or will parts need to be replaced?
- What else could be causing the F85 fault code to be displayed?
- Is there a way to determine if the PCB is at fault on a DIY basis?
What does the F85 fault code mean on my Vaillant boiler
According to Vaillant, the F85 fault code relates to a temperature sensor fault.
Each boiler is fitted with two temperature sensors. These are called NTC thermistors.
What causes the F85 fault code to be displayed on my boiler
The fault lies with the:
- NTC thermistors (faulty part)
- Incorrect installation
- Loose connections to or from the PCB
- Damaged wiring
How do the NTC thermistors work in my Vaillant boiler
NTC thermistors are temperature sensors that give off an electrical resistance reading.
When tested at 20C they should produce a reading of 12k ohms. The PCB (circuit board in your boiler) receives the signal in ohms, and converts it to a temperature reading.
There are two thermistors. One on the flow side of your boiler (where hot water leaves) and one on the return (where water returns to be reheated).
These resistance readings tell the PCB what to do next, for instance:
- When temperature isn’t at its desired level coming from the heat exchanger (and the flow), it asks the gas valve to provide more fuel to the burner, to heat water to a higher temperature
- If the difference between the flow and return temperature is too high, the boiler locks out as it’s a sign as a possible blockage
So, if there’s a problem with the thermistors or their installation, your Vaillant boiler is going to work intermittently.
What happens if the NTC thermistors are faulty, or they are incorrectly fitted
The resistance readings (which are then converted into temperature readings) tells the PCB what to do next.
So, if the readings are at or above the temperature demanded by the thermostat, it shuts off the boiler. If they’re below the required temperature, it turns the boiler on.
When there’s an incorrect installation, it’s likely you’ll encounter problems such as:
- Boiler locks out immediately as there’s a permanent difference in temperature between the flow and return
- Boiler cycles on and off randomly, as the signal between the PCB and thermistors is weak
- Radiators and hot water don’t get up to temperature, even though there’s demand from the thermostat
A boiler that works at randomly is likely to have an electrical issue.
Is an NTC thermistor expensive to replace, and is it better to buy a new boiler
Replacing both NTC thermistors on your Vaillant boiler including labour, should cost £150-200. That’s going to include a diagnostic, replacement parts and installation.
And even for old boilers, it’s cost effective to repair rather than replace the boiler.
However, you should always have your boiler engineer diagnose the NTC thermistor fault, but check for other faults too. It’s a common occurrence that when one fault occurs, it’s coupled with (or followed by) another.
If this additional fault was with an expensive part such as the heat exchanger, then a replacement boiler starts to become a more cost-effective option.
The heat exchanger could cost upwards of £350 to replace. Coupled with the NTC thermistors replacement, that’s a lot of money to invest in an old, inefficient and deteriorating boiler.
Can I fix the F85 fault without special tools, or a boiler engineer
You shouldn’t work on a gas appliance without Gas Safe accreditation, even if you have the tools.
The main reason is safety.
Also, diagnosing the incorrect fault will lead to an expensive repair bill, but you still being left with an unreliable boiler.
Always call a Gas Safe engineer for assistance.
How will the engineer know if the NTC thermistor is at fault
They can manually check readings for both temperature and resistance.
Your Vaillant boiler should be providing a 12k ohms reading at 20C. If it’s not, there’s either a problem with the thermistor, or the wiring.
The engineer will need to use a multi meter to test all sections of the wiring, wired connections and each of the two NTC thermistors independently. Just because one thermistor is faulty, doesn’t mean the other is.
Is it hard to determine whether the NTC thermistors have been incorrectly installed
Although Vaillant list one possible cause for the F85 fault as incorrect installation, this is incredibly uncommon.
For a start, it’s unlikely the boiler would work. And that means whoever fitted them didn’t test and sign off the boiler correctly.
Most likely, it’s a manufacturing default where the NTC thermistors have been fitted on:
- The same side
- The wrong sides
If they’ve both been fitted on the same side, this is extremely obvious.
What happens if the NTC thermistors are fitted on the wrong pipes
The PCB will recognise readings from the flow thermistor, and recognise them from the return thermistor. What it won’t recognise is whether the thermistors are fitted on the correct side of the boiler.
So, if the return thermistor is fitted on the flow side of the boiler, the PCB still reads the resistance reading as being the return (i.e. it reads the reading based on the thermistor, not the side it’s fitted on).
The return should always be colder than the flow. But, as the return thermistor is fitted on the flow side, the PCB is receiving a hotter reading than it should for that thermistor. It sees this as a fault, locks out and displays a fault code.
Are loose and damaged connections easy to fix, or will parts need to be replaced
Loose and damaged connections are common on older boilers. Typically, they’ve either been subject to water damage, or the vibrations from boiler parts have helped to work them loose.
Most loose and damaged connections are obvious to spot. But to be certain there are no more faults, a boiler engineer will test each connection and section of wiring with a multi meter.
Unfortunately, new NTC thermistors don’t usually come with new sections of wiring. So, if it’s the wiring causing a communication issue with the PCB, this will need to be purchased separately.
What else could be causing the F85 fault code to be displayed
When there’s an electrical fault, the PCB can never be ruled out without being tested first.
The PCB gives and receives signals to every electrical part in your boiler. And without it, your boiler wouldn’t work.
The fault code systems in older boilers aren’t particularly complex. So, there might not be a fault code for the specific issue with the PCB.
Make sure your engineer checks your PCB before replacing any parts.
Not just to ensure the F85 fault code is fully cleared, but to check for damage. If the PCB is broken, a replacement could be £400 or more.
Is there a way to determine if the PCB is at fault on a DIY basis
Electrical faults are always hard to diagnose. But, if your PCB is operating intermittently and causing problems it’s likely you’ll have noticed:
- Water leaking from the boiler – this can drip onto electrical components and cause damage
- The LED display flickers on and off
- The boiler cycling on and off
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