Vokera Boiler A08 Fault Code


The Vokera fault code A08 boiler NTC thermistor on return Q & A or require an engineer for an emergency boiler repair.


The Vokera Boiler Fault Code A08 Error Common Q&A


Just below, we have a list of common questions relating to the A08 fault code on the Vokera boiler.

Vokera Boiler A08 Fault Code Error

  • What does the Vokera boiler A08 fault code mean?
  • What is the return NTC thermistor on my boiler?
  • What can cause a fault with the return NTC thermistor, and the A08 fault code to be displayed?
  • When I reset my boiler it works for a brief period, can I keep doing this?
  • How will the boiler engineer know if the NTC thermistor is faulty?
  • If the NTC thermistor is faulty, how much should it cost to fix?
  • What can cause damaged or loose wiring, and the A08 fault code to be displayed on my boiler?
  • Can the wiring and connection based faults be found easily by a Gas Safe engineer?
  • How can the pump cause problems that lead to the A08 fault code being displayed?
  • What can be done to fix a clogged heating pump?
  • If I need a new pump, how much will it cost?
  • What has the PCB got to do with the NTC thermistor, and the A08 fault code?
  • How much will it cost to replace the PCB to fix the A08 fault code on my Vokera boiler?

What does the Vokera boiler A08 fault code mean?


When a boiler’s fault finding system finds a problem it will display a fault code on the display panel.

In this case, it’s the A08 which is related to the NTC thermistor on the return side of the boiler.

The fault lies with the temperature being too hot or cold, creating an incorrect temperature differential between the flow and return thermistors.


What is the return NTC thermistor on my boiler?


The NTC return thermistor monitors the temperature of the water in the heating system that’s coming back to be re-heated.

The boiler heats water, pumps it and it passes the flow NTC thermistor.

It then travels around the central heating system, cools down and comes back via the return pipe (and passes through the return NTC thermistor).

The thermistors on boilers monitor temperature.

The readings come in the form of resistance. And, these resistance readings are passed to the printed circuit board (PCB) which then converts them into temperature.


What can cause a fault with the return NTC thermistor, and the A08 fault code to be displayed?


There are several reasons that the temperature reading could be incorrect, including:

  • Faulty NTC thermistor
  • Damaged or loose wiring
  • Lack of circulation from the pump
  • PCB fault

When I reset my boiler it works for a brief period, can I keep doing this?


It’s extremely dangerous to be working on a gas appliance if you’re not qualified. In fact, in many scenarios it’s illegal.

A boiler locks out because there’s a threat to either the people’s safety in a property, or the boiler’s components (so, a lock out prevents damage).

A reset is designed to be used to clear the fault code once the problem has been fixed, but not before.


How will the boiler engineer know if the NTC thermistor is faulty?


As the NTC thermistors on boilers provide readings in ohms, the unit can be tested using a multi-meter.

Typically, boilers will read 12Kohms @ 20C. However, the boiler engineer will need to check the manual for your specific Vokera boiler to ensure that’s the case.

If the NTC thermistor is not showing this reading, then it’s likely faulty.

By offering the wrong reading to the PCB, it’s suggesting a higher/lower temperature than it should be.

And, that means that the boiler is going to lock out and show faults codes, even when the operation of the boiler is 100% safe.


If the NTC thermistor is faulty, how much should it cost to fix?


Assuming the damage is limited to the NTC thermistor, the cost of this repair is small in boiler repair terms.

Parts can be picked up for around £20, and a few hours labour should see the job completed by a competent engineer. So, the issue could be solved for as little as £100.

Once the new NTC thermistor has been fitted, the engineer will be able to reset the boiler to clear the A08 fault code on your Vokera boiler.


What can cause damage or loose wiring, and the A08 fault code to be displayed on my boiler?


Boilers (especially old ones) can vibrate a lot. Over years, this can work sections of wiring and connections loose.

If this happens, the communication between the NTC thermistor on the flow and the PCB can be interrupted. If this is the case, it’s going to start working sporadically.

But, problems with wiring and connections aren’t limited to them just being loose. We find that issues can also occur thanks to water damage.

This water comes from small leaks from components such as the heat exchanger and the pump. Any section of wiring (or any connections) that are subject to water damage and causing problems will need to be replaced.


Can the wiring and connection based faults be found easily by a Gas Safe engineer?


The first thing a boiler engineer is going to look for, are the visual signs of problems.

This includes the likes of burnt out connections that have likely met water, and connections/wiring that look old and frayed.

If connections/wiring can be re-secured, that should fix the problem. However, if the problem persists, they can test each section of wiring and each connection using a multi-meter.

The resistance readings given by the multi-meter help to determine where the problem really lies.


How can the pump cause problems that lead to the A08 fault code being displayed?


The pump’s job is to give a constant supply of hot water to radiators, towel rails and taps.

However, central heating systems are prone to sludge build-up. This is gunk that builds-up in the system from rust and minerals in water.

If the system hasn’t been cleaned out for a while, there’s a good chance parts of this sludge are blocking up the heating pump.

If this happens, regardless of the speed setting on the pump, the water won’t be pumped round the system quick enough.

This will create a differential in temperature between the NTC thermistors that is too large (due to water moving slowly and having more time to cool before it hits the return thermistor), so the PCB locks out the boiler.


What can be done to fix a clogged heating pump?


On some occasions, a clogged pump will have put the shaft, bearings and seals under too much pressure. If they are excessively worn, the whole unit might have to be replaced.

Assuming the pump is in good condition, the pump can be removed and cleaned. After it’s refitted, the boiler engineer can dose the system with inhibitor – this helps to break down sludge.

Finally, the engineer will fit a magnetic boiler filter which will catch the sludge, so it doesn’t clog up the pump in the future.


If I need a new pump, how much will it cost?


If your pump isn’t in good condition, it makes sense to replace it rather than invest the cash in labour trying to fix it.

Typically, a high-quality pump from a brand such as Grundfos is likely to cost more than £250 to replace (including parts and labour).

Obviously, the exact price depends on the size of the pump that your boiler uses.


What has the PCB got to do with the NTC thermistor, and the A08 fault code?


The PCB controls all the electrical parts in the boiler. For example, it tells the pump to run, the gas valve to open and receives signals from the NTC thermistors.

So, when there’s an electrical fault on a boiler, it makes sense that the PCB could be the culprit.

Like testing the wiring and connections on the boiler, the engineer will be able to use their multi-meter to test wiring, connections and the PCB itself.

Hopefully, this is a case of loose wiring or connections that can then be re-secured to fix the problem.

However, if the PCB isn’t showing any signs of life through the multi-meter, there’s a good chance it’s going to need replacing.


How much will it cost to replace the PCB to fix the A08 fault code on my Vokera boiler?


The exact price of a PCB depends on the model of boiler you own. The price can vary from £350-500 including parts and labour.

Other Vokera boiler issues


For more boiler issues regarding the Vokera then please visit the Vokera Boiler Problems And Cures page.