Nest thermostat

[Yes1Yes2Yes]

The thermostat was working fine , then it just stopped. The heat link  is still working and that’s how we controlling our heating system by turning it on and off by the heat link button . But the thermostat will not connected to the heat link link . We have tried factory reset , reconnect , pairing , press the heat link button for 20 seconds and release button flashes blue, press the heat link button for 30 seconds then release light flashes blue . Took the thermostat from it base plate attached it to an external power source came on for a min then error H71 came back on again 

[Frustratedddd · Accepted solution]

The Heat Link I assume has power still? What is the colour of the light showing on the Heat Link? When you reset the Heat Link did you pressand hold the button for well over 30 seconds as I describe on this forum? Press the button and keep held for well over 30 seconds until you get a steady pulsing blue light, do not release for a good 40 seconds, do that twice.

The H72 is a power issue, what is the battery voltage?

Remove the display from the base and charge with micro USB for an hour or so.

Your system is it wired or wireless?

[Frustratedddd]

Look in the H71 screen, and go to the next page.  Look at Heat Link information, and then make a note of the 6 digit code shown there. Now go to your Heat Link and look at the left hand fixing screw under the main cover. You will see a 6 digit code, do they match?

[Yes1Yes2Yes]

Now I have H72 fault I believe it’s unfixable and I need a replacement 

[Yes1Yes2Yes]

I did this the numbers are the same now I have fault code H72 battery. It keeps shutting now . Unusable 

[Yes1Yes2Yes]

I check this number are the same , thermostat still not working 

[Frustratedddd · Accepted solution]

The Heat Link I assume has power still? What is the colour of the light showing on the Heat Link? When you reset the Heat Link did you pressand hold the button for well over 30 seconds as I describe on this forum? Press the button and keep held for well over 30 seconds until you get a steady pulsing blue light, do not release for a good 40 seconds, do that twice.

The H72 is a power issue, what is the battery voltage?

Remove the display from the base and charge with micro USB for an hour or so.

Your system is it wired or wireless?

[Yes1Yes2Yes]

Thank you for your suggestion , I’ve tried on numerous occasions holding the heat link button for 40 seconds until it flashes blue , I’ve move the thermostat until it’s right beside the heat link button, the  thermostat won’t connect . I’ve tried all  the connections methods on the thermostat . I ‘ve charged the thermostat using a micro usb and was able to obtain this information before it shutdown Battery 3.727V, Vusb 0.01 V , VIN 0.00V , lin. 200mA (o) . All the screen shows now is a spanner crossed with a screwdriver 

[Yes1Yes2Yes]

Also the thermostat is not fixed to the wall but on its stand and plugged into a wall . The device links to the heat link wireless 

[Frustratedddd]

OK, those voltages are sort of Ok, but you need a little higher on the voltage really.

The spanner page is normal for H71/H72, but you should be able to get past that to the Tech page. You don't really need to.

Your display I think is in a confused mode due to all the reset/pairing etc., but there is another method we can try.

However, when did the problem actually start, day/date for example?

[Yes1Yes2Yes]

Problem started Tuesday the 12th of August 2025

[Frustratedddd · Accepted solution]

That answers everything! Good old Google have been updating the software for 3rd gen thermostats, and I expect this has created a problem like you have.

You might even find IF using the GOOD original Nest app, the thermostat has dissapeared.

Anyway are you handy with a screwdriver and do you have a couple of bits of wire? You don't need anything fancy even an old flex, data cable or anything where you can get 2 cores?

[Yes1Yes2Yes]

Yes fairly handy in a diy capacity, Since I last informed you battery is now showing 3.947 V , Vusb 5.03V and vin 0.00 V and lin 200ma (o) 

[Yes1Yes2Yes]

My broadband is really good 

[Frustratedddd]

What does that mean?

Your Broadband has nothing at all to do with the connection beteen the H/L and display!

I would rather know the answers to my question.

[Yes1Yes2Yes]

You mentioned that Google was updating software and was I good with a screwdriver answer yes

[Frustratedddd]

OK, what you need to do is try the following.

Get 2 cores of wire or anything that will work, you are only connecting 12v. You only need short wires.

[Please note the connections under the H/L cover are live, so turn of the isolator for the Heat Link]

Remove the base plate from your thermostat stand - you need that in your hand.

Remove the cover on the H/L and connect a core to T1 and another to T2

Then connect the other ends to T1 & T2 on the base plate. Doesn't matter which way round, they are not polarity worried.

This is all temporary so no need to make it look anything, you will just hold the base plate in your hand while standing next to the H/L.

When connected, replace the display onto the base plate in your hand.

Now turn on the supply, and the H/L lights will appear.

Now see if anything happens, if not press the display against the base for a reset, but keep pressed until you see the Google logo, then release.

See if that works, you might have to rotate the dial, but see what happens first.

When all done, remove the temporary connections, and place back on the stand.

Leave connected on the stand for a good 24hrs, then do a factory reset.

I hasten to add, make sure you have at least 3.76v min before you start, so charge with USB before you commence. Oh and don't keep looking at the voltage as that takes longer to charge.

Does that all make sense??

[Yes1Yes2Yes]

Ok will tried this , I’ll back to you in 24 hours 

[Frustratedddd]

Mods remove this pointless remark, and the other post they have replied to.