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Thanks for this, turning the back light off worked for me as well, although I cant run both water and heating at the same time but one at a time is working, just hope the auto settings work when water and heating is set to run in the early hours of the morning, apparently theres a capacitor that fails on these so it cant bare the load and trips, thanks for this amazing video

Hi simon a couple of years ago this solution worked by switching the light off. 2 years on and the panel is switching on and off again. I tried putting light back on and see if it would stop the flickering but no use. I guess i need to but a new panel unless you have another solution?

Can you tell me please if there is any battery inside to replace?

Excellent ! Problem solved thanks for that

Thank you very much for posting this video. You not only help us in restoring the hot water and heating but saved us from paying for call out costs in this cost of living crisis. I have bought exact make and able to replace it myself.

Thanks so much for this Simon. We have just bought a house with one of this fitted. My problem is setting the daily programmer … I have the manual … I set the 6 times, press ok and it always defaults to Monday which isn’t a concern. However no amount of button pushing on ‘next day ‘ or ‘copy’ does anything. In effect we don’t have a program for the next 6 days. I’d like to fix it but it’s good to know I can replace it … thanks to you.

Your an absolute superstar for posting this, it's just before Xmas and my controller had the same issue and getting zero heat, cant believe turning off the backlight got it working again...tahnks.

Excellent - as a landlord I had a beleaguered tenant who needed hot water for her children’s bath time after the plumber had reset the boiler. This solution sorted it. You’re a gem

What a bloke! My programmer was shutting of when trying to put water on at same time as heating. Turned back light of and we are good to go again !! Cheers

Thanks! Just found this and what you've shown is exactly what mine is doing as no heating or hot water this morning (cold showers - brrrrrr!) and this was clicking on and off! So off to the "fixer of screws" shop this morning to pick up one to replace with. Was nearly about to call an engineers in .

Amazing! Thank you so much for posting. The switching off backlight trick worked straightaway to get our hot water back on!!

Thank you. Even though the controller was wired to the wall, this video gave me the confidence to remove it and look inside. I saw there was a 2032 lithium battery disc which I swapped for a spare that I had to hand and hey presto! An enormous feeling of empowerment and satisfaction, thank you so much!

You don't say what's wrong with it and lots of mumbling.

thank you Simon for the video to show me that i could replace the unit myself despite that it was 6 years ago . My apt has the same unit to control the boiler, however I'm not sure when the mode switch button on the heating side stopped working. Now I can't change the heating to auto, once or cont modes... but the hot water side works. Do you think replacing the st9400c unit will solve the problem? because it seems that only one button isnt working... so i am wondering if there 's another problem causing it.

Turned the backlight off, unfortunately didn't stop it rebooting constantly. Thanks for the video though

Thank you for posting this video. Who would have thought that turning off the backlight would overcome the on/off cycle fault!

I haven’t the same problem, my boiler and program unit were installed at the same time. My fault is that when I turn off the ST9400 power was still getting through to the boiler, which would fire up even when turned off. I’m going to get a new ST9400, now full of confidence that it’s easy to install. Kudos Simon.

Simon, thank you for posting this problem with the ST9400C programmer and for the brilliant demonstration of how to replace it. I had exactly the same issue with mine after 11 years of use, and it was such a relief to see that it is a known problem and so easy to resolve. Credit also to Honeywell for designing the product in a way that allows an amateur to replace the main item while leaving the professionally installed backplate in situ. Very happy now.

I can't get mine to work on auto mode for zone 2, but zone 1 works fine.

Cheers, just managed to get our heating on by turning the back light off. Now I know it's only the controller I'll get a new one and not get someone out for the boiler.

Hi Simon do u know how to get in to the installer mode for a service plus s17r programmer

thanks, backlight off and it worked, great temporary fix :-) Update - instead buy a new unit, I managed to repair it by adding a capacitor th-cam.com/video/tM4P39KxtyM/w-d-xo.html (the small capacitor)

Thank u mine was doing the same took back light off figures crossed until I can replace the control

Omg amazing!

Thanks, Simon. Very clear and helpful.

What does your wiring to the backplate look like? Looking at general google question/answer, it is normal for the boiler to fire up even when neither hot water or heating is on but after about 4 years and managing to sort out a configuration for the wiring, we can leave the programmer on instead of manually turning on/off. Primary wiring is ok using live/neutral/earth but the wiring from the thermostat is strange using a brown/black/grey colour wires

Thanks for this just saved me a call out fee. Mine stopped working so emboldened by your video I bought a new one and replaced. Simple.

Thanks you so much for sharing its been extremely helpful. You saved me a huge gas Engineers’s bill.

Sinon! What a guy! This has just solved our problem too and I now look like a genius to my boyfriend. I did tell him I’d watched this video so I gave you full credit. But you saved us a weekend call out fee and an additional £300 on a new panel they wanted to install, so thank you!

Great Video. Thank you. Our boiler wouldn't turn off and as I had this unit because of a previous fault I fitted it as a first step. Just a small point. Our unit was mounted on a solid wall and the replacement unit wouldn't close properly. It took a while to realise that because the connecting wires came out of the front I had to 'nibble' at the wire entry points to make holes with a pair of pliers. There are lines cut in the unit if you need to do this.

Many thanks, worked a treat. Same issues as well with hot water and heating causing this switching on and off.

If you're good at electronics here's a fix: th-cam.com/video/tM4P39KxtyM/w-d-xo.html

I'm havng trouble with the hot water overide switch which doesn't seem to work when heating is set to "on" even if heating not required. I set heating to "off" and then change hot water to permanently "on". This seems work but is annoying. The unit is about 9 years old so I guess time for a new one. Thanks for the video.

I noticed my ST9100 would not work in override mode but was working in program mode. Did the backlight off trick and now it is working in all modes. Great . Thank you for taking time to document this.

A useful post that highlights that also highlights how the ST9420C can be a bit tricksy. I had problems binding a new DTS92E thermostat - followed he instructions to the letter and cleared the old thermostat but it just woudn't bind to the new one. Solution - don't just clear it but power down the ST9420 programmer completely for 30 seconds by switching off - then switch it on and go to the binding menu - worked first time then! The rule is: always factor in switching it off and switching it on again.....

Thanks for this video, kept our heating going over Christmas.

Slim, QUIET, works great. I wanted modern and out of the way. This is it!

Many many thanks for your video Simon. Mine failed today and followed your video to check and replace it. Took the option in the Installer menu to set the backlight to 'on when worked' which I'm hoping prolongs the life of my new unit. Thanks again.

Fab temp fix. Clicking was driving me crazy 🙄 as was in hall. Silence is golden.

Thank you Simon, solved the problem for now. Cheers Rob

I was cleaning mine when it just suddenly turned off all it shows is wrong time and date I can't get it to turn back on can u help plz

Thank you so much !!!

not sure how or why but this seems to of fixed mine for today 👍

Great help, many thanks!

Thank you Simon, I was able to sort it out following your tips. You’re a super star!

Desperate for a bath, turning the the backlight off somehow works....? Thank you.

Thank you!!

Thank you, really helpful - had exactly same problems !

unfortunately didn't fix my unit by turning the backlight off... off to screwfix I go as its about 1degrees today..

Thank you so much for this video. It's winter and snowing and I've managed to get my boiler back on! A replacement unit isn't going to break the bank too much either. Legend.