That is the target room temperature and if its showing -20C then its not getting a temperature signal from the thermistor.

Are you using the main controller to measure the temperature, i.e. the unit in your image or do you have a wireless controller as well?

If you are using the main controller go to initial settings, then into room sensor settings, sensor settings and it should show MAIN RC, if its showing something else then that is what you have changed and its needs changing back to MAIN RC.

@gary Firstly thanks for you reply. Iv been in a bit of a panic as apparently I fiddle too much according the the better half and that one day I’m genna muck something up😉

I do have a wireless controller that I control the room temperature from and I’m using auto adapt.

When browsing through the settings I’m sure it was on TH1 but I have this morning tried already the MAIN RC and right now everything seems fine.

Watch this space!!👍

And fair play to Mitsubishi support as they’ve already replied to my email from last night with the reply below.

A P1 error where the controller is looking for a sensor which doesn’t exist on the system, it’s a default setting which normally takes place if there has been a power cut

Carry out a power reset on the system to clear the fault from the indoor & outdoor unit

Power reset here

Ask the caller

What is controlling the Zone? The main controller, 3^rdparty controller, Mitsubishi wireless stat or a thermistor (A thermistor is a cable which is connected to the system to read the temperature)

1. Press and hold the POWER/HOLIDAY button in the bottom right hand corner of the Mitsubishi controller until SYSTEM SHUTDOWN comes up and select YES
2. Press the bottom left MENU button

1. Using the directional arrows highlight the INITIAL SETTINGS icon and select with the TICK  button
2. Scroll down to the option ROOM SENSOR SETTING and select with the tick button
3. Select the SENSOR SETTING in the options with the tick button

If it’s a 2 zone set up, an extra setting will show up here to select which zone

1. Select the SENSOR SETTING option with the tick button

1. Change the sensor option by pressing the cycle button until it displays either
   MAIN RC – If using the main controller or any third party thermostats
   ROOM RC (1 to 8) – If using a Mitsubishi wireless thermostat
   TH1 – If using a thermistor (This will default after a power cut, we don’t normally use it in the UK)

1. Confirm using the TICK button
2. Once confirmed use the back button to exit the menu and turn the system back on with the Power/briefcase button
3. Make sure the heating mode is correct  Changing Heating Modes

If the ROOM RC isn't there, it needs dipswitch SW1-8 by your installers. If that’s on go into ROOM RC Select and make sure the appropriate zone is selected

If you have a Mitsubishi wireless controller then it should be set to Room RC not Main RC, if you use Main RC it will now be using the main controller's temperature to control the house temp where as before it sounds like it was using the wireless controller.

My mistake. Room RC is what I selected 👍👍