I replaced the batteries in my wireless honeywell Cm61ng thermostat but it doenst' seem to be working with the unit next to the boiler. i think it may need pairing / re-programming. how do i go about doing this?

we had one of these and found out the battery inside had failed. the battery is none replaceable. did try to replace it but it lost its memory and was only good as a switch. think when the battery fails it loses it memory and things start going wrong. so would say change it. now thinking on it should of changed the battery why having the old one on and taken the old one off after installing the new one.

Probably not. There are replacement models available.
The T6661B programmable room thermostat is also known as CM61.
CM61 1 day wireless programmable thermostat is replaced by CM921.
CM61 1 day wired programmable thermostat is replaced by CM901.

If the power to the boiler is off, and you have a power stealing thermostat (tstat with a common), there may be battery backup. In which case, the thermostat is flashing because it is on battery backup. Not sure what you mean by replaced amp now.

Make sure all the pins line up, and your thermostat wire is not causing the binding.

Um...., you changed the controller and it worked, then put the old one back and it doesn't work >> replace controller!

Check the reset button if this doesn't work replace T-stat

The red flashing light means that there is an issues with the RF comms between control unit and receiver. You can reset by switching the boiler off and back on and then re-sending the temperature signal. You need to check batteries in you thermostat and also the position as the signal could be interfered with or it may be too far away from the receiver.

Instructions on how to turn off your optimum start:-

Turn off ‘Optimum Start’
Step 1 Put the thermostat in the OFF position.

Step 2 Press 3 buttons together (at the same time) which are the INFORMATION BUTTON (i) and PROGRAM BUTTONS 3 & 4.
This will show a code of ICL 1 – now let go of the buttons.

Step 3 Press PROGRAM BUTTON 2.
This will now show a code of 30t 1

Step 4 Press the + BUTTON (on the left of the display), tap through until you reach 15 OP 1

Step 5 With the TEMPERATURE DOWN BUTTON; change the 01 to a FLASHING 0. Confirm the FLASHING 0 with the INFORMATION BUTTON ( i ). The 0 will now be static.

Step 6 Put the Thermostat back in the ‘AUTO’ position.
‘OPTIMUM START’ is now switched off.

You have wired the cooling (yellow) and the heating (white) backwards.