Hi,

I have a GMP100-4 that blows cold. The pilot light is on and the burners burn but only for a few seconds before turning off again. The blower is on all the time so I'm getting little to no heat around the house.

I've replaced the filter, I assume there is only one and it is situated below the blower in the bottom compartment of the furnace. This happened a few months ago and the gas man changed the filter and then it worked fine, not this time though.

Any help greatly appreciated.

You need to clean the flame sensor it's the rod at the end of the orange wire take the screw out then polish it with a brillo pad and reinstall

Is the flamer sensor part of the ignitor, the thing that glows and lights the burners?

No its usually at the other end of the burners it looks like a rod that sits in the flame

There is a picture in the sticky https://www.askmehelpdesk.com/heating-air-conditioning/heating-cooling-maintenance-repair-troubleshooting-frequently-asked-questions-58313.html