I have a gas forced-air furnace 3-5yrs old. Honeywell setback type thermostat (set to "HOLDTEMP")

My problem is the furnace seems to shut off before it reaches set point (ussually 4-6 degrees below set point)

I changed the thermostat and still the same issue. If I turn the power off to the furnace and turn it back on the furnace starts up and runs fine for 15, 20 maybe 25 minutes, but eventually shuts off again short of set point. Doesn't seem to be any pattern as to how long it runs or how near set point it is.

I might be saving gas cost but the bride is putting on too many sweaters and a man can only wear some many pairs of socks.

Any help?

Exact model number of thermostat.

Honeywell RTH6300B

If your furnace is faulting out it should have a flashing light you can see.-- lower door on the furnace with a small glass window count the blinks it will give you a code telling you what's wrong.

Read under settings. If not set properly the unit will cycle off early. You might have yours set to hot water instead of forced air. Read and follow directions exactly. Link below for your stat.

http://customer.honeywell.com/techlit/pdf/69-0000s/69-1717.pdf

If your furnace is faulting out it should have a flashing light you can see.-- lower door on the furnace with a small glass window count the blinks it will give you a code telling you whats wrong.

All right, the fault code tells me it's one of 3 limits (2 ROL & 1 Limit). I've physically inspected the 2 ROL can't tell anything and the limit switch seems stuck to the unit. I removed the screws but one side of the plastic circuit board like switch is stuck to unit. That switch must be normally closed because when I disconnected it the fan started running but I thought I'd better not run it that way.

I'm not sure if this limit switch has some type of mech tab holding it to the wall of the furnace or is it just stuck something else, I'm fearful of breaking it and having no heat.

I've physically inspected the 2 ROL can't tell anything and the limit switch seems stuck to the unit.

You have to test these with a meter.