Hi,my unit just act up about 10 days ago,while run on heat it keep running the only way to stop it is to shut down the main switch . Thermostat is useless at this time no matter what position setting fan still running ac won't come on the same to heat.Just change the circuit board still won't help , there're no voltage on the board thermostat terminal . I saw a fire burn mark on the wire with fuselink to the limit switch but my friend check and he said that it has power through.Should I call a contractor ? Thank's

Check the primary limit switch. If open, it may reset with a few taps from the butt of a screwdriver between the terminals. If the fuse link looks burned, double-check it.

My friend checked the limit switch and he said that it's OK.

With power applied, check for 24v between "SEC-1" & "SEC-2" on board. If not present, transformer has failed.

I forgot to give the unit model , it's a 48NLT roof top unit . The old board have the S1 to 5 on the right and N1 to 5 on the left , no fuse on it .

Also the board number is HH84AA017

Here's a link to the manual. http://www.xpedio.carrier.com/idc/gr.../48nt-20si.pdf Page 21 is a troubleshooting guide.

I'll try tomorrow after work.

Double check the fusible link and that's the problem . Thank's KC13.

The fusible link was the SYMPTOM. Why it failed is the actual problem. Flame roll-out is the most common cause.

Still wait for part , when install the F L , I will check and clean the flame sensor replace if need.

How old is the unit? If the fusible link failed due to a faulty heat exchanger, operating the unit with it bypassed is dangerous. A replacement link will fail in short order as well.

20 years old and no the unit is not operate at this time , I just bypassed the F l to make sure it's not the board problem . Will inspect the heat exchanger . Thank's for the advise.

Heat exchanger look good , flame sonsor look a bit dirty , just clean it , still wait for the F-L .

Install new fusible link , work fine now

Just finish repair my unit from won't stop running by replace the fuseible link and it work fine for a week or so until now come another problem . Last night just come back in town , turn on the heater all I hear is the humming sound from the draft inducer motor and nothing come on ,today I turn on the unit , draft inducer come on , the ignitor clicking for about 30 seconds then stop , no gas hissing sound at all , no fire . Give me some idea . Thank's

I forgot my unit is HVAC Carrier model 48NLT036300 rooftop unit.

Is it doing anything? How about when you set tstat to fan on, does the blower then blow cold air,, if not have you checked the breaker

The fan come on when turn on the tstat , ac also on when turn to cool , breaker OK , I think the flame sensor or pilot ignitor maybe the problem . Found new part online reasonable price , will replace the pilot burner assembly since it's too old anyway .

In many/most cases you can clean flame senser and not have to replace also pilot assembly