home heater furnace blower doesn't turn on [SOLVED]

Question

FOR SOLUTION LOOK TO END OF POST

My furnace's exact model is MMHB-077ABHR-06 ,Controller is UT-1018

I have a nordyne 7076700 gas furnace for a mobile home, and the blower never starts up when heater starts up.

Steps:

1. gas is on, furnace power on, thermostat is on. ON/OFF/FAN Switch is set to ON
2. i hear transformer hum
3. i hear and see combustion motor work
4. maybe 10-20 seconds pass
5. furnace ignites
6. a few clicks of relays
7. after about 30s furnace turns off
8. then repeats back to step 4, without blower ever turning on

Tests I have preformed

1. gas is on
2. transformer gives 24V
3. Relay is new and operates properly
4. when disconnected/cold, limit switch is close (~0 ohms)
5. when disconnected/cold, fan switch is open ( inf resistance) ???Is this ok???
6. control board has no blinking LEDs
7. blower fan turns ON when ON/OFF/FAN is set to FAN
8. when thermostat is set to lower than room temp, all operations stop
9. when thermostat is set to higher than room temp, the steps above occur

Im not sure if the fan switch should be normally open or closed, maybe this is broke, they part is NORDYNE Part # 626337 https://www.amazon.com/626337-Nordyne-Furnace-Replacement-Switch/dp/B00FG9FNA2

i am cold, please help.

More Tests ive done

i tested a bunch of places with multi-meter voltage AC

1. transformer voltage is stable
2. none of the sensor/switches change state
3. Relay does not switch (voltage stays low.)
4. thermostat green to white wires show 24V when on 0.2V when off
5. thermostat green to red, 24V with thermostat setting above or below
6. turning fan on from thermostat works
7. when thermostat is set to Heat ON and FAN ON, The fan blows constantly, but the furnace still cycles through the same process described above. flame going on and off.
8. Turn off the gas input and controller led blink the proper error message

Another thing

this might be in my head, but in the past when the flames turned on I think the flames burning heating sound was louder, more forceful. but i might be making it up in my head.

Schematic

Igniter/flame sensor

this is a photo of the igniter. as mentioned below also operates as the flame sensor. It looks pretty clean, but I'll give it a light cleaning before reinstalling it. What should be the measurement for the 'flame sensor' when the igniter(mode) is not active and the flame is on?

Update

I read somewhere that taping the gas valve unit with a hammer might unjam some stuff in the gas valve unit. After i did that, the flame burns for about 3 min instead of the 30 second. This only occurs on the first loop. after that is burns for 30 seconds before resetting. But is is a noticeable change, and I'm not sure what it means. I could have just knocked some dust out.

Another update

Ive used my air compressor to blow and dust out of the burner assembly, igniter/flame sensor, flame chamber, and anything else i could easily. Then put the burning assembly/etc back. Also tightened some connectors that had a little more play than i liked.

The result. Now the burner goes for about 15min before resetting and continues to burn in 15min intervals. but the blower never goes on.

Solved (Solution)

Problem:

1. small leak in Gas Valve

Symptoms:

1. Furnace starts for a while, blower never goes on, then resets
2. small smell of gas. not as strong as unlight stove or unlight pilot light
3. furnace never gets hot as normal (forced blower on via thermostat, and heat on, only heats house a little bit)
4. Burner doesn't sound as strong as normal

What I confirmed wasn't the problem

1. all switches are normal
2. blower works when forced via thermostat
3. igniter/flame sensor is clean
4. All voltages across component normal
5. Relay works

What shows the problem.

1. Used very soapy water (1 to 4 dish soap to water)
2. Cover any possible leaky point.
3. Find any bubbles, in my case a hex-screw thing on the bottom of furnace gas valve

Solution:

1. Seal the leaky area

Answer 1

You did great work so far.

Based on that I would say it is the "Flame Sensor" not working.

Steps:

gas is on, furnace power on, thermostat is on. ON/OFF/FAN > > Switch is set to ON

i hear transformer hum

i hear and see combustion motor work

maybe 10-20 seconds pass

furnace ignites >>> here is where the flame sensor goes on to report flame is on

a few clicks of relays

after about 30s furnace turns off (missing flame sensor report)

then repeats back to step 4, without blower ever turning on

It looks like this, held by one screw. Take it out and clean it, and try again, if not replace it.

source https://daycon.com/product/nordyne-flame-sensor/