Reznor UDAS Header Debugging

[dlpoht]

I've been banging my head against my Reznor UDAS-60 unit heater that I installed new in my garage back in 2019. It was working well when I stopped using it for the summer (turned off power and gas). I turned it on for this season and it is exhibiting strange behavior that I cannot debug.

Behavior: On the call for heat, unit engages venter motor, opens gas valve, ignites properly, and flame is sensed (yellow LED). 30 seconds after ignition, as per the manual, the blower motor engages. At the same time (within 1 second) of engaging the blower motor, the flame cuts out. Then the blower runs for 2 minutes and the process repeats all over. Based on the manual, it seems that the heater is successfully getting through prepurge and ignition, but barely reaches the normal operating stage before failing.

Troubleshooting done:

1. No error code LED pattern (LED remains flashing throughout to indicate call for heat)
2. Verified that the limit control switch and flame rollout switch remain closed throughout
3. Verified that the flame sensor is intact, clean, and ceramic bushing is not cracked or corroded
4. Verified that the pressure switch remains closed while venter motor is operational
5. Checked the blower motor is drawing rated capacity 1.2 amps

Not done:

1. I have not tested the natural gas pressure. I don't have a manometor. However, the flame ignites and is detected by the flame sensor. Natural gas to the rest of the house seems fine (grill, home furnace, water heater).
2. Tested the current running across the flame sensor (should be 1 microamp)

What else could I be missing? Is my next step to replace the circuit board?

Thanks!

 

[TwoToque]

1) 1 uADC for a flame sense sounds too low to me. Clean the flame sense rod with a scotchbrite pad or emery cloth.
2) When the fan kicks on makes it sound like the roll out switch. Are you testing the operation with a multimeter? Are there any signs of a cracked heat exchanger? I know you said you confirmed the operation.
3) High limit switch. Is the temperature getting too high before the fan kicks in?
4) Last but not entirely dumb, are you losing your call for heat from a loose wire connection?

 

[fitter30]

Didn't see that there wasn't a light for flame rod or pressure switch if it makes at start up but then opens after. Check the pressure switch tubing and flame rod. Page 19 starts ignition

https://www.google.com/url?q=https://documents.alpinehomeair.com/product/Reznor%2520Installation%2520Instructions%2520UDAS%2520-%2520UDAP.pdf&sa=U&ved=2ahUKEwiDkprE4Kv7AhVokYkEHR-4BAUQFnoECAIQAg&usg=AOvVaw3M9mH7eWNofFW1VK0ZkEKX

 

[trevkers]

Hey, did you find anything out yet? I have a UDAP45 with very similar issues. Other than mine will only run the venter motor, then as soon as the ignition starts or tries, everything shuts down and then restarts. No codes, not locked out.
Any suggestions>?

Thanks.

 

[trevkers]

Here is a clip of this occuring. Still not working.
It is cold cold..
I've tried unplugging the blue wire on gas valve and then tried. It does try to light after that. . Does that gas valve have a feedback stating it's open??
Also the thermostat is unplugged and jumpered out.

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