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Hello my name is ***** ***** will be my pleasure to assist you. It sounds like it may just be a dirty flame sensor. If you locate the flame sensor and take it out and clean it with a piece of fine grit sand paper or scotch brite then put it back in and reset the power to see if the burners will sustain flame. It is located in the burner section of the furnace and sits in front of the burners it has a single wire going to it usually located on the left side of the burners. You may have to take the cover off the burners to get to it will be the cover with the glass hole in it held on by a few 5/16 screws.
Is there an intake and exhaust on the furnace?
if the intake is not glue in place try disconnecting the intake pipe. You may have an issue where the intake air is cooling the sensor off.
it could be the issue I would check the termination where it goes outside bees like to build nests in themand leaves and grass will clog them up.
its funny that the furnace runs fine with the cover of the burners off. It seems like it would get plenty of air from the intake if it is right in the room off the furnace. Be sure there is nothing blocking the intake either.
I will be off for an hour or so I will get back to you as soon as I get back on. Mean while check the intake to be sure it is not blocked and check the exhaust pipe outside. with the furnace running if you are outside you should be able to feel air coming from the pipe outside.
I am a different expert, Usually when these high efficiencey furnaces fail but will work with panel removed, the issue is a pressure switch issue.Removing the panel changes vacuum which operates the switch. Can you provide complete model on furnace (on label inside burner compartment)
OK lets start by turning power off. Thermostat off. The power back on (This resets board). Then call for heat and tell me how far in sequence it goes and as soon as it fails, first error code we get.
Sounds like you may have a failing board.The issue with running with panel off is usually a pressure switch error, But if that is the case, the board should give us a 3 flash code. Sounds like board is completely bypassing any error codes and going directly to a lockout saying it has tried 3 times when it has really only tried once
Lets do that.Also make sure to turn high voltage completely off so board resets.
This sure leans towards a failing board, but I hate to condemn without being onsite.I think I would call in an onsite the before relacing
Not saying definitely pressure switch can not be the issue, but if it is, the board is still failing to do its job by giving the pressure switch error.
Everything points to board,since it is not showing a pressure switch code, but goes straight to lockout saying it has attempted max number of times on first failure. These boards can be a few hundred bucks is why I hate to tell you to buy board without being onsite. Sometimes onsite I may see something obvious that I do not see via internet
I often see so called experts on online sites that tell people to replace board or replace inducer, etc as a guess.Then if that does not prove to be the issue, customer is out all that money and upset.I have been in this business since 1974,but do not like guessing.
Exactly all you had was a transformer a couple of relays, gas valve and a motor.