Lennox LGA180SH1J heat issues

[iceage]

Hi, been having a problem with a Lennox rtu from a service call. Two heat sections that run at same time. First sections ventor motor runs but trips off on air switch, had spare switch so changed out with same result, will run on heat with switch jumped out. Checked heat exchanger( no cracks) cleaned flame sensor etc. Put digital manometer on venter motor initially starts out at 1.47 inches water and runs down to .6(in about 6-10 minutes). Not jumped out will then short cycle off on pressure switch. I also pulled burners out as well and brushed out quite of a bit of crap out of heat exchanger tubes with boiler brush. Ventor motor rated for . 6 amps Was running with .5 amps

Second burner also had similar air switch issue but not as consistent. The problem with this section is a error code saying gas valve not energied after 2 min after call and error saying secondary limit open heat 2. I was able to change limit with another working one so side no longer locked out but limit error code still appears every five minutes or so. I opened up economizer to get some cooler air in to get limit to stay closed. Can’t say because I couldn’t access space if some diffusers are closed.

Any thoughts on what I am possibly not seeing?

[iceage]

Just to follow up, I replaced ventor motor about a week ago and the rtu is still heating.

[timebuilder]

Yes. You need to understand the components that make the heat come on in this type of unit, and studying the wiring diagram will help to firm up these ideas and the interaction of the IMC board with the ignition modules.

Do some pinpoint testing with the meter. For example I have received a limit indication which was not founded in a lack of voltage to the IMC board from the limit switch in question, so you need to confirm that if it's saying the limit switch is open, that it actually is open....and you can do that right at the IMC board. All of the information needed to be able to do that is printed right on that wiring diagram inside the door.

You said you replaced one of the inducer motors. Was that the motor that had the pressure switch indication problem?

What is the serial number of the unit in question?

[timebuilder]

So...12 days have passed. Can I get an answer to my questions?

[timebuilder]

No update...?