Lennox RTU LGA120SH1Y LOW PRESSURE TRIPS

[mturan]

I've got a lennox unit that is giving me a code 24 or low pressure trip on circuit #2. THis is wired into the A57 board directly. I've tried about everything from changing the low pressure switch, changing the boards, jumped the switch right at the board wiring, but nothing seems to stop this from tripping the unit with this 24 code on the A57 input. Any help would be appreciated.

[timebuilder]

What is the actual low side pressure?

[mturan]

The actual pressure was 68 psi and it never droppped less than 60 psi the entire day when it was going out on the low pressure trip.

[hvacpope]

the wiring/harness could be bad. you could set ECTO 4.14 to 8 counts (max) to disable compressor lockout due to LPS

[timebuilder]

If you placed a jumper at the board inputs and you still get the code, you could do as suggested above and adjust the ECTO parameters, but I would replace the IMC.

[mturan]

Ok thanks for the advice guys I will try those both and see if this works for us. Have you ever seen this situation before on this unit? Not really familiar with the Lennox units. Thanks again!

[timebuilder]

Ok thanks for the advice guys I will try those both and see if this works for us. Have you ever seen this situation before on this unit? Not really familiar with the Lennox units. Thanks again!

What I typically see is when there is a leak, the stage will be locked out on the low pressure switch opening three times. Look on the left inside door panel for the code list, and you will see that code right after the code you mentioned.


Reset the code by holding in the black square buitton for three seconds. Run the unit and take a voltage reading at the two wires where they enter the terminal strip at the A55 board. You will see that one side of the connector body next to the wire has a little more room than the other side, and that's where you can slip in a slim test probe. Monitor that voltage, which should be near zero, and if a code sets while monitoring a zero voltage, you will know that the pressure switch and wiring is good, and that the board CPU or some ancillary device on the board is failing.

If, however, that voltage goes higher and then the code sets, you could have a bad pressure switch.

This problem you are having is unusual, so take your time and verify what you are doing. Make sure the sw # mentioned in the code list is indeed the switch you are looking at on the stage where you are taking the pressure readings. If you confirm that the circuit is closed at the board connectors, yet you have a code shown, it's time to replace the IMC.

[mturan]

Thanks timebuilder for taking the time to help me with this. I will check on these items when I can get back to the site and let you know what I found on this. Thanks again for the help.

[hvac wiz 79]

op u said u jumped it out at the board & issue repeated tho .. id say that pretty much sums that up . . no?

[mturan]

Yea I jumped the switch at the A57 input cause its on circuit #2 and still got the diagnostic so i eliminated the switch and the wires. I have also changed out the A55 and A57 boards on this. Not sure what the IMC is but i can look into the ecto settings and try and do something there. I just appreciate the dialogue on this as I'm not that familiar with the lennox unit. Thanks everybody.

[timebuilder]

For someone who is familiar with the unit, yes, that would sum it up. But, I have seen new guys who believe they have taken steps such as those, and yet, made some error along the way.

My idea is to double check the work before a pricey board gets replaced.

[allstar08]

I would data log the suction pressure, DAT, and RAT, and possibly duct static, so I would know when the fault is happening. Possible low load, low air flow, etc. It sounds like you may be going down a alternate path.

[hvacpope]

I have worked in hundreds of LG's, I'm yet to find the first bad A55 board, cant say It doesn't happens but the boards are pretty much bulletproof.

[hvac wiz 79]

hvacpope in all my years & 100's n 100's L series units I found 1 defective A55 ... it was M1-5 .. I've encountered many misdiagnosed controllers tho lol

[timebuilder]

hvacpope in all my years & 100's n 100's L series units I found 1 defective A55 ... it was M1-5 .. I've encountered many misdiagnosed controllers tho lol

I've replaced one that I knew was bad.

AND....one that Lennox tech support SAID was bad.

[timebuilder]

Yea I jumped the switch at the A57 input cause its on circuit #2 and still got the diagnostic so i eliminated the switch and the wires. I have also changed out the A55 and A57 boards on this. Not sure what the IMC is but i can look into the ecto settings and try and do something there. I just appreciate the dialogue on this as I'm not that familiar with the lennox unit. Thanks everybody.

IMC is a name for the A55. If you have already replaced it and you still have the problem, hang onto the old board. You may want to put it back after a closer look.

Wish I could be there to look at it, but all we can do is follow your reporting.

[timebuilder]

Let's assume for a moment that you are still having the same problem as before.

Now, step back and look at this like it has just been found. Look at the codes. Use the "recall" switch setting and write them down, then clear them by holding in the black button till you see "0." Then, move the recall switch back to its normal position.

If there is another similar unit nearby, make sure all the DIP switches on the board are set up correctly, such as phase, gas, etc.

Look at all the terminal strip jacks and plugs. Is everything mated together evenly?

Does the switch mentioned in the code agree with the switch designation of the stage where you have your guages?

I would get my meter on the low pressure circuit and monitor the voltage while observing the low pressure while the unit runs, and look for the switch to open. If that switch never opens and you get a code, and you are using a new board, I would go back over the connections and wiring again.

Another test I would do is to disconnect the switch at its harness while the unit runs, and confirm that the board will respond to the opening of that circuit with the same code.

Let us know what you find.

[mturan]

Great Info on this unit. I like the recall information as i didn't know about that. I will get back to this unit eventually as it is quite a distance from where I live. I will be back there eventually though. Thanks again for all the help.