Lennox error code error

Buck_99

I have a Lennox Ml193df about 10 yrs old giving 3 flash error code indicating pressure switch problem. I've checked P-SW multiple times with a meter & pressure switch is closing during cycle. I've verified the 24 VAC is present at the connector to control board but still getting P-SW error code. Regardless of error in error, changed pressure switch anyway. Not surprisingly, same code. Must be control board, so changed out control board. Same code. (no used parts. All new). Randomly furnace operates normally for few days. Longest was couple if weeks. No idea what changed that made it decide to run. Have done various other winter time maintenance unrelated to pressure switch. Sucked out condensate line. Checked for frozen drain etc. Pressure switch closes within few seconds of inducer start. Measure 24VAC right at backside of connector to control board. Only thing can come up with is P-SW error is bogus but whatever error doesn't have it's own code. Pick one. Had help from professional Hvac, who agree with steps taken, but can only suggest reroute condensate line, & throw more parts at it. Looking for any advice from anyone who may have seen this kind of problem.

ratio

I've seen limits tied in to the 'pressure switch' signal, check for an intermittant limit switch. Also could be a poor connection as well, unplug & plug all the connections a few times.

The schematic should give some clues.

At least one mfgr's control board gives an odd flash code if the gas valve is open circuit—like if the switch is off. 'Course, that's one of the errors you're supposed to Just Know—that particular diagnosis doesn't appear in the manual.

worldclasshvac

Many manufacturers have other things tied into the error circuits. I've seen plenty of pressure switch codes that were all drain related. Only takes a split second for the switch to flutter and set a code as well. Need a manometer to verify you have the proper draw and the wiring diagram will show you what else is in this circuit. Often the install manual will tell you what else can cause the code in the troubleshooting section

pecmsg

that doesn’t indicate a pressure switch problem it indicates Something is preventing the pressure switches from opening/closing.

pedmec

How did you determine pressure switch issue? lennox control board has two leds that work in conjugation to give you a code fault. Red and Green. Nothing in the manual for that model says 3 flashes. Red off, Green slow flash for pressure switch error. I think you need to be more specific.

EdTheHeaterMan

By looking at the wiring diagram for thet furnace, it appears the the S10 limit and the S18 Combustion air proving switch are on the same circuit and can both offer up the 3 blink error. The S10 switch connects to Pin 1 and pin 7 on the 12 pin plug with blue wires. From the S10 switch a purple wire feeds S18 proving (pressure) switch. the other side of the proving switch has a purple wire that connects to pin 10 on the 12 pin connector.

The software on the PC Board will determine what code to flash bed on the electrical measurements as the sequence of operation travels thru the PC board. when there is a stop in the action, that will generate the code.

So look at the S10 limit switch to see if there is a problem with that. use your meter to see if the switch stays open once in a while?

If it is not the Limit switch, then maybe the # 10 pin in the Molex 12 pin connector is loose and does not make contact once in a while. Try to bend the pin just a veeeeerrrrrry litttttlllle bit and see if that stops the problem.

Low voltage path of the sequence of operation starts on the R from the transformer (blue wire) and goes through the spill switches near the burners (S47) then feeds 24 VAC to R on the PC Board.  R connects to the thermostat R and on a call for heat that connects to thermostat W and sends the 24 VAC to W on the PC board.  W on the PC board connects to the number 1 pin on the 12 pin plug.  That feeds 24 VAC to the S10 limit switch and from there 24 vac goes to pin 7 on the 12 pin plug and to the pressure switch S18. Once the pressure switch closes to prove that the combustion air fan is operating, then 24 VAC goes to pin 10 on the 12 pin plug.   If that #10 pin does not receive 24VAC then the 3 flash code will be sent.   That is why I believe the #10 connection in the 12 pin plug may be the problem.  

BE very careful when adjusting the pins in the male and the receiver in the female plugs; they can be very delicate.

EdTheHeaterMan

After looking at the details of that Lennox ML193 DF manual, your control may be from a different year than the control in the manual. As mentioned by @pedmec the error code for the fan proving switch is Red off, Green slow flash. There is no 3 flash code in that control. Perhaps you can take a photograph of the wiring diagram of your furnace and I can take a close look at it.