No furnace heat, unknown error code?

Mooncatt

Hello and fingers crossed.

I have a Trane XV95 furnace/AC unit and the furnace isn't kicking on. Symptoms are when it calls for heat, the inducer fan kicks in, the hot surface igniter gets bright orange, then I hear a click at the circuit board and no flame. The inducer fan remains on, igniter goes out, the main blower motor turns on, and I get a solid red led on the board. It'll stay that way until I shut it off at the main switch.

I have tested both pressure switches, the limit and roll out switches, and igniter. All tested good, low resistance, switches reset as designed, etc. When testing the gas valve, I get zero voltage throughout the cycle when probing the common to 1st stage pins, both at the valve itself and at the board. I'm assuming for now that the board isn't sending voltage to energize it. When I probe the flame sensor from the plug or at the board to ground, I get 3.8V (I read it should be around 90V) and it triggers a flash code on the circuit board. That code is "flame sensed when no flame should be present." To be clear, this is without the burners actually lit. If I don't probe the flame sensor, this code doesn't trip and it just kicks off as described at the start.

Here's another kicker. If I leave it alone until it trips, the red diagnostic led goes solid red, and the green keeps flashing fast to signal a call for heat. That isn't listed on the diagnostic code chart. There is an entry for both red and green solid to indicate a bad circuit board, but no entry for just a solid red. I have tried to look this one up and no luck.

So my immediate questions are:

Am I testing the flame sensor correctly?

If so, what could explain the low voltage and could that be the problem?

Am I probing the gas valve correctly?

Any clue about the solid red only led?

I'm open to any and all advice. I'm about ready to call a tech out, but wanting to try to figure this out on my own if possible. I'm hoping it's not a circuit board issue as it was just replaced this past Spring when the AC wouldn't kick on.

EBEBRATT-Ed

Sounds like the circuit board is not powering the gas valve. when you here it click it should power the valve

mattmia2

Has the burner ever fired since the board was replaced?

Mooncatt

mattmia2 said:

Has the burner ever fired since the board was replaced?

No, but then again we never had reason to attempt it. Can't say if it would've fired right after the replacement or not, and I wasn't home at the time to see if the techs tested that.

mattmia2

Is the wire for the main valve plugged in to the board?

Mooncatt

mattmia2 said:

Is the wire for the main valve plugged in to the board?

Are you referring to the gas valve being plugged in? Yes, and verified continuity.

mattmia2

yes, on the circuit board end. and to the correct terminals, with the connector the right way if applicable although it sounds like you only have a main valve, no intermittent pilot valve.

A picture of the schematic would help. Is there a fuse or circuit breaker on the board? Is the transformer on the board or is it separate?

pedmec

Normally that click noise you hear is the gas valve. put your hand on the gas valve when it tries for ignition. if its the gas valve you will feel it when it clicks. If you feel the gas valve move make sure you have the gas cock on. Everything you describe is the basic sequence of operation. The hot surface igniter will glow after the pressure switch has closed and all the safety's are in there correct position. The only thing left is the gas valve. Make sure the gas is ON. There is an on/off switch on the gas valve

"Solid red LED error codes will not be displayed". This is in there diagnostic literature.

Mooncatt

pedmec said:

Normally that click noise you hear is the gas valve...

 "Solid red LED error codes will not be displayed". This is in there diagnostic literature.

 There's a fair bit of space between the board and the gas valve, and I'm certain that click is at the board when it goes into the shut down/protection mode. And yes the switch is on at the valve and the main gas line.

Figures about that error code.

unclejohn

Maybe wrong board or incorrect wiring.

Mooncatt

mattmia2 said:

yes, on the circuit board end. and to the correct terminals, with the connector the right way if applicable although it sounds like you only have a main valve, no intermittent pilot valve. A picture of the schematic would help. Is there a fuse or circuit breaker on the board? Is the transformer on the board or is it separate?

Yes, only a main valve and no pilot. Fuse is good, and transformer looks to be separate. Here's the schematics. 

Mooncatt

Just to let everyone know, I had a tech come look at it today, and it required another circuit board. Apparently whoever installed the system didn't really think things through, as the main duct exhausts directly above the unit, and the openings of the floor vent are just the right size for kids to drop little things through, like a bead that was found clogging the AC evap drain. Apparently water had overflowed it recently (temp changes happen quick here), allowing it to work its way down inside the main body of the furnace and onto the back side of the circuit board. In other words, no warranty and a $745 bill.

I do still appreciate the attempts at helping, even if it was all for nothing in this case. New project: protecting that vent opening, which isn't a common size that is easily replaced with a better designed register.

mattmia2

Or perhaps the previous tech failed to find and fix the water problem that damaged the previous board?