I have a Trane XB80 furnace. For the last few years, it would intermittently fail to ignite and go into lockout mode. Cleaning the flame sensor has always resolved the issue. The “fix” would typically last the heating season and then I would need to clean it again the following winter.

This year it is doing the same thing. However, cleaning the flame sensor did not resolve the issue. I bought a new flame sensor and that did not fix the problem either.

The symptoms are:

- The first time the furnace fires up for the day it lights successfully and runs consecutively for about an hour.

- After running for about an hour the temperature usually isn’t at the thermostat setting yet. The furnace goes through the startup process again and has always done this.

- The HSI works and the burners light for about 2 seconds. However, the furnace isn’t seeing the flame and cuts off the gas. It repeats the process 3 times and then goes into lockout. The code is two blinking lights which translates to ignition failure.

- If I reset and let the furnace sit for a little while it will repeat the same process. The furnace works fine for a while but the next time it tries to come on it fails.

- I’m pretty sure the biggest clue to what is wrong is that it is always working on the first fire up but failing on subsequent ones. However, I have no idea what this means and am turning to you for assistance.

Thanks for your help!

 

Measure the AC voltage being applied to the flame sensor. Usually it should be 24 VAC, sometimes 120 VAC.

The circuit board may be defective and applying less voltage than it should, or no voltage at all.

 

Thank you for the recommendation, Seattle. I tried measuring the AC Voltage being applied to the flame sensor but didn’t get a reading. I’m pretty sure this is due to being a novice instead of the furnace. Below are my steps and please let me know if I am doing something incorrectly:

1. Took the front covers off the furnace.
2. Held down the safety switch so the furnace would have power.
3. Turned the thermostat to heat but set it below current temperature so the furnace wouldn’t try to light. Shouldn’t there be AC Voltage even when the furnace isn’t calling for heat?
4. Set the meter to read Voltage (AC).
5. Stuck the red end of the multimeter in the piece that connects to the back of the flame sensor.
6. Touched the black end of the meter to the flame sensor (place where the wire clips on to).

 

You need to have the inducer motor running while you check the voltage on the flame sensor.

If that gets no voltage try again during and after the HSI heats up.

 

 

Tried with the inducer motor running and while the HSI heats up. Still did not receive a VAC reading. Not sure if I'm not doing it right or if there isn't a reading. It seems odd that there would be no reading since the furnace works properly the first time it fires up and then fails on subsequent runs.

FYI - I posted a picture of the connection into the board of the flame sensor wire. On this connection you can see a few bare wires (white one in the middle). Is that normal? I noticed the other connections didn't appear to have bare wires showing.

Thanks again for your help.

 

your other lead needs to be to neutral or gorund and not to the flame sensor.

so 1 lead to the flame sensor wire, the other lead to neutral or ground.

 

Just as HVAC Tech says...


you can also use the chassis sheet metal of the furnace as ground.

So one connection to the furnace sheet metal or C terminal and the other to the flame sensor.

 

That makes sense to ground it. I tried that and came up with a very low voltage reading (around 1 volt with the meter turned to VAC). The furnace quickly went to a 5 flashing light pattern lockout which means flame present when there should not be. Unfortunately, I'm not sure what the volt reading means or if I am doing the test correctly.

SIDE NOTE: While testing, I was running the furnace with the doors off. It was FINE AND RAN SUCCESSFULLY WITH MULTIPLE STARTUPS.

Please let me know if you agree with this logic: With the doors on, the furnace is alright on first startup. It runs for awhile and the board gets hot. The next time the furnace tries to start it fails because the board is bad and can't make a good connection due to heat.

Do you think replacing the board is my best bet? Many thanks for all of your assistance.

 

Let's ignore the door removal issue in favor of continuing a logical diagnostic process

Your multimeter will need to measure down to less than one microamp DC (1,000,000) for the next test.


Put the meter in series with the flame sensor and the wire to the flame sensor. Measure the DC microamps while the furnace goes through the ignition sequence. Report of the reading you get.

 

Set the meter to read DC microamps. The furnace started up and ran (with flames) while I had the leads connected. The meter consistently showed 1 microamp.

 

The manual for your furnace is at

http://www.butcherdistributors.com/I...erior/tue1.pdf

Page 19 lists the diagnostic codes that may signal problem on your furnace. See if your furnace has that diagnostic system and if so what the codes read.


Typically a furnace will produce 4-5 microamps of flame rectification current when everything is working normally. For that reason I'd consider your 1 microamp read to be low.

One microamp is about where the furnace ignition control starts shutting off the gas. So I'm not surprised that you are getting erratic operation of the furnace with that reading.

I'd cycle the furnace through the ignition cycle 20-30 times times and see what range of flame rectification currents you get. Ideally you'd be measuring the current when the flame shuts off when it shouldn't.

Since you are getting marginal flame rectification current with a good flame sensor, I suspect you have a bad ignition control module that needs to be replaced. Running the flame rectification tests described above would help confirm that conclusion.

Erratic flame current is what I would expect to find. You may find that when the burners shut off you are getting a reading of .5 microamps or less. The 1 microamp read might cause reliable ignition or it might cause erratic shutoffs ---- hard to say with a low read like that.

Anyway, more data points with more ignition cycles would provide useful additional information.

[just as an aside, when I was doing furnace repair work I'd probably be at this point about 15 minutes after starting work on the furnace!]


But don't feel bad, DIY takes lots longer. You are doing well!

 

Many thanks for all of your help Seattle. The furnace does have the diagnostic system referenced in the manual. When the furnace fails to properly ignite the board gives two blinking lights which translates to “Retry Limit Exceeded”. This is when it goes through the startup sequence three times in a row and doesn’t recognize the flame.

I am going to see if I can borrow (or buy) a good Fluke level multimeter before I do too much additional testing. The one I have is cheap and doesn’t read down to tenths of a DC microamp. If I continue to get low or inconsistent flame rectification current reads I will try changing out the ignition control module.

Yes - this process is definitely taking a million times longer than a pro could do it However, I am learning lots and guess that is the tradeoff for DIY… Thanks again for the assistance.

 

>



Yep. It takes me a lot longer to DIY on a project when I'm learning too!

 

The flame rectification current flows from the flame sensor, through the flame to the furnace sheet metal (ground) and from the sheet metal back to the ignition control box.

So the ignition control needs a good ground. Some ignition controls have a wire to the furnace sheet metal, others have an obscure means of grounding. If there is a wire to the furnace sheet metal, check it and perhaps take it out and reinstall it if it's held in place by a screw.



A poor ground could be an explanation for a low read, so this is a possible solution that is worth trying.

 

Thanks - I remember seeing a copper wire about the size of a coat hanger attached with a screw. I'm assuming that is the ground and will check/reinstall it. If that doesn't work I will try to get some additional DC microamp readings with a more accurate meter and possibly put in a new ignition control module.

 

I borrowed a good Fluke meter (instead of using my cheap one) and have been taking new readings. Going from the flame sensor to ground I'm getting around 9 VAC. I believe this is supposed to be 24 VAC so it seems to be low.

The reading from flame sensor in series is 0.4 microamps DC. This number should be around 3 microamps DC and is also very low. When the furnace fails the number is even lower than 0.4 and the furnace cuts off the flame.

I did unscrew the ground wire and reattach. I was getting around 12 VAC and now get 8-9 VAC but I'm not sure if that is related.

Should my next step be replacing the ignition control module?

 

>



Yes, a good flame rectification current read is typically 5-6 uamps.

.4 uamps is way too low and is why the burner is shutting off.

Since you have a good ground and flame sensor, the explanation for that deficiency must be the circuit board. So you need to replace that.

 

Put in a new circuit board (with help from my cousin). The flame sensor measurements are now within the "normal" range and the furnace is working great. Many thanks Seattle for all of your help!

 

Hi
i have a trane xb80 furnace that was blowing cold air and as i read in this forum and according to the led code with flashing 4 times i found that i should change my swith limit , so i took out the switch it was l190f30 but the shop gave me l180f40 and told me it should work . I installed it on the furnace but it was blowing again colx air and whenever i was trying to put the temp above 23 it was stoping blowing air . And after a while it completely stop working . Now it's not working at alland it seams ghat there is no power in the furnace and the light is completely off . Please help we what to do ?

Read more: http://www.doityourself.com/forum/ga...#ixzz3MwAMDF7B