Nice explanation. Similar error condition I came cross on Goodman GHM95. After several roll-off limit switch changes and several days intermittent 4-blinks problem on and off. After lots of frustrations I found the problem was on board 12 pins connector. During investigations moving wires was correcting the problem but I thought it was corrected by changing switch. Finally I pull out the connector and put dielectric grease on each contact pins and place it back on board. Now no intermittent for more than one week. Several years ago I had similar intermittent problem with the gas valve connector on other furnace. I remember I applied the similar power contact grease (used by the electrician) and that fixed the intermittent gas valve problem on Goodman furnace. So my conclusion is these furnace control harness connectors pins contacts are not gold or tin coated or not under good contact mating pressure and by time it develops oxidation or loose contacts. The dielectric grease helps to maintain good contact i.e. solve intermittent contact problem. So if you see such intermittent issue then may try this.

I know this video is old but I wanted to thank you for saving me THOUSANDS of dollars by showing me this. Much love from California

I so enjoy your videos as I find myself verbally saying what needs to be done next and then watching you do it! Awsome technician you are!

Love the videos. Between you and mr.lavimoniere you guy's have the best videos. Straight forward and not a lot of dialogue like a few others on KZfaq. Keep up the great work.

Thanks for making this posting. If I didn't have my Fluke on the flame sensor input when it faulted, I would have been scratching my head for another day. The tech from the local hvac charged me 300 bucks to change out a motor start capacitor without fixing the problem. You helped me fix my problem in less than 24 hours whereas I was left without a solution before. I'm ready for winter!

Thanks for the video im a mechanic and was testing resistance on the whole high circuit and it was fine so I was lost, finding this video helped me find the problem thanks.

Another great video Ted. Keep them coming. I’m a Trane dealer too over a thousand miles away from you yet learning from some of your videos and enjoying watching them all.

Thank you for the video. I had the same board in my Trane furnace. I had suspected a problem with the board during the summer with the AC but it seemed to start working again. I had already ordered a replacement board but I put it on a shelf. Now the heat was doing what you showed. I tested the sensors (good). I made sure the power was off and replaced the board. (One wire at a time from old board to the new board). We have good heat now. 🙂 I would not do this myself but I'm fairly handy. Lol. I worked for Trane (manufacturing) for 12 years.

Thanks for this video ! Solved my no heat condition this morning by jumpering the blower assembly high limit switch - now I just need to find a replacement

I’ve ran into this same issue on a hotshot garage heater. Board was sending out an unstable low voltage signal on the limit circuit, throwing high limit codes. The most annoying part was that power cycling the board made the problem disappear, which is probably what happened in your case too. If you were to measure the voltage to ground on that circuit while it’s showing an error code, you’d probably find very low voltage. I measured as low as 9 volts AC. Took 2 return trips to figure it out, now I know. Catch the problem in the act! Don’t power cycle the board!!

Great video! Helped me troubleshoot & temporarily bypass my switch until the store opens in the morning! I will be saving this video as I have this exact model furnace in the mail part of my house. Thanks!

You taped the door switch, I'm calling the cops lol. Good diagnosis Ted. The extra stuff keeps them calling you back.

Thank you for the awesome video. Exactly the same problem, was going to replace reverse flow limit ,out of stock. Pick up and replaced board (kit) comes with new ignitor. Thanks again Merry Christmas!!

I carry a contactor off an condensing unit with two long leads on it and I put one lead at "c" on circuit board and use the other lead to test for 24 volts. You don't need to look because you can hear the clicking. I wire them into high limits and you can hear it drop out without holding leads there the whole time. A meter will sometimes show 24v but doesn't tell you how well it will flow under load (amps). A chattering relay in that instance will tell you everything you need to know. When testing many things in a system the relay is soooo fast. Go to a roll out "click" next test other side "click", etc. you can test a lot of things in a hurry without ever having to try and see the screen in a dark or cramped area.

This is EXACTLY my issue! Even same error code. I'm the home owner, I tested the limit switch which was fine but I replaced it anyways. Board still had same issue and no flame. Thanks! I can replace a part but I'll have the service tech out to wire a new board up.

Good job your the best teacher and technician, God bless sir,

2 screws = no call back from you causing the squeak. Doesn't get any better, smart move from your years of experience. 👍🏻👏

Mr.Cook. Excellent tip ,and video! Appreciate your time sir,and showing possible code error on fan - limit.,( computer board), Thank you for explaining and demonstrated in your clip..

Mine is intermittent and I had then same code 4 and checked all the switches and sensors which were good. I changed the board and did not even turn on. Put back old one and worked fine for an hour. When it was inop, I tapped on the board and started working. Apparently The new board required a jumper from R01 to Ro2, so i changed it back in and put the jumper. It now just turns on the main fan, nothing else. I put the old board back in and works for a while then I tap on board and works. Very frustrating as I know the Board is bad but cant get the new one to work. Same company, White-Rodgers unit too!

It's nice when KZfaq videos teach you something and get to the point instead of watching a a guy trying to figure something out

Thanks for vid. I noticed the control board was a White/Rodgers board. Did Trane use White/Rodgers boards as original equipment? Just a thought

I've run into this many times on this board. I believe the molex pins go bad inside the board and get loose which breaks the circuit. I find you wiggle the molex you can start the furnace up proving the board is faulty

Some jumpers on that board are corroded, the blue discoloration is corrosion. It looks like it's caused by the silicone applied over it which is possibly corrosive. You'd need to dig those jumpers out and replace them.

Here's one. Got sent to a apartment to check out a wall heater that was putting out high carbon monoxide according to the electric company. Got there and opened it up so I could check it out. Found signs of rollout by the burner orifice and upon lighting the heater and turning on I discovered the burner was 99% clogged. Cleaned it out really good so there was no longer any plugged areas and then had roll out across the whole burner constantly. Tried to turn the burner pressure down and it didn't matter what I did I couldn't get the gas pressure to go down at the valve. Wound up replacing the whole heater instead of just the valve. Not fun when you also have to remove a ball of dust bunnies the size of a cantaloupe from the area under the wall heater. That scared me when I pulled all that out from there because that's years worth of fuzz and the tenant hadn't lived there that long.

Anti DIY HVAC, You Sir are Quality, do it right, fix the little stuff at no charge, show and tell your experience. Good Customers always appreciate that level of attention, I do the same thing. I usually clean up the equipment too, that helps me see later on if anything else has happened especially with condensate, water leaks and so on. I’m trade as well, licensed Master Journey in De & Md, but Middle East Coast, USA, building codes are a bit more strict, but not as strict as say Massachusetts or California. Y’all southern folks do a lot of stuff with flex and duct board that we don’t because of code. I don’t see it as a problem it’s more a regional building thing, if your house is gonna float out to sea every 20 years in a hurricane why bother? Lol kidding. Duct board has the advantage of being easy to work with on site and not requiring a metal shop that needs a mechanic with a good bit of experience in metal fab. I’m not a duct board fan, I’m a tin knocker 3rd generation, so I’m a bit prejudiced about that.

Boards with intermittant and hard limit errors are usually a result of a break in a solder joint (cold solder break). Not easy find visually since they don't burn, just crack around a pin leg. Often caused by thermal expansion/vibration + insufficient solder from the factory. Solder breaks are completely recoverable and only the cost of solder and time. Prohibitive though becuase of warranty/return call risk/time=money.

Great video, thanks for sharing, I really liked following your diagnosis and have the same issue. Thanks also to bunch of commenters here! We were prepping for a pretty chilly night and morning, but then I saw comments like "push on the wire connections to the board" and "flick the chips on the board." I did that and the heat came on! haha We'll have a warm night and a new board tomorrow, so awesome.

Those blue ceramic discs almost never fail; they are x1/y2 safety capacitors, and made to take AC line voltage (they're high reliability). That high tolerance +/- 5% resistor bank has generated its share of heat, quite possibly, that's where the error is. The IC appears to be used in a comparator function, if those resistors are off tolerance, the chip will read errant signal/voltage, and most certainly have triggered the default safety position you've encountered.

I had one just like that. I ohmed out the limit string which was fine. I noticed that the 2 yellow wire connectors were loose in the molex plug. Tightened them up and all was good no call back.

I had a situation where a low voltage wire harness was loose at the molex connection on the board and the board was a giving a high limit code. Any time you’d move the harness the furnace would operate normal. Changed the board and no issues since. American Standard furnace btw.

i have seen that board run the inducer non-stop like you described many times... if your ever in a pinch and parts are not on hand, you can flick the relays around the inducer motor jack plug to get it to run normally, usually for a limited time since the relay is pitted and will eventually fall back into the stuck closed position. sometimes it doesnt but most customers don't like the unreliability and opt to replace the board. i would guess your fiddling with the wires was fixing it temporarily.

What is the part number for the new board??

Do you not see the brown areas of concern on the back of that old board?

I had the same model, same issue. shot both open limit sensors but still getting 4 flash error code. Replaced control board and it worked at once. The control board is dang hard to find though

Firstly, your video's are great!! As a Licensed HVAC Contractor here in Connecticut I'll weigh in on your taping door switches. Here we don't have many crawl spaces with foundation vents, we have basements. If Joe Homeowner tapes a door switch and leaves the Blower Door off it will pull the basement into a vacuum. Causing roll-outs on gas, melted fuel pump couplings in oil burners, Carbon Monoxide in the home. So, I'd just ask that you consider your audience. Everyone watching doesn't have your level of training and doesn't know what you and I know. They're just trying to save a buck or maybe just enjoy being self sufficient and doing their own repairs. Anyhow...............Your a big boy fully capable of making your own choices. Best of luck

This is exactly what is happening to me. Error code 4 but the limits rang out with no issue. So frustrating. I could not understand why it was working and then not working, I thought each time I had fixed it, but now I know. I am going to go ahead and change the board.

That is an ongoing problem I have had to deal with even in California! A defect!

I had a similar failure in a similar-vintage Trane furnace just a couple weeks ago. Actually it started in November - I was getting a high limit warning. The high limit switch seemed to be all over the place as far as continuity so I went ahead and replaced it. Then about 3 weeks ago I started having lockouts...my ignition sequence would run normally, except that when I got the relay click which was supposed to open the gas valve, the gas valve didn't open. I reseated the 12 pin connector a couple times, the issue would resolve for a few days, and then the problem would return. It was weird because whenever I shoved my voltmeter probes into the pins for the gas valve, the issue would resolve and I would have voltage. I did put the voltmeter on the gas valve and found that even with the click of the relay, I wasn't getting voltage to the valve. So I figured either a failing board or cold solder connections for those pins on the board. I thought about taking the board out and trying to melt down the solder connections...but I didn't trust myself to make that a lasting repair. So, after this happened 3 or 4 times I bought a replacement board and installed it. No troubles since. Yes, I am a DIYer, so skewer me...but I have significant experience testing and troubleshooting electrical circuits (I am still what you'd probably call a "hobbyist" though.)

how do you like the klein 8 in one hvac screwdriver? one of my favorite tools in the box lol

Try A voltage drop measurement; I can’t imagine there’s any great amperage running through that limit, but you never know.

Home owners do pull the doors off. I believe that is why the door switch is there in the first place.

The Ruud/Rheem blower limit if it gets any dirt goes bad and has to be replaced it will not send 24v to gas valve.

Blue and yellow outdoors.....a true Trane Man.

How much was this board

A digital multi-meter can occasionally mess you up. In my experience on commercial aircraft I have seen continuity using a digital meter and high resistance using an analog meter. The digital uses less current and responds quicker than an analog meter. Just food for thought.

Yeah I've had the exact same thing happen to me before, ended up having to go back out to change the limit. Now I just change them if there's any question.

Good job Ted - like your vedios!

Trane XB80 f.a.g.heater 4 flashing codes say bad high limit ( same thing as yours ). I checked the HL and rollout switches for continuity all good. Has no hsi glow action, hsi has continuity

Why is there a high limit on the blower. Would this be if the motor failed?

I’m a homeowner and fix all my own. That switch would be wired past. I haven’t been beat yet. I studied for the hvac test and might take it so I could buy Freon.

Came across this same problem today, It was the 12 pin plug... Unplug it blew it out plugged it back in, walla

Did the fan shut off when the wire fell off the jumper at 9:45 or was that the switch? Mine runs without the sensor with wires loose not jumpered at blower. All 4 sensors/rollouts have ohm readings when cold. Wire end has burn mark in on clear covering but sensor is clean.

Copper pipe on Natural Gas oxidizes from contact with the gas and throws off flakes of oxidized copper that can plug the gas valve. Yes, there's the dirt trap, the extended and capped piece of black pipe where the copper connected, but there's no sense in taking a chance. I realize you didn't install the pipe, just a general observation. You tape door switches and leave them taped, documenting your negligence should any mishap happen at the residence. The door switch is to keep the unit from running without the door. On most forced air furnaces, an open blower compartment will draw combustion gases in with the air rushing in the removed, or displaced door, that may cause serious harm to the occupants, or worse. Televising that you regularly defeat a safety feature just doesn't seem like a good thing to do. Lawyers would have a field day. A side note, remember that pulling air from an open furnace door might even pull flue gases from a nearby water heater. Not something that should occur beyond the diagnoses and repair. Limit switches. I've talked to techs that were surprised that limit switches can be diagnosed by taking voltage measurements across the limit, or across a group of limits while you narrow down the culprit. Assuming only one limit switch is bad, and that they're all N.C., the open one will show the full, or nearly full, control voltage across it. With limits that exposed, it can make short work of diagnosing a bad limit switch. In any case, good video, nice work.

The factory should put a support brace on the left side of circuit board aluminum panel.

I have a question if you don't mind. I lived in Texas before retiring to the Philippines. We want to get a part time place back in the states. The place we are looking at has a HEIL system throughout. In Texas had the same but really it was a"made in HELL"unit. What's anyone's opinion on that brand? Thanks.

At 16:39 looks to be a varistor (voltage dependent resistor) usually marked on boards as RV.

I had a magnet fall off a magnet tray . I use it to hold the door switch while I diagnose now

as a HVAC person , do you carry a supply of limit switches? Im a saturday mechanic. Im also wondering about ingiters? Do you carry a supply?

Why did you change board when high limit was bad?

Great fix I wish you would have stopped the squeak first sounded like a low batt in a detector

So the limit switch was good and the board was bad. Have you found what’s wrong with the board? Can I short limit switch to make it alway close?

why 2 furnaces in the attic? shouldnt 1 be in the basement?

Had a tech at my aunts place say she needed a new board, baught a new board because the high limit switch showed continuity right after it would cut off. had four flashes but I pulled it out and let it run out of the hole and it wouldn’t trip so all it that was wrong was the high limit switch. The place needs a return trunk line put in the house was set up for a common return but threat turned out to be a mess.

I looked at a newer high efficient train gas furnace and it kept flashing open limit or roll out I tested every limit and roll out and they were all closed I said it needed a new board the owner got a train guy over there and he said he got it going without replacing the board do you know what he did

a good learning moment!

When did screws become free! JK in Colorado you must use a flex connecter after valve.

I always start off doing continuity tests on the limits, maybe jumpering them is better? 🤷

Are these circuit boards still available? The guys who services my furnace says so, and I’m not trying to by a new furnace.

Ever had the main blower work but have to reset switch? Seems to happen on one I've worked on many times. Shows limit switch open in light code.... test them and they're fine...jump... still doesn't work... reach back far enough to push a reset on the main blower....works....🤷

I usually just check the safety’s for continuity right off the bat. Takes less than a minute. Beats jumping every one out.

Hello sir, where can I find this limit switch I try many store but I can't find

Have a fan limit switch at 2 ohms is that good?

Had the same squeak on a american standard zip screw work ed for me also

Why does my furnace have. 2 High limit switchs on the Blower Fan Then one one Furnace?

Dude, where the hell is your multi meter

Was that a board that required the igniter be replaced as well?

Why the tee and pipe pointing down and tube fitting to top?

Thanks. Saved me some cash

If it helps anyone, on my XB80, after all my limit switches tested out OK and I determined it was the control board, in my case I got it working by tapping on the black relays on control board (go figure). Ordering a new board but we have heat in the meantime.

I had a similar issue with a Trane package unit checked static pressure was good temperature split was good gas pressure was right went to HL switch jumped it worked a few times the. Kinda guessed it was the bored, any tips to determine it's the board quickly rather then testing all things?

even with the new board,I still notice a flash code blinking!couldn't tell how many flashes they were blinking

Good work as usual.

I am getting a false positive code what do I change out its an older model trane xr80 013

I thought the big copper loop was for earthquake protection. Dont usually see gas in copper anymore in the NE... my area anyway

I don’t think you tested that bimetal switch correctly. Those switches are a bimetal snap switch based off heat. They are normally closed or normally open switch and can only be tested with a match or lighter to snap them and measure resistance.

Thanks for this video, it's just what I needed!!

B340793P06 Where can I find this limit switch

Nice job again like always

Great job 👍

You’re putting your fingers on the leads the meter is reading continuity through your body.

Then there is always the possibility of the NEW board being faulty, especially if care is not taken not to zap it with static electricity, with bare sweaty finger tips. Besides, there is no need for that high tech board anyway... all the functions of a gas furnace or even a heat pump can be covered with simple relays. If you really want to get technical you can just add a simple timer chip, and use that as a delay so as not to cause excessive wear on the compressor. Too much unnecessary technology, in places that its not needed, is nothing more than additional points of falure.

Second time watching 10/10/2020

Red and White was back in the old T87F days...LOL

Great video ty

Great video

Do you endorse that I, a homeowner clean my outside coil with NuCalgon foaming spray and a garden hose?

Would not leave the door switch taped especially in Ca. Your just asking for a lawsuit :)

I got the same code but the problem wasn't the board it was wire harness plug

I'm having the same issue with white Roger surelight, jumped all the safeties and still won't start.

i got a board coming may be my issues