Ask Me Help Desk - Open High Limit Device

I have a Trane XB80 furnace that has two high limit devices. About 2 months ago, I had both switches replaced because the furnace would not go into a fault condition with the technician there (of course). He said that these high limit devices are automatically resetting which makes it nigh impossible to determine which of the two swithches was faulty. After replacing both switches, everything worked fine until now. I was there when the technician replaced them and the temperature readings he was getting were well within parameters so I don't believe it is a filter or blower issue.

I am now getting the same fault - 4 flashes of the LED. I can flip the switch at the circuit breaker to reset the furnace and it works normally for about 24 hours and then I get a repeat of the fault. Very strange. My guess is that it is a circuit board problem. Would this be an expert's opinion as the most likely problem at this point or could it be another faulty high limit device (which would seem to want to fault every time the flames flipped on).

Quote:

Originally Posted by ac7rt

I have a Trane XB80 furnace that has two high limit devices. About 2 months ago, I had both switches replaced because the furnace would not go into a fault condition with the technician there (of course). He said that these high limit devices are automatically resetting which makes it nigh impossible to determine which of the two swithches was faulty. After replacing both switches, everything worked fine until now. I was there when the technician replaced them and the temperature readings he was getting were well within parameters so I don't believe it is a filter or blower issue.

I am now getting the same fault - 4 flashes of the LED. I can flip the switch at the circuit breaker to reset the furnace and it works normally for about 24 hours and then I get a repeat of the fault. Very strange. My guess is that it is a circuit board problem. Would this be an expert's opinion as the most likely problem at this point or could it be another faulty high limit device (which would seem to want to fault every time the flames flipped on).

My guess is that it is a circuit board problem. Would this be an expert's opinion

Usually a expert would test the control systems,check the gas pressure and firing rate to make sure the unit is not overfiring from too much gas pressure. If all checks OK then and only then would a circuit board be replaced. I believe your problem is NOT a circuit board.

Read inside the blower door diagram what 4 flashes mean. Then post back.

You might also clean the falme sensor if so equipped.

The four flashes indicate an error called "open high limit device". The technician that came to my house a couple of months ago did several tests. I don't know all that he did other than knowing he was testing the temperature in the vicinity of both of the sensors and he remarked that the temperatures he was getting weren't even close to causing the sensors to have a fault. Since we couldn't get the furnace to repeat the error while he was there, he suggested that we might just want to replace both temperature sensors to solve the problem.

On this circuit, there are only 5 items: 2 temperature sensing devices that have both been replaced, 1 pressure sensor that appears to have a separate report of 3 flashes if it fails (so it has apparently not failed), and finally 2 sensors of some sort that my technician said are not "resettable". I understood if one of these 2 sensor failed, it would be obvious which one would need fixing and the furnace would not work at all. Therefore, everything on this circuit seems to be working correclty. I suppose one issue could be that one of the replacement parts was also faulty, but again, it is odd that this furnace works fine for extended periods and only randomly has an issue.

Therefore, I am left with thinking that his tests were valid, leaving only one component - the circuit board. I'm just trying to see if other, more knowledgeable people, come to the same conclusion or think I should have the technician test other things.

I do not believe that parts are to be replaced unless they are proven defective. The error of Open High limit device means that the furnace is overheating OR the high limit control is defective or out of range allowing the unit to shut down when it should not. Since this is the case I would replace the high limit control device and see how it performs. If that fixes it OK and if not go to the next step. I can only offer common sense since I am not there to do the testing. The high limit is much cheaper than the circuit board to start with and the high limit is causing the problem from what you said.

I thought I should share this with the group. I found the problem, or I should say the technician found my suspected problem. It was the circuit board. They removed the circuit board and the back of it had some burn stains on it. Thank you all for your ideas.

OUCH circuit boards are expensive. I was hoping that it would be the cheaper part.