GE Profile Electric Convection Oven, intermittent overheating, even after fix - Page 1
Forum section: electronics / repair

author: incf
author_rank: Frequent Contributor
timestamp: May 17, 2025, 04:05:11 pm
content: I've got a GE Profile electric convection oven. It uses mechanical relays to control its heating element.This sort of convection oven has the following components:A front panel display PCB module (with 7-segment display, capacitive buttons, and microcontroller) that sends command to the heating element control PCB via serialA heating element control PCB (with microcontroller) that has several large relays powering the heating elements and several small relays operating fans, lights, etc. It also is responsible for reading the temperature sensorA (100 ohm?) Platinum RTD temperature probeA few fans, a lightbulb, etc.This oven was formerly used by a commercial bakery. Within its lifetime it has accrued thousands of hours of service. I frequently use it for long duration low temperature / slow cooking of foods.One day, while it was operating, it overheated and smoke stated issuing from it (bits of grime/food residue on the bottom of the oven chamber getting carbonized). I turned off the breaker powering the oven.The next day I turned the breaker back on, but never 'turned on' the oven. It again heated up to the point of generating smoke (due to small amounts of food residue burning off). This occurred despite it not being 'turned on' (to start the oven you have to select a particular oven mode, enter a temperature, and press start).I decided that the relay powering the heating element was likely welded shut. I ordered a used replacement heating element control PCB.(But I never directly measured the heating element control PCBs relay contacts, I just assumed they were welded shut due to the previous test.)The replacement PCB arrived, I replaced the old heater control PCB. The issue disappeared.The oven ran flawlessly for one or two weeks, then the oven overheated again. (I turned off the breaker)However this time, it is intermittent. The heating element does appear to be stuck on this time (or at least, if it ever was stuck on, it became unstuck when I went to test it the next day)This is why I am posting. I am no longer sure of my diagnosis or my remedy.As a sanity check, I decided to check the relays on the original PCB I had declared defective. None of the relay contacts appeared to be welded shut. This contradicts my original test where the oven heated up without me selecting a mode and pressing "start."----------------This raises the following questions:Can relays with contacts that are welded shut come unstuck by themselves? (eg. is the jostling of removing a PCB from a chassis really sufficient to do that? seems unlikely to me)After the first failure, could a second independent failure have occurred. For example: Could the relays on the replacement PCB be good and the wiring connecting to RTD temperature sensor have become intermittent?(Could I have fooled myself with my original diagnostics? Either reaching the wrong conclusion, or even making incorrect observations? seems possible)(Could some other failure mode explain my original diagnostics other than a stuck relay?)When the oven powers on (ie. the breaker is turned on) relays clack inside. When it is put into oven mode more and the user presses "start" more relays click inside. I have to wonder if it performs a self test or something at power on (which would provide an opportunity for something other than a bad relay to explain my initial findings).There are no error codes. The display unit seemed oblivious during each of the three overheating faults. (I never recorded what the display showed any of the three times, but I judged it to be in the correct mode and displaying the expected mode or temperature setpoint numbers each the time)edit: attached example photos of what a "heating element control PCB" looks like (taken from an ebay listing, not photos of my particular replacement boards, but similar enough) Model WB27X25554.edit 2: the oven is a "GE Profile™ Series 30" Built-In Single Convection Wall Oven Model #: PT7050SF2SS"

author: Xena E
author_rank: Frequent Contributor
timestamp: May 17, 2025, 08:38:50 pm
content: Those relays can certainly weld their contacts.And yes also become unstuck with handling.Can you replace the relevant relay?It may be worth checking if the relay drive is disappearing but the element supply is being held on... would require a little surgery to bring the relevant connections out of the device to check...X

author: incf
author_rank: Frequent Contributor
timestamp: May 17, 2025, 08:49:53 pm
content: Quote from: Xena E on May 17, 2025, 08:38:50 pmI need to dig up a wiring diagram (or see if there is one inside the oven chassis someplace)I don't know which relay failed (if any). And as far as I can tell, it almost looks like there would have to be two relays in series conducting at the same time for the oven to fail in the way that I observed the second time.Another challenge is that both PCBs (new and the replacement) have a thick conformal coating, and testing it requires it being buried inside the chassis of the oven.

author: floobydust
author_rank: Super Contributor
timestamp: May 17, 2025, 09:22:01 pm
content: You might be on a goose chase. Last time I had a problem like this, it was the temperature sensor failing. Some develop an internal short (even to GND as well) that gives bad temperature readings, and the oven might think it's simply below setpoint. Check the RTD or just replace it.

author: incf
author_rank: Frequent Contributor
timestamp: May 17, 2025, 09:26:32 pm
content: Quote from: floobydust on May 17, 2025, 09:22:01 pmI was wondering about that. I've ordered one just in case.

author: incf
author_rank: Frequent Contributor
timestamp: May 17, 2025, 09:28:45 pm
content: Another thought just popped into my head. Could flipping a breaker while the unit has the heating elements energized have caused the relays to weld shut the first time around?Maybe there might have been contact bounce (or some equivalent) when manually opening the breaker. And maybe the relay coils would not have been driven properly (fully on or fully off) while being exposed to that abnormal condition where the power is being cut by a slow moving mechanical contact.

author: Xena E
author_rank: Frequent Contributor
timestamp: May 17, 2025, 09:35:53 pm
content: Quote from: incf on May 17, 2025, 08:49:53 pmYour initial fault seemed like it fitted the stuck relay explanation...yes a schematic would be revealing and save a bit of time fault finding.Having slightly different faults after changing the board would suggest that the replacement could be faulty as well perhaps.As for a control fault, can the oven display an approximate temperature measurement while cooking, or just the setpoint?Most quality breakers have a good snap action to prevent arcing so I dont think that would actually cause a fault.X

author: incf
author_rank: Frequent Contributor
timestamp: May 17, 2025, 10:01:25 pm
content: Quote from: Xena E on May 17, 2025, 09:35:53 pmWhile it is preheating it alternates between current temperature and setpoint.But once it reaches the setpoint I do not recall ever having seen the number change, even after opening the door. (But I could be wrong, it would be helpful for diagnosing a sensor fault if it did display current temperature all the time)

author: floobydust
author_rank: Super Contributor
timestamp: May 17, 2025, 10:46:15 pm
content: Do confirm the ground on the control board LV/sensor side. Not sure where that connection is. What is R238? by the ground faston, is that a fusible resistor?I have see a ground fault or open happen from wiring or a melted switch, which causes problems with the RTD amplifier with too much AC hum.Some control boards are even not isolated from mains, with those a ground fault in the oven can make it eat control boards.I think this board is isolated, thankfully.