LG Range LRE30451ST/02 Oven Overheating Despite new main PCB, Thermistor. Relay board tests OK.

[PABbles]

Model Number

LRE30451ST/02

Brand

LG

Age

6-10 years

Help!
I have an LG LRE30451ST/02 that has me stumped. The oven is overheating. Set for 350F, actual oven temp will be over 380F and the oven display is telling me it is at 230F. I have the service manual and went through the troubleshooting for this issue. I tested (tested OK) and replaced the thermistor which didn't fix the issue. I tested the relay board to check for welded relay - tested OK. As a result, I replaced the main PCB thinking that it was not properly reading the resistance input from the thermistor, and, in turn, was not telling the relay board to turn off the bake/broil elements (which also both tested OK) when necessary. My understanding is that the relay board on the back of the oven is "dumb" in that it takes its commands from the main PCB re: when to turn on or off. No temperature decisions take place there. The kicker here is the incorrect temperature being displayed on the oven display. If it were reading correctly, I'd think that the relay board was bad, but with it still reading the wrong temperature, I can only think that either I got a bad replacement thermistor, or a bad replacement PCB, or both. I tested the new thermistor and the room temp resistance is in-specification.

Before I go and spend another $120 on the relay board, is there anything else I should be trying? Based on the wrong temp displayed, I can't get myself to believe that this is a relay board issue. I've already checked out this thread: https://www.applianceblog.com/mainforums/threads/lre30451st-lg-range-oven-overheating.64827/ and it doesn't cover what I'm looking for.

Any help is appreciated.

 

[PABbles]

I've been chasing an oven over heating issue on an LG LRE30451ST electric range. Set at 350F and runs way past to over 500F. Placing a thermocouple in the oven, the oven (display) thinks it is at a lower temperature than it is actually at (thermocouple). First troubleshooting step was to test resistance of the thermistor. It tested @ 1080 Ohms @ 70F (normal), but 1330 Ohms @ 350F and 1440 Ohms @ 500F. Appliance thermistor resistance charts available to me indicate that 350F should be +/- 1654 Ohms and about 1950 Ohms @ 500F. I replaced the thermistor which didn't fix the issue, so I then replaced the main (upper, not the relay board) board. That was the next step per the LG service manual. Issue still persists. Relay board tests fine, as do the elements. Also, continuity check from the thermistor plug to the main board plug was fine. Wires are good, no cut wires...etc.
Here's the question: How often do you see out of box failures on new thermistors? I'm going to run the same test on the new thermistor tonight to see how it compares to the (allegedly bad) one I replaced. It tested at 1080 Ohms @ room temp but I have not temp cycled it yet like the old one to see what it does over temp range. This all before laying out another $100+ for a new relay board which I don't believe will solve the issue. Since the oven display is showing wrong temp (vs. thermocouple reading), I'm thinking that the thermistor is still bad (or the new main board is bad ). If I test new and resistance tracks the same as the old, then I either have 2 good thermistors & a bad main board (unlikely), 2 bad thermistors and a good main board, or 2 good thermistors, a good main board and a bad relay board (unlikely due to the display temp readout being wrong). Any advice appreciated.