LG LST5651SW stove: oven not heating properly

[Curious808]

LG LST5651SW stove: The oven has a hidden bottom (bake) element that seems not to be heating up, but tests OK.

PROBLEM DESCRIPTION: Set oven to 400. The display shows it heating up normally to about 365, and stops there. The top (broil) element is visible and heated. Can't see the bottom element, but the bottom of the oven isn't extremely hot. A wet cloth on the bottom doesn't sizzle and doesn't really heat up. The stovetop burners seem to be working fine.

TESTING SO FAR:

1) Removed the back panel and leads to the bottom element. Using multimeter, continuity test is positive (continuous tone). Ohm test reads about 16.5 ohms; the product sheet says it should be 17 ohms.

2) Removed the sensor for testing. Plug openings too small to fit multimeter probes, so inserted wires into the back of the plug where the sensor wires go in. No continuity tone, but ohm reading is 1,100; product sheets says it should be 1,090.

It seems the bottom element is not heating, but is OK. Not sure why the sensory continuity test fails, but the ohm reading seems OK.

Any idea what to do next? Thanks!

[CapLeaker]

What were the oven settings during the test? The bottom element only comes on in the bake and convect bake function.

[stj]

you metered the element cold, it could go open as it heats.
put a high wattage lamp in series with it to see if it draws current continuously
or use a clamp-meter

[Curious808]

Originally Posted by stj

you metered the element cold, it could go open as it heats.
put a high wattage lamp in series with it to see if it draws current continuously
or use a clamp-meter

I don't have either of those. Is there another way to check?

I haven't actually removed the element, only disconnected the wires. Would I be able to get a more useful reading if I plugged it back in and turned on the oven for a few minutes before testing? Or if I removed the element, would it likely show physical damage?

[petehall347]

you could check the voltage is getting to it

[Curious808]

I removed the element. There's no damage, corrosion, anything like that. It looks perfect. There's no visible damage and the resistance is to spec -- does that mean that it's fine?

[R_J]

It looks like there is a oven relay control board in this stove, I would check the relay solder connections, then check the relay operation.

[redwire]

LG LST5651SW Parts with 6871W1N012B oven relay board like https://www.reliableparts.ca/product/inv_16906185
You should be able to hear the relay clunk, or swap the wire with the bake/broil heating element.
edit: weird the board shows no "BROIL" spade terminal, even the 6871W1N012A with extra relays doesn't show it.

I would do a visual for bad solder joints on the relay board, including the connector to the (other) oven control board.

[Curious808]

More info:

No error code shows on the display. On the product sheet, there's an error code table, with "F7 Main oven heating error" that covers both oven heating elements, sensor, and PCB.

It seems to get up to around 365 and stops. I'd been setting it to 400 450 to test. Now I set it at 300. When it gets to 300 and the preheat complete alert -- beeping and flashing oven light -- goes off as normal.

[redwire]

It sounds like there's simply not enough heat to get the oven past 365° with only the broil element puttin' out.
As was mentioned, the bake heating element (or relay circuit) could be going bad, open-circuit when it's hot.
You're going to have to troubleshoot it. I could not find the LG service manual which has a test mode and wiring diagram to make repair easier.

Post pics of the relay board backside, I suspect a intermittent connection to the bake heating element or a bad relay.

The temp sensor if bad will show the wrong temperature or an error code. It looks fine. A multimeter continuity test is for wires and connections, readings under 100 ohms beep so it's normal for a thermistor to fail a continuity test at 1,000 ohms.

[Curious808]

Originally Posted by redwire

Post pics of the relay board backside, I suspect a intermittent connection to the bake heating element or a bad relay.

Here are photos of the back and a close-up of the relay board.





I marked the relay board photo with three red lines. Any combination of these test OK for continuity. Of the six connecting posts on the board, no other combinations test OK for continuity. If that means anything...

Please excuse my ignorance. I have pretty good common sense, but little electrical/electronics knowledge. I'm learning as I go.

The big sheet that was attached to the back of the stove has diagrams but doesn't mention a test mode, only instructions to test for continuity, and what the ohms reading should be for all the components.

How do I properly test the relay board?

[stj]

wanna try something questionable?

in theory if you wrap a few turns of insulated wire around a power cable and then hook the ends to a voltmeter it will show current-flow as volts.
but i dont know the turns-ratio so you could get mV or you could get much higher

[redwire]

Yeah that's an old trick of seeing a millivolt voltage drop across a wire or fuse which indicates current flow. We don't know if there is power to the bake heater or current flow when the oven is hot and craps out.

Hey, I looked a bit more and apparently LG ovens have thermal limiters (fuse) i.e. https://www.reliableparts.ca/product...al Safety Fuse
OP, can you post a picture of the back wall of the oven?
You would be able to see one of these, or spot bad connections etc.
Usually a bad connection or relay contact will make heat and burn/melt things as well as a zzzzzzt sound. They can stand out.

edit: there was an LG recall for this stove. "The burners on the electric ranges can fail to turn off after being switched off or the temperature setting can increase unexpectedly during use, posing burn and fire hazards to consumers. To determine whether your electric range is being recalled, please follow the instructions below."
https://www.lgelectricrangerecall.com/

[stj]

Originally Posted by redwire

Yeah that's an old trick of seeing a millivolt voltage drop across a wire or fuse which indicates current flow.

no, i'm talking about creating an induction coil to obtain some mV based on the current flowing into the heater

[R_J]

Most ovens will turn on both the main element and the broil element to get the oven up to temperature faster (preheat), In this case it seems that only the broil element is being tuned on to get the temp to 365, If the lower element is not operating it is either open or not receiving any voltage.
Locate the relay board and check it.

[CapLeaker]

Originally Posted by R_J

Most ovens will turn on both the main element and the broil element to get the oven up to temperature faster (preheat), In this case it seems that only the broil element is being tuned on to get the temp to 365, If the lower element is not operating it is either open or not receiving any voltage.
Locate the relay board and check it.

I side with R_J on this. My broil element has no problem heating up the store way past the 365. Matter effect, that’s also how the oven clean function works too. The bottom element isn’t always on. I don’t think there is a short either, rather an open on either the element, a bad relay or whatever turns the relay on. I am not sure maybe a temperature control of some sort sounds viable too, but that should throw some code, I would imagine.

[R_J]

The first thing I would do is inspect the solder side of the relay board for poor solder connections on the relays

Can you provide the part number off the relay?

[Curious808]

Originally Posted by R_J

The first thing I would do is inspect the solder side of the relay board for poor solder connections on the relays

Can you provide the part number off the relay?

Is that it on top left of the board: 6870WIN12A?

[R_J]

Originally Posted by Curious808

Is that it on top left of the board: 6870WIN12A?

NO, on the black plastic relay itself. but I think I figured out the pinout
Have you removed the board and checked the solder connections?

It looks like the smaller relay (RY6) is the one that controls the bake (lower) element

Does that sheet diagram show how the elements are connected to the relay board?

[Curious808]

The two black plastic relays are marked Potter & Brumfield T9AS1D22-12.

The solders look good to me. They are all shiny and reflective though the photo doesn't capture that. Some have a dull spot in the middle.