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Disclaimer

I'm not very sure about the stack where I'm suppose to post my question (between here and "Home Improvement" stack.)

Question

A few days ago my oven started beeping repeatedly when it was powered on.

The beep is the power-on bit.

After taking the oven apart I noticed that what I am assuming to be a heat probe had a burnt wire.

This probe is supplied directly from the current input (brown wire), this same wire then continues to the oven control console. If I unplug the probe the oven does not work (which makes sense.)

I have two question:

  • How can I test this probe to be sure that it is not the one causing the problem?
  • Can I find this part on the internet if I have to change it?

Pictures

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JRE
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Hadock
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  • Which oven? Make? model? – Kartman Feb 13 '21 at 17:28
  • Those are simple mechanical switches which are controlled by a bimetal spring. Thermostat z42k gives enough results. It should be closed when cold and open when hot. You could put it into another oven, heat it up, then test for conduction while it's 200°C hot. Don't know if you want to try that. – Janka Feb 13 '21 at 17:52
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    That burnt plastic housing looks like bad news to me. They use heat and flame resistant plastics in oven parts that may get hot. That part looks as though it was exposed to open flame or got very, very hot. So I think you need to figure out why that happened. It could just be a bad crimp or whatever. But check it out. – user57037 Feb 13 '21 at 19:15
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    IMO, no device is malfunctioning except the bad crimping. – Marko Buršič Feb 13 '21 at 19:43

3 Answers3

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It appears to be a contact resistance failure of the 1/4" Quick Disconnect type plug causing a temperature exceeding the hotplate to melt the plastic.

enter image description here The plastic used must withstand the thermal protection switch temperature and then some just as the PVC of the wire , which appears to be in tact.

When this contact loosens from vibration and increases it's contact resistance sufficiently at the heater current to cause a temperature rise, oxidation process increases and at some point resulted in a thermal runaway that caused failure.

The solution requires a very low resistance wire crimp to be made which requires a tool and some skill to ensure either wire or wire + insulation crimp is made so that the contact is gas tight.

Find a technician who knows how to crimp these very common equivalent plugs. The exact same is unlikely to find easily with this plastic at right angle.

however one can replace it with enter image description here

The wire must pass a 5 pound (>2kg) pull test from the connector to ensure a reliable connection. The plug must fit tight on the thermal switch. If the vertical orientation is a problem, it may be pre-bent with care.

Tony Stewart EE75
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If that part were broken, then the oven wouldn't turn on at all.

Since it is a simple mechanical temperature switch, it can't be the source of the beep. There's no intelligence in it to signal that it isn't working right, and no way for the oven to test it for proper function.

The switch conducts when cold, and opens when it gets too hot. Since the oven turns on, it is conducting. The only way to see if it will open when it gets too hot is to heat it up. It should not get hot enough to open in normal operation, and you do not want to be playing with that thing when the oven is operating - it switches the main power supply for the oven, and you don't really want to be touching that when the oven is on.

There's something else wrong. The burned wire is a bad sign, but doesn't point to the cause.

This thermo switch is probably good.

The connector might be the problem. It is obviously getting hot. If it has bad contact, that would make it get hot and it would make the oven "think" that the line voltage is too low.

I'd fix the connector, and make sure that there's nothing close to the connector that could have burned it.

You could test it by clipping it to an ohmmeter and heating the front side (with the smooth circular part.)

  • When cold, it will conduct.
  • If you get it hot enough, it will go "click" and the ohmmeter will read an open circuit.

The trick is getting it hot without destroying it. The barrel of a soldering iron might get hot enough.

JRE
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These are normally closed temperature controlled bimetallic switches, so the first thing to do is grab a multimeter and test continuity between the two pins. It should ring. But you already did that test unknowingly, since you say:

If I unplug the probe the oven does not work (which makes sense.)

So it lets current through, which means it's probably okay. If you really worry about it, put it in a frying pan, metal face down and pins up, and cook to about 250°C. It should click. Let cool, it should click again. Triggering temperature should be a bit above the maximum temperature of the oven.

Since this type of switch is not adjustable, it's most likely there to cut off power if the actual oven thermostat (or whatever controls the heating) fails shorted and causes the heating element to run continuously. In other words, it's a simple and rugged fire prevention device.

The incinerated fast-on connector should be replaced and re-crimped properly so it makes good contact with the wire and does not heat up under load. If the contact was bad, that may be the cause of the trouble. If the oven still beeps once that's done, then the problem is somewhere else. If the thermostat is electronic, maybe the actual temperature sensor is disconnected or has failed.

bobflux
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