Plumbing

I have a bathroom that is usually odor-free but occasionally sewer odor creeps in. When it does, it’s usually not intense but on rare occasions it’s intense enough to smell from the next room.

Any ideas? ~~I cannot connect it with any activity.~~¹ It seems to hit randomly. Traps are good. It seems to close to the toilet connection to the pipework but there are no waste water leaks in sight.

Do I need to remove the toilet and dig up the tiles? I could hire a plumber to scope the drain, but that would likely cost more me digging up the bathroom myself. Should I look into renting a drain camera? Or would it make sense to rent an infrared camera and pour hot water in the drain?

I think the kitchen drains may be upstream from the bathroom.

¹ (update) the odor seems to hit after I run hot water in the kitchen just upstream to the bathroom.

Has anyone encountered this? I didn’t talk directly to the plumber but was told they will not flush a 30+ year old tank. I wonder if the plumber is concerned that it’s so fragile that flushing would cause leaks.

This diagram is from the service manual of a combi boiler. It’s a flow sensor which detects whether hot water is running, which is then used to trigger on-demand heat and switch a diverter to take radiators out of the loop.

In English, the diagram shows:

• X ⅔ red wire (+5V)
• X 2/2 black wire (ground)
• X 2/6 green wire (signal)

I need to know what those fractions mean. I took the voltage measurements in this video:

• https://iv.ggtyler.dev/watch?v=d8ucufoyUlQ

I cannot necessarily trust the model in that video to have the same specs as mine. My voltmeter detected 4.68 V on the red input wire showing that the sensor is well fed. The green “signal” wire is supposed to be 0 V at rest and 2 V with water running (or I think the reverse of that is used in some models). In my case the green wire is ~1.33 V at rest and ~0.66 V when water is running. I need to know if these readings are normal as I troubleshoot this problem.

update

As the responders point out, the strings in the diagram represent labels for where the wires land on the motherboard. The underlying problem was also solved with the help of someone in a cross-posted thread.