Which part should be tested? Flame sensor Expansion valve Solenoid valve Gas valve
A toilet is cycling on and off, and the tank is refilling every few minutes. What should you check first?
1) flush valve
2) fill valve
5) overflow tube
Q1 - Flame sensor
First, turn off the power to your furnace. Check all wires and ground wires to make sure they have good tight connections and are not burnt. Next, remove the screw or screws that hold your flame sensor in. Please do not confuse the flame sensor with an Ignitor. We do not want to touch the ignitor at all. If you touch an ignitor it will shorten the life of the ignitor or it will break. Below this paragraph, we have a picture comparing an ignitor with a flame sensor. Clean your flame sensor with steel wool or emery cloth (fine sand paper). You might need a new flame sensor, but most of the time they can be cleaned and your furnace will work well after cleaning. I have seen the flame sensor go out. Most of the time when they go out the white porcelain is cracked. You might want to have a spare flame sensor on hand.
Q2 - Flapper
If your toilet keeps refilling at 15-minute intervals, it probably has a leak. In the vast majority of cases, the problem is with the flapper, which is the rubber seal between the tank and the bowl. When it's worn, or the chain is too short, water seeps into the bowl, and the fill valve cycles on as soon as the float falls below the cutoff level. There's a more serious possibility, however, and that is that the tank is leaking. If so, you'll see water on the floor. Some tank leaks require replacement of the toilet.