The sensor wire is not sensing the flame. Check the size of the flame to make sure it is making the metal glow. If it is the ignitor box may be bad.
The person asking the question asked why it is just clicking and not firing. That has nothing to do with the flame sensor.
Obviously, the pilot is not allowing gas through so the spark can ignite it. I would replace the ignitor. The gas orifice in it is more than likely clogged. The hole in the orifice is at maximum, 15 thousandths of an inch in diameter. They tend to get clogged if dirty gas gets into the system (happens often with LP(propane) but also happens with Natural Gas. If not sure of how to do this job, call an appliance service person or company. It may cost more but it will be done safely and correctly .
Have you turned the control from Pilot to On? Have you increased the temperature setting?
my boiler wont ignite
THERMOCOUPLE or defective gas valve
The thermocouple is bad. The thermocouple is the small metal tube that sits in the pilot light flame and runs to the control valve. The thermocouple generates a very small electrical current that keeps the gas valve open. They go bad regularly. **** I would have to say to check to make sure the valve is not in the "Pilot" position. If it is then turn it to the "On' position. If it still isn't working then you may have a bad Gas Control Valve. If the thermocouple was bad the pilot light would not stay lit. If the pilot light stay lit, it is doing what it is supossed to do.
Have it checked for codes.
Most likely the thermocouple is bad. The thermocouple is the little tube that sits in the flame of the pilot light. Runs from the pilot light to the control valve. Very simple to replace. Costs about $6.
Could be lots of things. If you have a standing pilot light, one that stays on all the time, the thermocouple may be bad. A thermocouple is a small rod that sits in the flame of the pilot light. It then runs to the gas valve. This produces a small amount of electricity that keeps the valve open to let the gas in. If the pilot light goes out it shuts the valve so the house doesn't blow up. Easy enough to replace. If the pilot light is still on and the furnace will not light, it has to be the thermocouple. Check fuses at the furnace and in the panel. Newer gas furnaces have a circuit board in them that can go bad. There are a dozen things that could keep it from working, it is just a process of elination.
I don't know if this will help, but I had a similar problem and it was a short in the light switch. Because the alarm was wired into the light switch. I wiggled the switch and it started. Change it.
Bad thermo,replace. The thermo senses that the pilot is lit and opens gas valve,also could be bad or blocked gas valve.
my alarm keeps going of and wont start.
This question is not specific enough for a good answer. What is the appliance? A furnace? A cooktop burner? An oven? The first thing to check would be the igniter, which is what typically "lights" the gas coming from the valve. You can check an igniter with a simple continuity test using a test meter. If the pilot is lit, then there is no igniter, or the igniter lites the pilot, so if the pilot is lit how could it be the igniter. Bad thermocouple, this tells the gas valve that the pilot light is lit, if the gas valve does not know the pilot is lit it will not allow gas to come out of the valve, this is a safety feature so your unit does not blow up.
i have no idea it wont tell me