Re: little spark to start my unvented 8 year old Rinnai....
Yes - most likely thermocouple, or if the pilot has an orange tip, it will not heat the thermocouple enough to keep the valve energised. Try a blast with canned air or contact cleaner down the tube, not 100psi air compressor.
If that doesn't help, time to ring a gasfitter.
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I have experience with coleman furnaces installed in mobile homes. There is a red switch on the gas valve. Some models require you to slide the red switch to the top and hold. Holding is allowing gas to the pilot. You have to stick a long match,grill igniter, to stick through the inspection hole (a little flap or door to see inside the burner area) and hold the match over the pilot while holding the switch upwards at the same time. Others have a pilot selection on the gas valve which you slide the red switch to pilot and push down and hold while liting the pilot. You may need help with holding the door open while doing this. If you can you can use the copper line for the thermocouple to hold the door open for you. Good luck
depending on how old the unit is varies... if you actually have a pilot assembly you will hold down the pilot valve located on the gas valve adn you would turn dial to pilot and push down red button or knob itself the stick a long match in pilot assembly area and lite...once lighted keep holding down for about 15 seconds to get thermocoupler hot adn then rotate dial to on...if it goes out star toperation again and if continues then the copper thermocoupler next to pilot assembly is dirty or bad... if you don't have a pilot assembly and unit is within last 10 years or so you may have a hot surface ignitor in there and it automatically glows on the call for heat and will lite..if your not seeing it its made of porcelein and could have cracked or went out.... some units also have spark ignition and originates from the board and then will go to a spark module ....repost if you have more questions a/c & heat servicer for 18 years...
The thing with these is over the summer months when they are not in use they get dust inside the ODS safety pilot. When that dust gets in there it cuts down on the oxygen inside and the safety simply shuts down because it was designed to do that. What you need to do is flush the pilot and allow it to dry. I use a product caller but you could use shop air or even a soda straw to blow out the pilot. I used to sell these thermocouples for ODS systems and they are not cheap. Most times they do not solve the problem either because the pilot is dusty. A brand new one won't solve your problem and a cheaper thermocouple don't work on these. More HERE
What do you mean by the pilot light is on but won't light. Do you mean that there is gasbut won't light. If so the spark may not be hitting the pilot properly or it may not be getting enough gas. If lighting but continuing to spark the sensor may need cleaning. This can be cleaned with steel wool (not sand paper), probably will need to disassemble the pilot asembly The pilot may also be dirty reducing gas flow. If this is the case dis-assembly will be needed and the pilot cleaned.
Possibly A bad thermocoupler. It is the part that looks like a stiff solid copper wire with on end sticking in the flame of the pilot light and is a safety feature if the pilot goes out it starts to cool and the resistance changes back you the module and the module shuts the gas off. I have seen these go intermediately faulty in waterheaters and do just what you described.