Sensor

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Manufacturer's instructions represent the basis of local codes, the foundation of warrantee and insurance. Manufacturer's instructions include: "Installer must be a trained, experienced service technician." It is best to follow the manufacturer's instructions to the letter; which are usually to the effect of calling a repair company authorized by the manufacturer and/or the utility. Manufacturer's instructions that accompany the product tell what a DIY can do and what is recommended to be done by a certified tech. No intelligent technician can advise against following such instructions and expect to stay in business; therefore you can identify a dangerous, inexperienced guesser by advice that ignores manufacturer's instructions and diverts you away from those instructions. Changing any part will not complete the checks for electrical connection tightness and cleanliness, gas pressure, and combustion efficiency to prevent CO, that the manufacturer recommends. The best a concerned, experienced tech, who is aware that leaving any step of the recommended service procedure undone creates danger, can do is to direct you to the instructions, therefore:

Electronic ignitions require that the flame be proved by a current through a flame to the sensor. If there is resistance to the electric flow due to coating , bad contacts, or a failing module, then the ignitor will stop, some will retry threee times, some repeatedly.

The most frequent and common cause of an electronic ignition preventing you from having a furnace explode is for the sensor not to sense a flame. Unfortunately, chlorine and other chemicals in the air, especially in a laundry area, coats the sensor so it can not sense a flame. Cleaning or replacement of the sensor is a periodic requirement. If you can not find it; call a pro.

Early fall is the best time to let a repair person tune up the furnace by testing what the manufacturer requires. It doesn't matter what source of fuel. All furnace/boilers require a yearly test and clean up.
The cost for the service tech to come to your house or location to check everything over is very small compared to the furnace breaking down in the middle of winter.

The average homeowner can change the air filter(s), keep the furnace/boiler area clear of debris, and give a quick look-over to make sure it is ok.

Ah, I miss the good old days when all the residential units used natural draft chimneys and either thermocouples or oil burners (just after the coal days with no complicated controls) and no dampers. The old units could be tinkered with by handymen and there were no code inspectors to check if anything was done right, so nobody could sue for being careless. Then, the idea of the level of difficulty matched the expectations of the novice.

Not so today - DIY's jump right into the stuff that is reserved for folks that have been basic repairmen in that particular field for some years, maybe a decade, and then catch an interest to troubleshoot electronic controls, so they go to schools for the expanded knowledge. Just like many kids, it seems so easy to start at the top and bypass all the courses first. "Tell me all you have learned in a lifetime - it should only take three minutes." - in this situation, in one paragraph.
http://www.scacdtechsupport.com/hot_surface_ignition.html

But, be kind, it is possible for a genious to learn the entire troubleshooting procedure and complete operation of all the controls involved in a few hands-on minutes. All he has to do is find one other genious who will take the risk of instructing someone he has never met and trusting that he can learn it all and won't kill himself in the process. Or if he does accidentally kill himself, no other will find out to sue him for being so foolish as not to know that it takes all kinds to make a world.

In other words: List your training that qualifies you as an authorized repairman, trained in their electrical controls, and authorized gas heat specialist, so the factory can continue the warrantee and take the legal responsibity for advanture. But then, you wouldn't need to ask.

The web is not the place to get that training, just the fundamentals of homeowner service.

http://www.heatincorporated.com/~technical_information/Troubleshooting/Flamerec.pdf

This is the most frequent question in the heating DIY sites, as it is a universal problem with flame sensors.

http://www.heatincorporated.com/~technical_information/Troubleshooting/Flamerec.pdf

Chlorine, sulphur and other chemical compounds, especially in laundry rooms, coat the sensor and create a resistacne to the current. Cleaning the sensor with a dollar bill or other scrub surface that will not leave a wire (steel wool is a culprit) to short out the sensor to ground might restore the sensor.

A useful practice is to keep a new sensor on hand and learn to replace it in the fall before the season starts. That should be a regular practice of a service company. Although the incentive is lessened considerably by being able to keep busy and wealthy all winter just cleaning or replacing sensors, there really isn't usually much more to do.