The symptom you're experiencing is that the "flame sensor" is not seeing a flame after a few seconds of operation. Most likely the cause of this failure is that there really is no flame present, thus the safety is doing its job. Oil heaters need regular maintainence, including inspecting the flue, changing the nozzle, changing the fuel filter, check belts if any and blower, replace air filter etc. This isn't that expensive to have done by a pro who has the parts on his or her truck.

There are a myriad of obscure issues with oil heaters of this type, one common source of a tripping safety is a small leak on the inlet side of the piping, which allows bubbles to form just behind the nozzle. These can be a bear to find and fix. Another issue you run into with these is if the piping from the tank to the pump is pinched or too small, the vacum of the pump drawing fluid can cause light oils within the fuel to go from liquid to vapor forming bubbles with the same result as an air leak. You can even get an occasional tank of fuel that's prone to this.

I'd suggest calling a tech. who doesn't mind the smell and has a few years under his or her belt.

take care

: This happened to my furnace as well. As it turned out, the transformer was bad. It cost $150 and a professional to replace the transformer.

:
: : I will try to explain this as best I can. I have no knowledge of furnaces. I turn up the thermostat, nothing happens. I push a red button on the furnace to reset it, and the motor starts to run, a few minutes later I hear a click and the blower comes on. The furnace will run shuting off when it reaches the set temp. But the furnace will not kick on by itself, I need to push the red button again. It has an old mercury switch that should be replaced? Is this the thermocouple, igniter? Is it something my husband can fix? Thank you.