Hi, I have a dual energy hot water furnace. When the temperature goes below -12°C, the oil furnace is supposed to start. 90% of the time it works. We have had 3 different technicians during the last 3-4 years. They have changed filters, nozzles, oil ignition control box, they have cleaned the electrodes and adjusted the mixture.
My question is could it be a problem with the transformer not producing enough voltage? And is there a test for this?

Transformer about 10,000 volts. There is a special high voltage probe to test that hi a voltage. OR option B is to space a screw driver grounded to one transformer contact about 1/4 of a inch away from the other contact and see how far a arc you can pull when you move the screw driver back away from the other contact point. You need a nice hard blue arc about 1/2 inch minimum in length. The longer the better.

The last way to test is really not that good but back in the 50's and 60's we did not have all the fancy tools of today to work with. If you really want to be accurate have the service tech use the high voltage probe. That is what the probe is designed for.