The burner won't fire on a cold morning. Start here.

Fuel first

Diesel gels. Not dramatically, and not always visibly, but enough to starve a burner on the first genuinely cold morning of the season. Check the filter before you check anything electrical — this is the single most common cold-morning burner call we take, and it costs nothing to rule out.

Water in the fuel does the same thing and is more likely than people assume if the tank has been sitting.

Then electrodes

Electrodes foul with soot and drift out of gap over a season of vibration. A burner that lit fine in September and will not light in December has often just drifted. Pull them, look at them, gap them to spec, and clean the porcelain.

Then the thermostat and the switches

Most machines will not fire the burner unless there is flow. If the flow switch or pressure switch is not closing, the burner never gets the signal — and the machine looks completely healthy otherwise because the pump side is fine.

Airflow last

A restricted intake or a soot-loaded blower wheel changes the fuel-air mixture enough to stop ignition. It is less common than the first three, but it is what is left when they check out.

Burner work is what our bench does most. If you have been through fuel and electrodes and it is still not lighting, bring it in rather than swapping parts at it — that gets expensive quickly and it is usually one thing.